Making proteins in the powerhouse

Understanding regulation of mitochondrial DNA (mtDNA) expression is of considerable interest as mitochondrial dysfunction is important in human pathology and ageing. Similar to the situation in bacteria, there is no compartmentalization between transcription and translation in mitochondria; hence, both processes are likely to have a direct molecular crosstalk. Accumulating evidence suggests that there are important mechanisms for regulation of mammalian mtDNA expression at the posttranscriptional level. Regulation of mRNA maturation, mRNA stability, translational coordination, ribosomal biogenesis and translation itself, all form the basis for controlling oxidative phosphorylation capacity. Consequently, a wide variety of inherited human mitochondrial diseases are caused by mutations of nuclear genes regulating various aspects of mitochondrial translation. Furthermore, mutations of mtDNA, associated with human disease and ageing, often affect tRNA genes critical for mitochondrial translation. Recent advances in molecular understanding of mitochondrial translation regulation will likely provide novel avenues for modulating mitochondrial function to treat human disease.VetenskapsrådetAccepte

Tracing the Trail of Protons through Complex I of the Mitochondrial Respiratory Chain

Mitochondria are the structures that produce the bulk part of the cellular energy currency ATP, which drives numerous energy requiring processes in the cell. This process involves a series of large enzyme complexes—the respiratory chain—that couples the transfer of electrons to the creation of a concentration gradient of protons across the inner mitochondrial membrane, which drives ATP synthesis. Complex I (or NADH-quinone oxidoreductase) is the largest and by far the most complicated of the respiratory chain enzyme complexes. The molecular mechanism whereby it couples electron transfer to proton extrusion has remained mysterious until very recently. Low-resolution X-ray structures of complex I have, surprisingly, suggested that electron transfer in the hydrophilic arm, protruding into the mitochondrial matrix, causes movement of a coupling rod that influences three putative proton pumps within the hydrophobic arm embedded in the inner mitochondrial membrane. In this Primer, we will briefly introduce the recent progress made in this area and highlight the road ahead that likely will unravel the detailed molecular mechanisms of complex I function

Structure Casts Light on mtDNA Replication

In this issue, Lee et al. (2009) present a crystal structure of the human mitochondrial DNA polymerase (POLγ). The structure of this heterotrimeric enzyme lays a foundation for understanding how POLγ mutations cause human mitochondrial disease and why some antiviral nucleoside analogs cause cellular toxicity

Understanding aging: revealing order out of chaos

AbstractAging is often described as an extremely complex process affecting all of the vital parameters of an individual. In this article, we review how understanding of aging evolved from the first analyses of population survival to the identification of the molecular mechanisms regulating life span. Abundant evidence implicates mitochondria in aging and we focus on the three main components of the mitochondrial theory of aging: (1) increased reactive oxygen species (ROS) production, (2) mitochondrial DNA (mtDNA) damage accumulation, and (3) progressive respiratory chain dysfunction. Experimental evidence shows a relationship between respiratory chain dysfunction, ROS damage, and aging in most of the model organisms. However, involvement of the mtDNA mutations in the aging process is still debated. We recently created a mutant mouse strain with increased levels of somatic mtDNA mutations causing a progressive respiratory chain deficiency and premature aging. These mice demonstrate the fundamental importance of the accumulation of mtDNA alterations in aging. We present here an integrative model where aging is provoked by a single primary event leading to a variety of effects and secondary causes

Mitochondrial DNA mutations in disease and aging

The small mammalian mitochondrial DNA (mtDNA) is very gene dense and encodes factors critical for oxidative phosphorylation. Mutations of mtDNA cause a variety of human mitochondrial diseases and are also heavily implicated in age-associated disease and aging. There has been considerable progress in our understanding of the role for mtDNA mutations in human pathology during the last two decades, but important mechanisms in mitochondrial genetics remain to be explained at the molecular level. In addition, mounting evidence suggests that most mtDNA mutations may be generated by replication errors and not by accumulated damage

