Shaping the mitochondrial proteome

Monitoring alpha-helical membrane protein insertion into the outer mitochondrial membrane in mammalian cells

Mitochondrial function is dependent on the correct localization and insertion of membrane proteins into the outer mitochondrial membrane (OM). In mammals, the OM contains ∼150 proteins, the majority of which contain α-helical transmembrane domains. This family of α-helical proteins has significantly expanded in metazoans and has evolved to mediate critical signaling and regulatory processes including mitochondrial fusion and fission, mitophagy, apoptosis and aspects of the innate immune response. Recently, the conserved OM protein MTCH2 has been identified as an insertase for α-helical proteins in human mitochondria. However, our understanding of the targeting, insertion, folding and quality control of α-helical OM proteins remains incomplete. Here we highlight three methods to monitor α-helical protein insertion both in human cells and in vitro. First, we describe a versatile split fluorescent reporter system that can be used to monitor the insertion of α-helical proteins into the OM in human cells. Second, we delineate a streamlined approach to isolating functional, insertion competent mitochondria from human cells that are compatible with in vitro import assays. Finally, we explain in detail how to reconstitute the insertion of α-helical proteins in a minimal system, by creating functional proteoliposomes containing purified MTCH2. Together these tools represent an integrated platform to enable the detailed mechanistic analysis of biogenesis of the diverse and physiologically essential α-helical OM proteome.

A Critical Analysis of the FDA's Omics-Driven Pharmacodynamic Biomarkers to Establish Biosimilarity

Demonstrating biosimilarity entails comprehensive analytical assessment, clinical pharmacology profiling, and efficacy testing in patients for at least one medical indication, as required by the U.S. Biologics Price Competition and Innovation Act (BPCIA). The efficacy testing can be waived if the drug has known pharmacodynamic (PD) markers, leaving most therapeutic proteins out of this concession. To overcome this, the FDA suggests that biosimilar developers discover PD biomarkers using omics technologies such as proteomics, glycomics, transcriptomics, genomics, epigenomics, and metabolomics. This approach is redundant since the mode-action-action biomarkers of approved therapeutic proteins are already available, as compiled in this paper for the first time. Other potential biomarkers are receptor binding and pharmacokinetic profiling, which can be made more relevant to ensure biosimilarity without requiring biosimilar developers to conduct extensive research, for which they are rarely qualified.

Advances in Human Mitochondria-Based Therapies

Mitochondria are the key biological generators of eukaryotic cells, controlling the energy supply while providing many important biosynthetic intermediates. Mitochondria act as a dynamic, functionally and structurally interconnected network hub closely integrated with other cellular compartments via biomembrane systems, transmitting biological information by shuttling between cells and tissues. Defects and dysregulation of mitochondrial functions are critically involved in pathological mechanisms contributing to aging, cancer, inflammation, neurodegenerative diseases, and other severe human diseases. Mediating and rejuvenating the mitochondria may therefore be of significant benefit to prevent, reverse, and even treat such pathological conditions in patients. The goal of this review is to present the most advanced strategies using mitochondria to manage such disorders and to further explore innovative approaches in the field of human mitochondria-based therapies.

Mitochondrial biology and dysfunction in secondary mitochondrial disease

Mitochondrial diseases are a broad, genetically heterogeneous class of metabolic disorders characterized by deficits in oxidative phosphorylation (OXPHOS). Primary mitochondrial disease (PMD) defines pathologies resulting from mutation of mitochondrial DNA (mtDNA) or nuclear genes affecting either mtDNA expression or the biogenesis and function of the respiratory chain. Secondary mitochondrial disease (SMD) arises due to mutation of nuclear-encoded genes independent of, or indirectly influencing OXPHOS assembly and operation. Despite instances of novel SMD increasing year-on-year, PMD is much more widely discussed in the literature. Indeed, since the implementation of next generation sequencing (NGS) techniques in 2010, many novel mitochondrial disease genes have been identified, approximately half of which are linked to SMD. This review will consolidate existing knowledge of SMDs and outline discrete categories within which to better understand the diversity of SMD phenotypes. By providing context to the biochemical and molecular pathways perturbed in SMD, we hope to further demonstrate the intricacies of SMD pathologies outside of their indirect contribution to mitochondrial energy generation.

The Pathophysiological Significance of "Mitochondrial Ejection" from Cells

Mitochondria have beneficial effects on cells by producing ATP and contributing to various biosynthetic procedures. On the other hand, dysfunctional mitochondria have detrimental effects on cells by inducing cellular damage, inflammation, and causing apoptosis in response to various stimuli. Therefore, a series of mitochondrial quality control pathways are required for the physiological state of cells to be maintained. Recent research has provided solid evidence to support that mitochondria are ejected from cells for transcellular degradation or transferred to other cells as metabolic support or regulatory messengers. In this review, we summarize the current understanding of the regulation of mitochondrial transmigration across the plasma membranes and discuss the functional significance of this unexpected phenomenon, with an additional focus on the impact on the pathogenesis of cardiovascular diseases. We also provide some perspective concerning the unrevealed mechanisms underlying mitochondrial ejection as well as existing problems and challenges concerning the therapeutic application of mitochondrial ejection.

The small PPR protein SPR2 interacts with PPR-SMR1 to facilitate the splicing of introns in maize mitochondria

Splicing of plant mitochondrial introns is facilitated by numerous nucleus-encoded protein factors. Although some splicing factors have been identified in plants, the mechanism underlying mitochondrial intron splicing remains largely unclear. In this study, we identified a small P-type pentatricopeptide repeat (PPR) protein containing merely four PPR repeats, small PPR protein 2 (SPR2), which is required for the splicing of more than half of the introns in maize (Zea mays) mitochondria. Null mutations of Spr2 severely impair the splicing of 15 out of the 22 mitochondrial Group II introns, resulting in substantially decreased mature transcripts, which abolished the assembly and activity of mitochondrial complex I. Consequently, embryogenesis and endosperm development were arrested in the spr2 mutants. Yeast two-hybrid, luciferase complementation imaging, bimolecular fluorescence complementation, and semi-in vivo pull-down analyses indicated that SPR2 interacts with small MutS-related domain protein PPR-SMR1, both of which are required for the splicing of 13 introns. In addition, SPR2 and/or PPR-SMR1 interact with other splicing factors, including PPR proteins EMPTY PERICARP16, PPR14, and chloroplast RNA splicing and ribosome maturation (CRM) protein Zm-mCSF1, which participate in the splicing of specific intron(s) of the 13 introns. These results prompt us to propose that SPR2/PPR-SMR1 serves as the core component of a splicing complex and possibly exerts the splicing function through a dynamic interaction with specific substrate recognizing PPR proteins in mitochondria.

