Replication stalling by catalytically impaired Twinkle induces mitochondrial DNA rearrangements in cultured cells

Item does not contain fulltextPathological mitochondrial DNA (mtDNA) rearrangements have been proposed to result from repair of double-strand breaks caused by blockage of mitochondrial DNA (mtDNA) replication. As mtDNA deletions are seen only in post-mitotic tissues, it has been suggested that they are selected out in actively dividing cells. By electron microscopy we observed rearranged mtDNA molecules in cultured human cells expressing a catalytically impaired helicase. As these molecules were undetectable by PCR, we propose that deleted mtDNA molecules in cultured cells are fragile and sensitive to heating. Further consequences of mtDNA replication stalling are discussed

What cost mitochondria? The maintenance of functional mitochondrial DNA within and across generations

The peculiar biology of mitochondrial DNA (mtDNA) potentially has detrimental consequences for organismal health and lifespan. Typically, eukaryotic cells contain multiple mitochondria, each with multiple mtDNA genomes. The high copy number of mtDNA implies that selection on mtDNA functionality is relaxed. Furthermore, because mtDNA replication is not strictly regulated, within-cell selection may favour mtDNA variants with a replication advantage, but a deleterious effect on cell fitness. The opportunities for selfish mtDNA mutations to spread are restricted by various organism-level adaptations, such as uniparental transmission, germline mtDNA bottlenecks, germline selection and, during somatic growth, regular alternation between fusion and fission of mitochondria. These mechanisms are all hypothesized to maintain functional mtDNA. However, the strength of selection for maintenance of functional mtDNA progressively declines with age, resulting in age-related diseases. Furthermore, organismal adaptations that most probably evolved to restrict the opportunities for selfish mtDNA create secondary problems. Owing to predominantly maternal mtDNA transmission, recombination among mtDNA from different individuals is highly restricted or absent, reducing the scope for repair. Moreover, maternal inheritance precludes selection against mtDNA variants with male-specific effects. We finish by discussing the consequences of life-history differences among taxa with respect to mtDNA evolution and make a case for the use of microorganisms to experimentally manipulate levels of selectio

New light on space hazards

In the last decade it has become increasingly clear that mitochondrial DNA (mtDNA) is not naked but associated with proteins in poorly defined structures called nucleoids that are essential for mtDNA maintenance. The function of nucleoids is not simply to package mtDNA but also to provide a stable environment for its replication, transcription and repair. Even though their properties and dynamics have begun to be revealed in recent years, their structural and molecular organization remains largely unknown in mammals. Although, there are a number of proteins identified to be nucleoid associated by using several biochemical isolation methods combined with mass spectrometric analysis, the main difficulties in the identification of these proteins are their low abundance and the assumed dynamic composition of nucleoids. Considering various purification methods, there is a thin line between the stringency and specificity in the identification of potential nucleoid associated proteins. In this review, the main focus is to provide a comprehensive comparison of the so far published purification and analysis methods to generate a list of potentially nucleoid associated proteins, but also, to discuss the disadvantages and possible improvements in proteomic analyses

DNA Sequences Proximal to Human Mitochondrial DNA Deletion Breakpoints Prevalent in Human Disease Form G-quadruplexes, a Class of DNA Structures Inefficiently Unwound by the Mitochondrial Replicative Twinkle Helicase

Contains fulltext : 138677.pdf (publisher's version ) (Open Access)Mitochondrial DNA deletions are prominent in human genetic disorders, cancer, and aging. It is thought that stalling of the mitochondrial replication machinery during DNA synthesis is a prominent source of mitochondrial genome instability; however, the precise molecular determinants of defective mitochondrial replication are not well understood. In this work, we performed a computational analysis of the human mitochondrial genome using the "Pattern Finder" G-quadruplex (G4) predictor algorithm to assess whether G4-forming sequences reside in close proximity (within 20 base pairs) to known mitochondrial DNA deletion breakpoints. We then used this information to map G4P sequences with deletions characteristic of representative mitochondrial genetic disorders and also those identified in various cancers and aging. Circular dichroism and UV spectral analysis demonstrated that mitochondrial G-rich sequences near deletion breakpoints prevalent in human disease form G-quadruplex DNA structures. A biochemical analysis of purified recombinant human Twinkle protein (gene product of c10orf2) showed that the mitochondrial replicative helicase inefficiently unwinds well characterized intermolecular and intramolecular G-quadruplex DNA substrates, as well as a unimolecular G4 substrate derived from a mitochondrial sequence that nests a deletion breakpoint described in human renal cell carcinoma. Although G4 has been implicated in the initiation of mitochondrial DNA replication, our current findings suggest that mitochondrial G-quadruplexes are also likely to be a source of instability for the mitochondrial genome by perturbing the normal progression of the mitochondrial replication machinery, including DNA unwinding by Twinkle helicase

Analysis of 953 human proteins from a mitochondrial HEK293 fraction by complexome profiling

Contains fulltext : 128506.pdf (publisher's version ) (Open Access)Complexome profiling is a novel technique which uses shotgun proteomics to establish protein migration profiles from fractionated blue native electrophoresis gels. Here we present a dataset of blue native electrophoresis migration profiles for 953 proteins by complexome profiling. By analysis of mitochondrial ribosomal complexes we demonstrate its potential to verify putative protein-protein interactions identified by affinity purification-mass spectrometry studies. Protein complexes were extracted in their native state from a HEK293 mitochondrial fraction and separated by blue native gel electrophoresis. Gel lanes were cut into gel slices of even size and analyzed by shotgun proteomics. Subsequently, the acquired protein migration profiles were analyzed for co-migration via hierarchical cluster analysis. This dataset holds great promise as a comprehensive resource for de novo identification of protein-protein interactions or to underpin and prioritize candidate protein interactions from other studies. To demonstrate the potential use of our dataset we focussed on the mitochondrial translation machinery. Our results show that mitoribosomal complexes can be analyzed by blue native gel electrophoresis, as at least four distinct complexes. Analysis of these complexes confirmed that 24 proteins that had previously been reported to co-purify with mitoribosomes indeed co-migrated with subunits of the mitochondrial ribosome. Co-migration of several proteins involved in biogenesis of inner mitochondrial membrane complexes together with mitoribosomal complexes suggested the possibility of co-translational assembly in human cells. Our data also highlighted a putative ribonucleotide complex that potentially contains MRPL10, MRPL12 and MRPL53 together with LRPPRC and SLIRP

Human Mitochondrial DNA-Protein Complexes Attach to a Cholesterol-Rich Membrane Structure

Contains fulltext : 152311.pdf (publisher's version ) (Open Access