No abstract

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.

Mutations in BCS1L, an assembly factor that facilitates the insertion of the catalytic Rieske Iron‐Sulfur subunit into respiratory chain complex III, result in a wide variety of clinical phenotypes that range from the relatively mild Björnstad syndrome to the severe GRACILE syndrome. To better understand the pathophysiological consequences of such mutations, we studied fibroblasts from six complex III‐deficient patients harboring mutations in the BCS1L gene. Cells from patients with the most severe clinical phenotypes exhibited slow growth rates in glucose medium, variable combined enzyme deficiencies, and assembly defects of respiratory chain complexes I, III, and IV, increased H2O2 levels, unbalanced expression of the cellular antioxidant defenses, and apoptotic cell death. In addition, all patients showed cytosolic accumulation of the BCS1L protein, suggestive of an impaired mitochondrial import, assembly or stability defects of the BCS1L complex, fragmentation of the mitochondrial networks, and decreased MFN2 protein levels. The observed structural alterations were independent of the respiratory chain function and ROS production. Our results provide new insights into the role of pathogenic BCS1L mutations in mitochondrial function and dynamics. Hum Mutat 31:–12, 2010. © 2010 Wiley‐Liss, Inc.

The oxidative phosphorylation system contains four respiratory chain complexes that connect the transport of electrons to oxygen with the establishment of an electrochemical gradient over the inner membrane for ATP synthesis. Due to the dual genetic source of the respiratory chain subunits, its assembly requires a tight coordination between nuclear and mitochondrial gene expression machineries. In addition, dedicated assembly factors support the step-by-step addition of catalytic and accessory subunits as well as the acquisition of redox cofactors. Studies in yeast have revealed the basic principles underlying the assembly pathways. In this review, we summarize work on the biogenesis of the bc complex or complex III, a central component of the mitochondrial energy conversion system.

Respiratory chains are crucial for cellular energy conversion and consist of multi‐subunit complexes that can assemble into supercomplexes. These structures have been intensively characterized in various organisms, but their physiological roles remain unclear. Here, we elucidate their function by leveraging a high‐resolution structural model of yeast respiratory supercomplexes that allowed us to inhibit supercomplex formation by mutation of key residues in the interaction interface. Analyses of a mutant defective in supercomplex formation, which still contains fully functional individual complexes, show that the lack of supercomplex assembly delays the diffusion of cytochrome c between the separated complexes, thus reducing electron transfer efficiency. Consequently, competitive cellular fitness is severely reduced in the absence of supercomplex formation and can be restored by overexpression of cytochrome c . In sum, our results establish how respiratory supercomplexes increase the efficiency of cellular energy conversion, thereby providing an evolutionary advantage for aerobic organisms.

We have analyzed the cellular pathways and metabolic adaptations that take place in primary skin fibroblasts from patients with mutations in BCS1L, a major genetic cause of mitochondrial complex III enzyme deficiency. Mutant fibroblasts exhibited low oxygen consumption rates and intracellular ATP levels, indicating that the main altered molecular event probably is a limited respiration-coupled ATP production through the OXPHOS system. Two-dimensional DIGE and MALDI-TOF/TOF mass spectrometry analyses unambiguously identified 39 proteins whose expression was significantly altered in complex III-deficient fibroblasts. Extensive statistical and cluster analyses revealed a protein profile characteristic for the BCS1L mutant fibroblasts that included alterations in energy metabolism, cell signaling and gene expression regulation, cytoskeleton formation and maintenance, and intracellular stress responses. The physiological validation of the predicted functional adaptations of human cultured fibroblasts to complex III deficiency confirmed the up-regulation of glycolytic enzyme activities and the accumulation of branched-chain among other amino acids, suggesting the activation of anaerobic glycolysis and cellular catabolic states, in particular protein catabolism, together with autophagy as adaptive responses to mitochondrial respiratory chain dysfunction and ATP deficiency. Our data point to an overall metabolic and genetic reprogramming that could contribute to explain the clinical manifestations of complex III deficiency in patients.

Despite considerable knowledge on the genetic basis of mitochondrial disorders, their pathophysiological consequences remain poorly understood. We previously used two-dimensional difference gel electrophoresis analyses to define a protein profile characteristic for respiratory chain complex III-deficiency that included a significant overexpression of cytosolic gelsolin (GSN), a cytoskeletal protein that regulates the severing and capping of the actin filaments. Biochemical and immunofluorescence assays confirmed a specific increase of GSN levels in the mitochondria from patients' fibroblasts and from transmitochondrial cybrids with complex III assembly defects. A similar effect was obtained in control cells upon treatment with antimycin A in a dose-dependent manner, showing that the enzymatic inhibition of complex III is sufficient to promote the mitochondrial localization of GSN. Mitochondrial subfractionation showed the localization of GSN to the mitochondrial outer membrane, where it interacts with the voltage-dependent anion channel protein 1 (VDAC1). In control cells, VDAC1 was present in five stable oligomeric complexes, which showed increased levels and a modified distribution pattern in the complex III-deficient cybrids. Downregulation of GSN expression induced cell death in both cell types, in parallel with the specific accumulation of VDAC1 dimers and the release of mitochondrial cytochrome c into the cytosol, indicating a role for GSN in the oligomerization of VDAC complexes and in the prevention of apoptosis. Our results demonstrate that respiratory chain complex III dysfunction induces the physiological upregulation and mitochondrial location of GSN, probably to promote cell survival responses through the modulation of the oligomeric state of the VDAC complexes.

Mitochondrial oxidative phosphorylation (OXPHOS) defects are the primary cause of inborn errors of energy metabolism. Despite considerable progress on their genetic basis, their global pathophysiological consequences remain undefined. Previous studies reported that OXPHOS dysfunction associated with complex III deficiency exacerbated the expression and mitochondrial location of cytoskeletal gelsolin (GSN) to promote cell survival responses. In humans, besides the cytosolic isoform, GSN presents a plasma isoform secreted to extracellular environments. We analyzed the interplay between both GSN isoforms in human cellular and clinical models of OXPHOS dysfunction. Regardless of its pathogenic origin, OXPHOS dysfunction induced the physiological upregulation of cytosolic GSN in the mitochondria (mGSN), in parallel with a significant downregulation of plasma GSN (pGSN) levels. Consequently, significantly high mGSN-to-pGSN ratios were associated with OXPHOS deficiency both in human cells and blood. In contrast, control cells subjected to hydrogen peroxide or staurosporine treatments showed no correlation between oxidative stress or cell death induction and the altered levels and subcellular location of GSN isoforms, suggesting their specificity for OXPHOS dysfunction. In conclusion, a high mitochondrial-to-plasma GSN ratio represents a useful cellular indicator of OXPHOS defects, with potential use for future research of a wide range of clinical conditions with mitochondrial involvement.