Intraspecies variation of the mitochondrial genome: An evaluation for phylogenetic approaches based on the conventional choices of genes and segments on mitogenome

Intraspecies nucleotide sequence variation is a key to understanding the evolutionary history of a species, such as the geographic distribution and population structure. To date, numerous phylogenetic and population genetics studies have been conducted based on the sequences of a gene or an intergenic region on the mitochondrial genome (mtDNA), such as cytochrome c oxidase subunits or the D-loop. To evaluate the credibility of the usage of such 'classic' markers, we compared the phylogenetic inferences based on the analyses of the partial and entire mtDNA sequences. Importantly, the phylogenetic reconstruction based on the short marker sequences did not necessarily reproduce the tree topologies based on the analyses of the entire mtDNA. In addition, analyses on the datasets of various organisms revealed that the analyses based on the classic markers yielded phylogenetic trees with poor confidence in all tested cases compared to the results based on full-length mtDNA. These results demonstrated that phylogenetic analyses based on complete mtDNA sequences yield more insightful results compared to those based on mitochondrial genes and segments. To ameliorate the shortcomings of the classic markers, we identified a segment of mtDNA that may be used as an 'approximate marker' to closely reproduce the phylogenetic inference obtained from the entire mtDNA in the case of mammalian species, which can be utilized to design amplicon-seq-based studies. Our study demonstrates the importance of the choice of mitochondrial markers for phylogenetic analyses and proposes a novel approach to choosing appropriate markers for mammalian mtDNA that reproduces the phylogenetic inferences obtained from full-length mtDNA.

Eukarya the chimera: eukaryotes, a secondary innovation of the two domains of life?

One of the most significant events in the evolution of life is the origin of the eukaryotic cell, an increase in cellular complexity that occurred approximately 2 billion years ago. Ground-breaking research has centered around unraveling the characteristics of the Last Eukaryotic Common Ancestor (LECA) and the nuanced archaeal and bacterial contributions in eukaryogenesis, resulting in fundamental changes in our understanding of the Tree of Life. The archaeal and bacterial roles are covered by theories of endosymbiogenesis wherein an ancestral host archaeon and a bacterial endosymbiont merged to create a new complex cell type - Eukarya - and its mitochondrion. Eukarya is often regarded as a unique and distinct domain due to complex innovations not found in archaea or bacteria, despite housing a chimeric genome containing genes of both archaeal and bacterial origin. However, the discovery of complex cell machineries in recently described Asgard archaeal lineages, and the growing support for diverse bacterial gene transfers prior to and during the time of LECA, is redefining our understanding of eukaryogenesis. Indeed, the uniqueness of Eukarya, as a domain, is challenged. It is likely that many microbial syntrophies, encompassing a 'microbial village', were required to 'raise' a eukaryote during the process of eukaryogenesis.

Origin and Early Evolution of the Eukaryotic Cell

The origin of eukaryotes has been defined as the major evolutionary transition since the origin of life itself. Most hallmark traits of eukaryotes, such as their intricate intracellular organization, can be traced back to a putative common ancestor that predated the broad diversity of extant eukaryotes. However, little is known about the nature and relative order of events that occurred in the path from preexisting prokaryotes to this already sophisticated ancestor. The origin of mitochondria from the endosymbiosis of an alphaproteobacterium is one of the few robustly established events to which most hypotheses on the origin of eukaryotes are anchored, but the debate is still open regarding the time of this acquisition, the nature of the host, and the ecological and metabolic interactions between the symbiotic partners. After the acquisition of mitochondria, eukaryotes underwent a fast radiation into several major clades whose phylogenetic relationships have been largely elusive. Recent progress in the comparative analyses of a growing number of genomes is shedding light on the early events of eukaryotic evolution as well as on the root and branching patterns of the tree of eukaryotes. Here I discuss current knowledge and debates on the origin and early evolution of eukaryotes. I focus particularly on how phylogenomic analyses have challenged some of the early assumptions about eukaryotic evolution, including the widespread idea that mitochondrial symbiosis in an archaeal host was the earliest event in eukaryogenesis. Expected final online publication date for the Annual Review of Microbiology, Volume 75 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

HERMES: An improved method to test mitochondrial genome molecular synapomorphies among clades

Mitochondrial chromosomes have diversified among eukaryotes and many different architectures and features are now acknowledged for this genome. Here we present the improved HERMES index, which can measure and quantify the amount of molecular change experienced by mitochondrial genomes. We test the improved approach with ten molecular phylogenetic studies based on complete mitochondrial genomes, representing six bilaterian Phyla. In most cases, HERMES analysis spotted out clades or single species with peculiar molecular synapomorphies, allowing to identify phylogenetic and ecological patterns. The software presented herein handles linear, circular, and multi-chromosome genomes, thus widening the HERMES scope to the complete eukaryotic domain.

Isolation and comparative proteomic analysis of mitochondria from the pulp of ripening citrus fruit

Mitochondria are crucial for the production of primary and secondary metabolites, which largely determine the quality of fruit. However, a method for isolating high-quality mitochondria is currently not available in citrus fruit, preventing high-throughput characterization of mitochondrial functions. Here, based on differential and discontinuous Percoll density gradient centrifugation, we devised a universal protocol for isolating mitochondria from the pulp of four major citrus species, including satsuma mandarin, ponkan mandarin, sweet orange, and pummelo. Western blot analysis and microscopy confirmed the high purity and intactness of the isolated mitochondria. By using this protocol coupled with a label-free proteomic approach, a total of 3353 nonredundant proteins were identified. Comparison of the four mitochondrial proteomes revealed that the proteins commonly detected in all proteomes participate in several typical metabolic pathways (such as tricarboxylic acid cycle, pyruvate metabolism, and oxidative phosphorylation) and pathways closely related to fruit quality (such as γ-aminobutyric acid (GABA) shunt, ascorbate metabolism, and biosynthesis of secondary metabolites). In addition, differentially abundant proteins (DAPs) between different types of species were also identified; these were found to be mainly involved in fatty acid and amino acid metabolism and were further confirmed to be localized to the mitochondria by subcellular localization analysis. In summary, the proposed protocol for the isolation of highly pure mitochondria from different citrus fruits may be used to obtain high-coverage mitochondrial proteomes, which can help to establish the association between mitochondrial metabolism and fruit storability or quality characteristics of different species and lay the foundation for discovering novel functions of mitochondria in plants.

MMPdb and MitoPredictor: Tools for facilitating comparative analysis of animal mitochondrial proteomes

Data on experimentally-characterized animal mitochondrial proteomes (mt-proteomes) are limited to a few model organisms and are scattered across multiple databases, impeding a comparative analysis. We developed two resources to address these problems. First, we re-analyzed proteomic data from six species with experimentally characterized mt-proteomes: animals (Homo sapiens, Mus musculus, Caenorhabditis elegans, and Drosophila melanogaster), and outgroups (Acanthamoeba castellanii and Saccharomyces cerevisiae) and created the Metazoan Mitochondrial Proteome Database (MMPdb) to host the results. Second, we developed a novel pipeline, "MitoPredictor" that uses a Random Forest classifier to infer mitochondrial localization of proteins based on orthology, mitochondrial targeting signal prediction, and protein domain analyses. Both tools generate an R Shiny applet that can be used to visualize and interact with the results and can be used on a personal computer. MMPdb is also available online at https://mmpdb.eeob.iastate.edu/.