TFAM forces mtDNA to make a U-turn

The mammalian mitochondrial transcription factor A (TFAM) is encoded in the nucleus and imported into mitochondria, where it functions as an activator of mtDNA transcription and packages mtDNA into DNA-protein aggregates called mitochondrial nucleoids. Two X-ray crystallography studies in this issue reveal that TFAM shapes mtDNA into a sharp U-turn, thereby providing a molecular mechanism for its dual roles in the expression and maintenance of mtDNA.VetenskapsrådetPublishe

Loss of LRPPRC causes ATP synthase deficiency

Defects of the oxidative phosphorylation system, in particular of cytochrome-c oxidase (COX, respiratory chain complex IV), are common causes of Leigh syndrome (LS), which is a rare neurodegenerative disorder with severe progressive neurological symptoms that usually present during infancy or early childhood. The COX-deficient form of LS is commonly caused by mutations in genes encoding COX assembly factors, e.g. SURF1, SCO1, SCO2 or COX10. However, other mutations affecting genes that encode proteins not directly involved in COX assembly can also cause LS. The leucine-rich pentatricopeptide repeat containing protein (LRPPRC) regulates mRNA stability, polyadenylation and coordinates mitochondrial translation. In humans, mutations in Lrpprc cause the French Canadian type of LS. Despite the finding that LRPPRC deficiency affects the stability of most mitochondrial mRNAs, its pathophysiological effect has mainly been attributed to COX deficiency. Surprisingly, we show here that the impaired mitochondrial respiration and reduced ATP production observed in Lrpprc conditional knockout mouse hearts is caused by an ATP synthase deficiency. Furthermore, the appearance of inactive subassembled ATP synthase complexes causes hyperpolarization and increases mitochondrial reactive oxygen species production. Our findings shed important new light on the bioenergetic consequences of the loss of LRPPRC in cardiac mitochondria

Proteolytic Processing of OPA1 Links Mitochondrial Dysfunction to Alterations in Mitochondrial Morphology

Many muscular and neurological disorders are associated with mitochondrial dysfunction and are often accompanied by changes in mitochondrial morphology. Mutations in the gene encoding OPA1, a protein required for fusion of mitochondria, are associated with hereditary autosomal dominant optic atrophy type I. Here we show that mitochondrial fragmentation correlates with processing of large isoforms of OPA1 in cybrid cells from a patient with myoclonus epilepsy and ragged-red fibers syndrome and in mouse embryonic fibroblasts harboring an error-prone mitochondrial mtDNA polymerase {gamma}. Furthermore, processed OPA1 was observed in heart tissue derived from heart-specific TFAM knock-out mice suffering from mitochondrial cardiomyopathy and in skeletal muscles from patients suffering from mitochondrial myopathies such as myopathy encephalopathy lactic acidosis and stroke-like episodes. Dissipation of the mitochondrial membrane potential leads to fast induction of proteolytic processing of OPA1 and concomitant fragmentation of mitochondria. Recovery of mitochondrial fusion depended on protein synthesis and was accompanied by resynthesis of large isoforms of OPA1. Fragmentation of mitochondria was prevented by overexpressing OPA1. Taken together, our data indicate that proteolytic processing of OPA1 has a key role in inducing fragmentation of energetically compromised mitochondria. We present the hypothesis that this pathway regulates mitochondrial morphology and serves as an early response to prevent fusion of dysfunctional mitochondria with the functional mitochondrial network

Preserved respiratory chain capacity and physiology in mice with profoundly reduced levels of mitochondrial respirasomes

The mammalian respiratory chain complexes I, III 2, and IV (CI, CIII 2, and CIV) are critical for cellular bioenergetics and form a stable assembly, the respirasome (CI-CIII 2-CIV), that is biochemically and structurally well documented. The role of the respirasome in bioenergetics and the regulation of metabolism is subject to intense debate and is difficult to study because the individual respiratory chain complexes coexist together with high levels of respirasomes. To critically investigate the in vivo role of the respirasome, we generated homozygous knockin mice that have normal levels of respiratory chain complexes but profoundly decreased levels of respirasomes. Surprisingly, the mutant mice are healthy, with preserved respiratory chain capacity and normal exercise performance. Our findings show that high levels of respirasomes are dispensable for maintaining bioenergetics and physiology in mice but raise questions about their alternate functions, such as those relating to the regulation of protein stability and prevention of age-associated protein aggregation