Unique, Diverged, and Conserved Mitochondrial Functions Influencing Candida albicans Respiration

Candida albicans is an opportunistic fungal pathogen of major clinical concern. The virulence of this pathogen is intimately intertwined with its metabolism. Mitochondria, which have a central metabolic role, have undergone many lineage-specific adaptations in association with their eukaryotic host. A screen for lineage-specific genes identified seven such genes specific to the CTG clade of fungi, of which C. albicans is a member. Each is required for respiratory growth and is integral to expression of complex I, III, or IV of the electron transport chain. Two genes, NUO3 and NUO4, encode supernumerary subunits of complex I, whereas NUE1 and NUE2 have nonstructural roles in expression of complex I. Similarly, the other three genes have nonstructural roles in expression of complex III (QCE1) or complex IV (COE1 and COE2). In addition to these novel additions, an alternative functional assignment was found for the mitochondrial protein encoded by MNE1 MNE1 was required for complex I expression in C. albicans, whereas the distantly related Saccharomyces cerevisiae ortholog participates in expression of complex III. Phenotypic analysis of deletion mutants showed that fermentative metabolism is unable to support optimal growth rates or yields of C. albicans However, yeast-hypha morphogenesis, an important virulence attribute, did not require respiratory metabolism under hypoxic conditions. The inability to respire also resulted in hypersensitivity to the antifungal fluconazole and in attenuated virulence in a Galleria mellonella infection model. The results show that lineage-specific adaptations have occurred in C. albicans mitochondria and highlight the significance of respiratory metabolism in the pathobiology of C. albicans IMPORTANCE Candida albicans is an opportunistic fungal pathogen of major clinical concern. The virulence of this pathogen is intimately intertwined with its metabolic behavior, and mitochondria have a central role in that metabolism. Mitochondria have undergone many evolutionary changes, which include lineage-specific adaptations in association with their eukaryotic host. Seven lineage-specific genes required for electron transport chain function were identified in the CTG clade of fungi, of which C. albicans is a member. Additionally, examination of several highly diverged orthologs encoding mitochondrial proteins demonstrated functional reassignment for one of these. Deficits imparted by deletion of these genes revealed the critical role of respiration in virulence attributes of the fungus and highlight important evolutionary adaptations in C. albicans metabolism.

Is mitochondrial DNA profiling predictive for athletic performance?

Mitochondrial DNA encodes some proteins of the oxidative phosphorylation enzymatic complex, playing an important role in aerobic ATP production; therefore, it can contribute to the ability to respond to endurance exercise training. The accumulation of mitochondrial mutations and the migratory processes of populations have given a great contribution to the development of haplogroups with a different distribution in the world. Several studies have shown the important role of gene polymorphisms in aerobic performance. In this review, some mitochondrial haplogroups and multiple rare alleles were taken into consideration and could be linked to the athlete's physical performance of different ethnic groups.

Relative timing of mitochondrial endosymbiosis and the "pre-mitochondrial symbioses" hypothesis

The origin of eukaryotes stands as a major open question in biology. Central to this question is the nature and timing of the origin of the mitochondrion, an ubiquitous eukaryotic organelle originated by the endosymbiosis of an alphaproteobacterial ancestor. Different hypotheses disagree, among other aspects, on whether mitochondria were acquired early or late during eukaryogenesis. Similarly, the nature and complexity of the receiving host is debated, with models ranging from a simple prokaryotic host to an already complex proto-eukaryote. Here, I will discuss recent findings from phylogenomics analyses of extant genomes that are shedding light into the evolutionary origins of the eukaryotic ancestor, and which suggest a later acquisition of alpha-proteobacterial derived proteins as compared to those with different bacterial ancestries. I argue that simple eukaryogenesis models that assume a binary symbiosis between an archaeon host and an alpha-proteobacterial proto-mitochondrion cannot explain the complex chimeric nature that is inferred for the eukaryotic ancestor. To reconcile existing hypotheses with the new data, I propose the "pre-mitochondrial symbioses" hypothesis that provides a framework for eukaryogenesis scenarios involving alternative symbiotic interactions that predate the acquisition of mitochondria. © 2018 The Authors. IUBMB Life published by Wiley Periodicals, Inc. on behalf of International Union of Biochemistry and Molecular Biology, 70(12):1188-1196, 2018.

The Origin and Diversification of Mitochondria

Mitochondria are best known for their role in the generation of ATP by aerobic respiration. Yet, research in the past half century has shown that they perform a much larger suite of functions and that these functions can vary substantially among diverse eukaryotic lineages. Despite this diversity, all mitochondria derive from a common ancestral organelle that originated from the integration of an endosymbiotic alphaproteobacterium into a host cell related to Asgard Archaea. The transition from endosymbiotic bacterium to permanent organelle entailed a massive number of evolutionary changes including the origins of hundreds of new genes and a protein import system, insertion of membrane transporters, integration of metabolism and reproduction, genome reduction, endosymbiotic gene transfer, lateral gene transfer and the retargeting of proteins. These changes occurred incrementally as the endosymbiont and the host became integrated. Although many insights into this transition have been gained, controversy persists regarding the nature of the original endosymbiont, its initial interactions with the host and the timing of its integration relative to the origin of other features of eukaryote cells. Since the establishment of the organelle, proteins have been gained, lost, transferred and retargeted as mitochondria have specialized into the spectrum of functional types seen across the eukaryotic tree of life.

Dealing with Stress: Defective Metabolic Adaptation in Chronic Obstructive Pulmonary Disease Pathogenesis

The mitochondrion is the main site of energy production and a hub of key signaling pathways. It is also central in stress-adaptive response due to its dynamic morphology and ability to interact with other organelles. In response to stress, mitochondria fuse into networks to increase bioenergetic efficiency and protect against oxidative damage. Mitochondrial damage triggers segregation of damaged mitochondria from the mitochondrial network through fission and their proteolytic degradation by mitophagy. Post-translational modifications of the mitochondrial proteome and nuclear cross-talk lead to reprogramming of metabolic gene expression to maintain energy production and redox balance. Chronic obstructive pulmonary disease (COPD) is caused by chronic exposure to oxidative stress arising from inhaled irritants, such as cigarette smoke. Impaired mitochondrial structure and function, due to oxidative stress-induced damage, may play a key role in causing COPD. Deregulated metabolic adaptation may contribute to the development and persistence of mitochondrial dysfunction in COPD. We discuss the evidence for deregulated metabolic adaptation and highlight important areas for investigation that will allow the identification of molecular targets for protecting the COPD lung from the effects of dysfunctional mitochondria.

Endoplasmic Reticulum-Mitochondria Calcium Communication and the Regulation of Mitochondrial Metabolism in Cancer: A Novel Potential Target

Cancer is characterized by an uncontrolled cell proliferation rate even under low nutrient availability, which is sustained by a metabolic reprograming now recognized as a hallmark of cancer. Warburg was the first to establish the relationship between cancer and mitochondria; however, he interpreted enhanced aerobic glycolysis as mitochondrial dysfunction. Today it is accepted that many cancer cell types need fully functional mitochondria to maintain their homeostasis. Calcium (Ca²⁺)—a key regulator of several cellular processes—has proven to be essential for mitochondrial metabolism. Inositol 1,4,5-trisphosphate receptor (IP3R)-mediated Ca²⁺ transfer from the endoplasmic reticulum to the mitochondria through the mitochondrial calcium uniporter (MCU) proves to be essential for the maintenance of mitochondrial function and cellular energy balance. Both IP3R and MCU are overexpressed in several cancer cell types, and the inhibition of the Ca²⁺ communication between these two organelles causes proliferation arrest, migration decrease, and cell death through mechanisms that are not fully understood. In this review, we summarize and analyze the current findings in this area, emphasizing the critical role of Ca²⁺ and mitochondrial metabolism in cancer and its potential as a novel therapeutic target.

Being right on Q: shaping eukaryotic evolution

Reactive oxygen species (ROS) formation by mitochondria is an incompletely understood eukaryotic process. I proposed a kinetic model [BioEssays (2011) 33, 88–94] in which the ratio between electrons entering the respiratory chain via FADH₂ or NADH (the F/N ratio) is a crucial determinant of ROS formation. During glucose breakdown, the ratio is low, while during fatty acid breakdown, the ratio is high (the longer the fatty acid, the higher is the ratio), leading to higher ROS levels. Thus, breakdown of (very-long-chain) fatty acids should occur without generating extra FADH₂ in mitochondria. This explains peroxisome evolution. A potential ROS increase could also explain the absence of fatty acid oxidation in long-lived cells (neurons) as well as other eukaryotic adaptations, such as dynamic supercomplex formation. Effective combinations of metabolic pathways from the host and the endosymbiont (mitochondrion) allowed larger varieties of substrates (with different F/N ratios) to be oxidized, but high F/N ratios increase ROS formation. This might have led to carnitine shuttles, uncoupling proteins, and multiple antioxidant mechanisms, especially linked to fatty acid oxidation [BioEssays (2014) 36, 634–643]. Recent data regarding peroxisome evolution and their relationships with mitochondria, ROS formation by Complex I during ischaemia/reperfusion injury, and supercomplex formation adjustment to F/N ratios strongly support the model. I will further discuss the model in the light of experimental findings regarding mitochondrial ROS formation.

Measurement of Mitochondrial Respiration and Motility in Acute Care: Sepsis, Trauma, and Poisoning

Metabolic biomarkers have potentially wider use in disease diagnosis and prognosis as well as in monitoring disease response to treatment. While biomarkers such as interleukins, microRNA, and lactate have been proposed for disease surveillance, there are still conflicting results regarding their clinical utility. Treatment of commonly encountered disease of acute care such as sepsis, trauma, and poisoning often relies on clinical diagnosis and therapy guided by use of surrogate markers of illness severity. The measurement of mitochondrial function, including respiration and motility, may offer superior alternatives to such markers. Assessing mitochondrial function in a clinical context has the potential to impact the area of acute care in terms of diagnosis, prognosis, and treatment. The study of mitochondrial bioenergetics has become critical in understanding the pathophysiology and treatment of complex diseases such as diabetes and cardiovascular disorders.

An Interactome-Centered Protein Discovery Approach Reveals Novel Components Involved in Mitosome Function and Homeostasis in Giardia lamblia

Protozoan parasites of the genus Giardia are highly prevalent globally, and infect a wide range of vertebrate hosts including humans, with proliferation and pathology restricted to the small intestine. This narrow ecological specialization entailed extensive structural and functional adaptations during host-parasite co-evolution. An example is the streamlined mitosomal proteome with iron-sulphur protein maturation as the only biochemical pathway clearly associated with this organelle. Here, we applied techniques in microscopy and protein biochemistry to investigate the mitosomal membrane proteome in association to mitosome homeostasis. Live cell imaging revealed a highly immobilized array of 30–40 physically distinct mitosome organelles in trophozoites. We provide direct evidence for the single giardial dynamin-related protein as a contributor to mitosomal morphogenesis and homeostasis. To overcome inherent limitations that have hitherto severely hampered the characterization of these unique organelles we applied a novel interaction-based proteome discovery strategy using forward and reverse protein co-immunoprecipitation. This allowed generation of organelle proteome data strictly in a protein-protein interaction context. We built an initial Tom40-centered outer membrane interactome by co-immunoprecipitation experiments, identifying small GTPases, factors with dual mitosome and endoplasmic reticulum (ER) distribution, as well as novel matrix proteins. Through iterative expansion of this protein-protein interaction network, we were able to i) significantly extend this interaction-based mitosomal proteome to include other membrane-associated proteins with possible roles in mitosome morphogenesis and connection to other subcellular compartments, and ii) identify novel matrix proteins which may shed light on mitosome-associated metabolic functions other than Fe-S cluster biogenesis. Functional analysis also revealed conceptual conservation of protein translocation despite the massive divergence and reduction of protein import machinery in Giardia mitosomes.

Organelles with endosymbiotic origin are present in virtually all extant eukaryotes and have undergone considerable remodeling during > 1 billion years of evolution. Highly diverged organelles such as mitosomes or plastids in some parasitic protozoa are the product of extensive secondary reduction. They are sufficiently unique to generate interest as targets for pharmacological intervention, in addition to providing a rich ground for evolutionary cell biologists. The so-called mitochondria-related organelles (MROs) comprise mitosomes and hydrogenosomes, with the former having lost any role in energy metabolism along with the organelle genome. The mitosomes of the intestinal pathogen Giardia lamblia are the most highly reduced MROs known and have proven difficult to investigate because of their extreme divergence and their unique biophysical properties. Here, we implemented a novel strategy aimed at systematic analysis of the organelle proteome by iterative expansion of a protein-protein interaction network. We combined serial forward and reverse co-immunoprecipitations with mass spectrometry analysis, data mining, and validation by subcellular localization and/or functional analysis to generate an interactome network centered on a giardial Tom40 homolog. This iterative ab initio proteome reconstruction provided protein-protein interaction data in addition to identifying novel organelle proteins and functions. Building on this data we generated information on organelle replication, mitosome morphogenesis and organelle dynamics in living cells.

Mitochondrial tolerance to single and repeat exposure to simulated sunlight in human epidermal and dermal skin cells

BACKGROUND

Sunlight represents the primary threat to mitochondrial integrity in skin given the unique nature of the mitochondrial genome and its proximity to the electron transport chain. The accumulation of mitochondrial DNA (mtDNA) mutations is a key factor in many human pathologies and this is linked to key roles of mitochondrial function in terms of energy production and cell regulation.

OBJECTIVE

The main objective of this study was to evaluate solar radiation induced changes in mitochondrial integrity, function and dynamics in human skin cells using a Q-Sun solar simulator to deliver a close match to the intensity of summer sunlight.

METHODS

Spontaneously immortalised human skin epidermal keratinocytes (HaCaT) and Human Dermal Fibroblasts (HDFn) were divided into two groups. Group A were irradiated once and Group B twice 7days apart and evaluated using cell survival, viability and mitochondrial membrane potential (MMP) and mass at 1, 4 and 7days post one exposure for Group A and 1, 4, 7 and 14days post second exposure for Group B.

RESULTS

Viability and survival of HaCaT and HDFn cells decreased after repeat exposure to Simulated Sunlight Irradiation (SSI) with no recovery. HDFn cells showed no loss in MMP after one or two exposures to SSI compared to HaCaT cells which showed a periodic loss of MMP after one exposure with a repeat exposure causing a dramatic decrease from which cells did not recover. Mitochondrial Mass in exposed HDFn cells was consistent with control after one or two exposures to SSI; however mitochondrial mass was significantly decreased in HaCaT cells.

CONCLUSION

Data presented here suggests that mitochondria in epidermal cells are more sensitive to sunlight damage compared to mitochondria in dermal cells, despite their origin, confirming a skin layer specific sensitivity to sunlight, but not as expected.

The proteome of higher plant mitochondria

Plant mitochondria perform a wide range of functions in the plant cell ranging from providing energy and metabolic intermediates, via coenzyme biosynthesis and their own biogenesis to retrograde signaling and programmed cell death. To perform these functions, they contain a proteome of >2000 different proteins expressed in some cells under some conditions. The vast majority of these proteins are imported, in many cases by a dedicated protein import machinery. Recent proteomic studies have identified about 1000 different proteins in both Arabidopsis and potato mitochondria, but even for energy-related proteins, the most well-studied functional protein group in mitochondria, <75% of the proteins are recognized as mitochondrial by even one of six of the most widely used prediction algorithms. The mitochondrial proteomes contain proteins representing a wide range of different functions. Some protein groups, like energy-related proteins, membrane transporters, and de novo fatty acid synthesis, appear to be well covered by the proteome, while others like RNA metabolism appear to be poorly covered possibly because of low abundance. The proteomic studies have improved our understanding of basic mitochondrial functions, have led to the discovery of new mitochondrial metabolic pathways and are helping us towards appreciating the dynamic role of the mitochondria in the responses of the plant cell to biotic and abiotic stress.

Recent events dominate interdomain lateral gene transfers between prokaryotes and eukaryotes and, with the exception of endosymbiotic gene transfers, few ancient transfer events persist

While there is compelling evidence for the impact of endosymbiotic gene transfer (EGT; transfer from either mitochondrion or chloroplast to the nucleus) on genome evolution in eukaryotes, the role of interdomain transfer from bacteria and/or archaea (i.e. prokaryotes) is less clear. Lateral gene transfers (LGTs) have been argued to be potential sources of phylogenetic information, particularly for reconstructing deep nodes that are difficult to recover with traditional phylogenetic methods. We sought to identify interdomain LGTs by using a phylogenomic pipeline that generated 13 465 single gene trees and included up to 487 eukaryotes, 303 bacteria and 118 archaea. Our goals include searching for LGTs that unite major eukaryotic clades, and describing the relative contributions of LGT and EGT across the eukaryotic tree of life. Given the difficulties in interpreting single gene trees that aim to capture the approximately 1.8 billion years of eukaryotic evolution, we focus on presence-absence data to identify interdomain transfer events. Specifically, we identify 1138 genes found only in prokaryotes and representatives of three or fewer major clades of eukaryotes (e.g. Amoebozoa, Archaeplastida, Excavata, Opisthokonta, SAR and orphan lineages). The majority of these genes have phylogenetic patterns that are consistent with recent interdomain LGTs and, with the notable exception of EGTs involving photosynthetic eukaryotes, we detect few ancient interdomain LGTs. These analyses suggest that LGTs have probably occurred throughout the history of eukaryotes, but that ancient events are not maintained unless they are associated with endosymbiotic gene transfer among photosynthetic lineages.

Late acquisition of mitochondria by a host with chimaeric prokaryotic ancestry

The origin of eukaryotes stands as a major conundrum in biology. Current evidence indicates that the last eukaryotic common ancestor already possessed many eukaryotic hallmarks, including a complex subcellular organization. In addition, the lack of evolutionary intermediates challenges the elucidation of the relative order of emergence of eukaryotic traits. Mitochondria are ubiquitous organelles derived from an alphaproteobacterial endosymbiont. Different hypotheses disagree on whether mitochondria were acquired early or late during eukaryogenesis. Similarly, the nature and complexity of the receiving host are debated, with models ranging from a simple prokaryotic host to an already complex proto-eukaryote. Most competing scenarios can be roughly grouped into either mito-early, which consider the driving force of eukaryogenesis to be mitochondrial endosymbiosis into a simple host, or mito-late, which postulate that a significant complexity predated mitochondrial endosymbiosis. Here we provide evidence for late mitochondrial endosymbiosis. We use phylogenomics to directly test whether proto-mitochondrial proteins were acquired earlier or later than other proteins of the last eukaryotic common ancestor. We find that last eukaryotic common ancestor protein families of alphaproteobacterial ancestry and of mitochondrial localization show the shortest phylogenetic distances to their closest prokaryotic relatives, compared with proteins of different prokaryotic origin or cellular localization. Altogether, our results shed new light on a long-standing question and provide compelling support for the late acquisition of mitochondria into a host that already had a proteome of chimaeric phylogenetic origin. We argue that mitochondrial endosymbiosis was one of the ultimate steps in eukaryogenesis and that it provided the definitive selective advantage to mitochondria-bearing eukaryotes over less complex forms.

Integration of Proteomics and Transcriptomics Data Sets for the Analysis of a Lymphoma B-Cell Line in the Context of the Chromosome-Centric Human Proteome Project

A comprehensive study of the molecular active landscape of human cells can be undertaken to integrate two different but complementary perspectives: transcriptomics, and proteomics. After the genome era, proteomics has emerged as a powerful tool to simultaneously identify and characterize the compendium of thousands of different proteins active in a cell. Thus, the Chromosome-centric Human Proteome Project (C-HPP) is promoting a full characterization of the human proteome combining high-throughput proteomics with the data derived from genome-wide expression profiling of protein-coding genes. Here we present a full proteomic profiling of a human lymphoma B-cell line (Ramos) performed using a nanoUPLC-LTQ-Orbitrap Velos proteomic platform, combined to an in-depth transcriptomic profiling of the same cell type. Data are available via ProteomeXchange with identifier PXD001933. Integration of the proteomic and transcriptomic data sets revealed a 94% overlap in the proteins identified by both -omics approaches. Moreover, functional enrichment analysis of the proteomic profiles showed an enrichment of several functions directly related to the biological and morphological characteristics of B-cells. In turn, about 30% of all protein-coding genes present in the whole human genome were identified as being expressed by the Ramos cells (stable average of 30% genes along all the chromosomes), revealing the size of the protein expression-set present in one specific human cell type. Additionally, the identification of missing proteins in our data sets has been reported, highlighting the power of the approach. Also, a comparison between neXtProt and UniProt database searches has been performed. In summary, our transcriptomic and proteomic experimental profiling provided a high coverage report of the expressed proteome from a human lymphoma B-cell type with a clear insight into the biological processes that characterized these cells. In this way, we demonstrated the usefulness of combining -omics for a comprehensive characterization of specific biological systems.

Evolutionary defined role of the mitochondrial DNA in fertility, disease and ageing

BACKGROUND

The endosymbiosis of an alpha-proteobacterium and a eubacterium a billion years ago paved the way for multicellularity and enabled eukaryotes to flourish. The selective advantage for the host was the acquired ability to generate large amounts of intracellular hydrogen-dependent adenosine triphosphate. The price was increased reactive oxygen species (ROS) inside the eukaryotic cell, causing high mutation rates of the mitochondrial DNA (mtDNA). According to the Muller's ratchet theory, this accumulation of mutations in asexually transmitted mtDNA would ultimately lead to reduced reproductive fitness and eventually extinction. However, mitochondria have persisted over the course of evolution, initially due to a rapid, extreme evolutionary reduction of the mtDNA content. After the phylogenetic divergence of eukaryotes into animals, fungi and plants, differences in evolution of the mtDNA occurred with different adaptations for coping with the mutation burden within these clades. As a result, mitochondrial evolutionary mechanisms have had a profound effect on human adaptation, fertility, healthy reproduction, mtDNA disease manifestation and transmission and ageing. An understanding of these mechanisms might elucidate novel approaches for treatment and prevention of mtDNA disease.

METHODS

The scientific literature was investigated to determine how mtDNA evolved in animals, plants and fungi. Furthermore, the different mechanisms of mtDNA inheritance and of balancing Muller's ratchet in these species were summarized together with the consequences of these mechanisms for human health and reproduction.

RESULTS

Animal, plant and fungal mtDNA have evolved differently. Animals have compact genomes, little recombination, a stable number of genes and a high mtDNA copy number, whereas plants have larger genomes with variable gene counts, a low mtDNA copy number and many recombination events. Fungal mtDNA is somewhere in between. In plants, the mtDNA mutation rate is kept low by effective ROS defence and efficient recombination-mediated mtDNA repair. In animal mtDNA, these mechanisms are not or less well-developed and the detrimental mutagenesis events are controlled by a high mtDNA copy number in combination with a genetic bottleneck and purifying selection during transmission. The mtDNA mutation rates in animals are higher than in plants, which allow mobile animals to adapt more rapidly to various environmental conditions in terms of energy production, whereas static plants do not have this need. Although at the level of the species, these mechanisms have been extremely successful, they can have adverse effects for the individual, resulting, in humans, in severe or unpredictably segregating mtDNA diseases, as well as fertility problems and unhealthy ageing.

CONCLUSIONS

Understanding the forces and processes that underlie mtDNA evolution among different species increases our knowledge on the detrimental consequences that individuals can have from these evolutionary end-points. Alternative outcomes in animals, fungi and plants will lead to a better understanding of the inheritance of mtDNA disorders and mtDNA-related fertility problems. These will allow the development of options to ameliorate, cure and/or prevent mtDNA diseases and mtDNA-related fertility problems.

Origin and evolution of metabolic sub-cellular compartmentalization in eukaryotes

A high level of subcellular compartmentalization is a hallmark of eukaryotic cells. This intricate internal organization was present already in the common ancestor of all extant eukaryotes, and the determination of the origins and early evolution of the different organelles remains largely elusive. Organellar proteomes are determined through regulated pathways that target proteins produced in the cytosol to their final subcellular destinations. This internal sorting of proteins can vary across different physiological conditions, cell types and lineages. Evolutionary retargeting - the alteration of a subcellular localization of a protein in the course of evolution - has been rampant in eukaryotes and involves any possible combination of organelles. This fact adds another layer of difficulty to the reconstruction of the origins and evolution of organelles. In this review we discuss current themes in relation to the origin and evolution of organellar proteomes. Throughout the text, a special focus is set on the evolution of mitochondrial and peroxisomal proteomes, which are two organelles for which extensive proteomic and evolutionary studies have been performed.

Two-component phosphorelays in fungal mitochondria and beyond

Prokaryotes, eukaryotic microorganisms and plants utilize two-component signal transduction pathways to detect and respond to various environmental cues. These signaling cascades were acquired by eukaryotes via horizontal gene transfer events from ancestral bacteria. Recent exciting discoveries have identified two-component signaling systems in mitochondria and chloroplasts of several eukaryotic microorganisms and plants, therefore providing important clues to the evolutionary transition of these signaling cascades from prokaryotes to eukaryotes. This review will focus on the role of two-component signal transduction pathways in fungal pathogenesis and also discuss key new discoveries of presence of proteins participating in these signaling pathways in mitochondrion. Before addressing these issues, I first briefly describe the magnitude and the economic impact of the healthcare problems caused by fungal pathogens.

Mosaic nature of the mitochondrial proteome: Implications for the origin and evolution of mitochondria

Comparative studies of the mitochondrial proteome have identified a conserved core of proteins descended from the α-proteobacterial endosymbiont that gave rise to the mitochondrion and was the source of the mitochondrial genome in contemporary eukaryotes. A surprising result of phylogenetic analyses is the relatively small proportion (10-20%) of the mitochondrial proteome displaying a clear α-proteobacterial ancestry. A large fraction of mitochondrial proteins typically has detectable homologs only in other eukaryotes and is presumed to represent proteins that emerged specifically within eukaryotes. A further significant fraction of the mitochondrial proteome consists of proteins with homologs in prokaryotes, but without a robust phylogenetic signal affiliating them with specific prokaryotic lineages. The presumptive evolutionary source of these proteins is quite different in contending models of mitochondrial origin.

How to deal with oxygen radicals stemming from mitochondrial fatty acid oxidation

Oxygen radical formation in mitochondria is an incompletely understood attribute of eukaryotic cells. Recently, a kinetic model was proposed, in which the ratio between electrons entering the respiratory chain via FADH2 or NADH determines radical formation. During glucose breakdown, the ratio is low; during fatty acid breakdown, the ratio is high (the ratio increasing--asymptotically--with fatty acid length to 0.5, when compared with 0.2 for glucose). Thus, fatty acid oxidation would generate higher levels of radical formation. As a result, breakdown of fatty acids, performed without generation of extra FADH2 in mitochondria, could be beneficial for the cell, especially in the case of long and very long chained ones. This possibly has been a major factor in the evolution of peroxisomes. Increased radical formation, as proposed by the model, can also shed light on the lack of neuronal fatty acid oxidation and tells us about hurdles during early eukaryotic evolution. We specifically focus on extending and discussing the model in light of recent publications and findings.

High variability of mitochondrial gene order among fungi

From their origin as an early alpha proteobacterial endosymbiont to their current state as cellular organelles, large-scale genomic reorganization has taken place in the mitochondria of all main eukaryotic lineages. So far, most studies have focused on plant and animal mitochondrial (mt) genomes (mtDNA), but fungi provide new opportunities to study highly differentiated mtDNAs. Here, we analyzed 38 complete fungal mt genomes to investigate the evolution of mtDNA gene order among fungi. In particular, we looked for evidence of nonhomologous intrachromosomal recombination and investigated the dynamics of gene rearrangements. We investigated the effect that introns, intronic open reading frames (ORFs), and repeats may have on gene order. Additionally, we asked whether the distribution of transfer RNAs (tRNAs) evolves independently to that of mt protein-coding genes. We found that fungal mt genomes display remarkable variation between and within the major fungal phyla in terms of gene order, genome size, composition of intergenic regions, and presence of repeats, introns, and associated ORFs. Our results support previous evidence for the presence of mt recombination in all fungal phyla, a process conspicuously lacking in most Metazoa. Overall, the patterns of rearrangements may be explained by the combined influences of recombination (i.e., most likely nonhomologous and intrachromosomal), accumulated repeats, especially at intergenic regions, and to a lesser extent, mobile element dynamics.

MitoCOGs: clusters of orthologous genes from mitochondria and implications for the evolution of eukaryotes

Background

Mitochondria are ubiquitous membranous organelles of eukaryotic cells that evolved from an alpha-proteobacterial endosymbiont and possess a small genome that encompasses from 3 to 106 genes. Accumulation of thousands of mitochondrial genomes from diverse groups of eukaryotes provides an opportunity for a comprehensive reconstruction of the evolution of the mitochondrial gene repertoire.

Results

Clusters of orthologous mitochondrial protein-coding genes (MitoCOGs) were constructed from all available mitochondrial genomes and complemented with nuclear orthologs of mitochondrial genes. With minimal exceptions, the mitochondrial gene complements of eukaryotes are subsets of the superset of 66 genes found in jakobids. Reconstruction of the evolution of mitochondrial genomes indicates that the mitochondrial gene set of the last common ancestor of the extant eukaryotes was slightly larger than that of jakobids. This superset of mitochondrial genes likely represents an intermediate stage following the loss and transfer to the nucleus of most of the endosymbiont genes early in eukaryote evolution. Subsequent evolution in different lineages involved largely parallel transfer of ancestral endosymbiont genes to the nuclear genome. The intron density in nuclear orthologs of mitochondrial genes typically is nearly the same as in the rest of the genes in the respective genomes. However, in land plants, the intron density in nuclear orthologs of mitochondrial genes is almost 1.5-fold lower than the genomic mean, suggestive of ongoing transfer of functional genes from mitochondria to the nucleus.

Conclusions

The MitoCOGs are expected to become an important resource for the study of mitochondrial evolution. The nearly complete superset of mitochondrial genes in jakobids likely represents an intermediate stage in the evolution of eukaryotes after the initial, extensive loss and transfer of the endosymbiont genes. In addition, the bacterial multi-subunit RNA polymerase that is encoded in the jakobid mitochondrial genomes was replaced by a single-subunit phage-type RNA polymerase in the rest of the eukaryotes. These results are best compatible with the rooting of the eukaryotic tree between jakobids and the rest of the eukaryotes. The land plants are the only eukaryotic branch in which the gene transfer from the mitochondrial to the nuclear genome appears to be an active, ongoing process.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-014-0237-5) contains supplementary material, which is available to authorized users.

Minimization of extracellular space as a driving force in prokaryote association and the origin of eukaryotes

BACKGROUND

Internalization-based hypotheses of eukaryotic origin require close physical association of host and symbiont. Prior hypotheses of how these associations arose include chance, specific metabolic couplings between partners, and prey-predator/parasite interactions. Since these hypotheses were proposed, it has become apparent that mixed-species, close-association assemblages (biofilms) are widespread and predominant components of prokaryotic ecology. Which forces drove prokaryotes to evolve the ability to form these assemblages are uncertain. Bacteria and archaea have also been found to form membrane-lined interconnections (nanotubes) through which proteins and RNA pass. These observations, combined with the structure of the nuclear envelope and an energetic benefit of close association (see below), lead us to propose a novel hypothesis of the driving force underlying prokaryotic close association and the origin of eukaryotes.

RESULTS

Respiratory proton transport does not alter external pH when external volume is effectively infinite. Close physical association decreases external volume. For small external volumes, proton transport decreases external pH, resulting in each transported proton increasing proton motor force to a greater extent. We calculate here that in biofilms this effect could substantially decrease how many protons need to be transported to achieve a given proton motor force. Based as it is solely on geometry, this energetic benefit would occur for all prokaryotes using proton-based respiration.

CONCLUSIONS

This benefit may be a driving force in biofilm formation. Under this hypothesis a very wide range of prokaryotic species combinations could serve as eukaryotic progenitors. We use this observation and the discovery of prokaryotic nanotubes to propose that eukaryotes arose from physically distinct, functionally specialized (energy factory, protein factory, DNA repository/RNA factory), obligatorily symbiotic prokaryotes in which the protein factory and DNA repository/RNA factory cells were coupled by nanotubes and the protein factory ultimately internalized the other two. This hypothesis naturally explains many aspects of eukaryotic physiology, including the nuclear envelope being a folded single membrane repeatedly pierced by membrane-bound tubules (the nuclear pores), suggests that species analogous or homologous to eukaryotic progenitors are likely unculturable as monocultures, and makes a large number of testable predictions.

REVIEWERS

This article was reviewed by Purificación López-García and Toni Gabaldón.

How the mitochondrion was shaped by radical differences in substrates: what carnitine shuttles and uncoupling tell us about mitochondrial evolution in response to ROS

As free-living organisms, alpha-proteobacteria produce reactive oxygen species (ROS) that diffuse into the surroundings; once constrained inside the archaeal ancestor of eukaryotes, however, ROS production presented evolutionary pressures - especially because the alpha-proteobacterial symbiont made more ROS, from a variety of substrates. I previously proposed that ratios of electrons coming from FADH2 and NADH (F/N ratios) correlate with ROS production levels during respiration, glucose breakdown having a much lower F/N ratio than longer fatty acid (FA) breakdown. Evidently, higher endogenous ROS formation did not hinder eukaryotic evolution, so how were its disadvantages mitigated? I propose that the resulting selection pressures favoured the evolution of a variety of eukaryotic 'innovations': peroxisomes for FA breakdown, carnitine shuttles, the linkage of beta-oxidation to antioxidant properties, uncoupling proteins (UCPs) and using mitochondrial uncoupling during beta-oxidation to reduce ROS. Recently observed relationships between peroxisomes and mitochondria further support the model.

The pre-endosymbiont hypothesis: a new perspective on the origin and evolution of mitochondria

Mitochondrial DNA (mtDNA) is unquestionably the remnant of an α-proteobacterial genome, yet only ~10%-20% of mitochondrial proteins are demonstrably α-proteobacterial in origin (the "α-proteobacterial component," or APC). The evolutionary ancestry of the non-α-proteobacterial component (NPC) is obscure and not adequately accounted for in current models of mitochondrial origin. I propose that in the host cell that accommodated an α-proteobacterial endosymbiont, much of the NPC was already present, in the form of a membrane-bound metabolic organelle (the premitochondrion) that compartmentalized many of the non-energy-generating functions of the contemporary mitochondrion. I suggest that this organelle also possessed a protein import system and various ion and small-molecule transporters. In such a scenario, an α-proteobacterial endosymbiont could have been converted relatively directly and rapidly into an energy-generating organelle that incorporated the extant metabolic functions of the premitochondrion. This model (the "pre-endosymbiont hypothesis") effectively represents a synthesis of previous, contending mitochondrial origin hypotheses, with the bulk of the mitochondrial proteome (much of the NPC) having an endogenous origin and the minority component (the APC) having a xenogenous origin.

Mitochondrial form and function

Mitochondria are one of the major ancient endomembrane systems in eukaryotic cells. Owing to their ability to produce ATP through respiration, they became a driving force in evolution. As an essential step in the process of eukaryotic evolution, the size of the mitochondrial chromosome was drastically reduced, and the behaviour of mitochondria within eukaryotic cells radically changed. Recent advances have revealed how the organelle's behaviour has evolved to allow the accurate transmission of its genome and to become responsive to the needs of the cell and its own dysfunction.

A metabolic scenario for the evolutionary origin of peroxisomes from the endomembranous system

A novel model for the evolutionary origin of peroxisomes and related organelles from within the endoplasmic reticulum is proposed.

Mitochondrial two-component signaling systems in Candida albicans

Two-component signal transduction pathways are one of the primary means by which microorganisms respond to environmental signals. These signaling cascades originated in prokaryotes and were inherited by eukaryotes via endosymbiotic lateral gene transfer from ancestral cyanobacteria. We report here that the nuclear genome of the pathogenic fungus Candida albicans contains elements of a two-component signaling pathway that seem to be targeted to the mitochondria. The C. albicans two-component response regulator protein Srr1 (stress response regulator 1) contains a mitochondrial targeting sequence at the N terminus, and fluorescence microscopy reveals mitochondrial localization of green fluorescent protein-tagged Srr1. Moreover, phylogenetic analysis indicates that C. albicans Srr1 is more closely related to histidine kinases and response regulators found in marine bacteria than are other two-component proteins present in the fungi. These data suggest conservation of this protein during the evolutionary transition from endosymbiont to a subcellular organelle. We used microarray analysis to determine whether the phenotypes observed with a srr1Δ/Δ mutant could be correlated with gene transcriptional changes. The expression of mitochondrial genes was altered in the srr1Δ/Δ null mutant in comparison to their expression in the wild type. Furthermore, apoptosis increased significantly in the srr1Δ/Δ mutant strain compared to the level of apoptosis in the wild type, suggesting the activation of a mitochondrion-dependent apoptotic cell death pathway in the srr1Δ/Δ mutant. Collectively, this study shows for the first time that a lower eukaryote like C. albicans possesses a two-component response regulator protein that has survived in mitochondria and regulates a subset of genes whose functions are associated with the oxidative stress response and programmed cell death (apoptosis).

The import of the transcription factor STAT3 into mitochondria depends on GRIM-19, a component of the electron transport chain

The signal transducer and activator of transcription 3 (STAT3), a nuclear transcription factor, is also present in mitochondria and regulates cellular respiration in a transcriptional-independent manner. The mechanism of STAT3 import into mitochondria remains obscure. In this report we show that mitochondrial-localized STAT3 resides in the inner mitochondrial membrane. In vitro import studies show that the gene associated with retinoid interferon induced cell mortality 19 (GRIM-19), a complex I subunit that acts as a chaperone to recruit STAT3 into mitochondria. In addition, GRIM-19 enhances the integration of STAT3 into complex I. A S727A mutation in STAT3 reduces its import and assembly even in the presence of GRIM-19. Together, our studies unveil a novel chaperone function for GRIM-19 in the recruitment of STAT3 into mitochondria.

Mitochondria: A mirror into cellular dysfunction in heart disease

Cardiovascular (CV) disease is the single most significant cause of morbidity and mortality worldwide. The emerging global impact of CV disease means that the goals of early diagnosis and a wider range of treatment options are now increasingly pertinent. As such, there is a greater need to understand the molecular mechanisms involved and potential targets for intervention. Mitochondrial function is important for physiological maintenance of the cell, and when this function is altered, the cell can begin to suffer. Given the broad range and significant impacts of the cellular processes regulated by the mitochondria, it becomes important to understand the roles of the proteins associated with this organelle. Proteomic investigations of the mitochondria are hampered by the intrinsic properties of the organelle, including hydrophobic mitochondrial membranes; high proportion of basic proteins (pI greater than 8.0); and the relative dynamic range issues of the mitochondria. For these reasons, many proteomic studies investigate the mitochondria as a discrete subproteome. Once this has been achieved, the alterations that result in functional changes with CV disease can be observed. Those alterations that lead to changes in mitochondrial function, signaling and morphology, which have significant implications for the cardiomyocyte in the development of CV disease, are discussed.

Cancer cell growth and survival as a system-level property sustained by enhanced glycolysis and mitochondrial metabolic remodeling

Systems Biology holds that complex cellular functions are generated as system-level properties endowed with robustness, each involving large networks of molecular determinants, generally identified by “omics” analyses. In this paper we describe four basic cancer cell properties that can easily be investigated in vitro: enhanced proliferation, evasion from apoptosis, genomic instability, and inability to undergo oncogene-induced senescence. Focusing our analysis on a K-ras dependent transformation system, we show that enhanced proliferation and evasion from apoptosis are closely linked, and present findings that indicate how a large metabolic remodeling sustains the enhanced growth ability. Network analysis of transcriptional profiling gives the first indication on this remodeling, further supported by biochemical investigations and metabolic flux analysis (MFA). Enhanced glycolysis, down-regulation of TCA cycle, decoupling of glucose and glutamine utilization, with increased reductive carboxylation of glutamine, so to yield a sustained production of growth building blocks and glutathione, are the hallmarks of enhanced proliferation. Low glucose availability specifically induces cell death in K-ras transformed cells, while PKA activation reverts this effect, possibly through at least two mitochondrial targets. The central role of mitochondria in determining the two investigated cancer cell properties is finally discussed. Taken together the findings reported herein indicate that a system-level property is sustained by a cascade of interconnected biochemical pathways that behave differently in normal and in transformed cells.

Mitochondrial evolution

Viewed through the lens of the genome it contains, the mitochondrion is of unquestioned bacterial ancestry, originating from within the bacterial phylum α-Proteobacteria (Alphaproteobacteria). Accordingly, the endosymbiont hypothesis--the idea that the mitochondrion evolved from a bacterial progenitor via symbiosis within an essentially eukaryotic host cell--has assumed the status of a theory. Yet mitochondrial genome evolution has taken radically different pathways in diverse eukaryotic lineages, and the organelle itself is increasingly viewed as a genetic and functional mosaic, with the bulk of the mitochondrial proteome having an evolutionary origin outside Alphaproteobacteria. New data continue to reshape our views regarding mitochondrial evolution, particularly raising the question of whether the mitochondrion originated after the eukaryotic cell arose, as assumed in the classical endosymbiont hypothesis, or whether this organelle had its beginning at the same time as the cell containing it.

The oxidative phosphorylation system in mammalian mitochondria

The chapter provides a review of the state of art of the oxidative phosphorylation system in mammalian mitochondria. The sections of the paper deal with: (i) the respiratory chain as a whole: redox centers of the chain and protonic coupling in oxidative phosphorylation (ii) atomic structure and functional mechanism of protonmotive complexes I, III, IV and V of the oxidative phosphorylation system (iii) biogenesis of oxidative phosphorylation complexes: mitochondrial import of nuclear encoded subunits, assembly of oxidative phosphorylation complexes, transcriptional factors controlling biogenesis of the complexes. This advanced knowledge of the structure, functional mechanism and biogenesis of the oxidative phosphorylation system provides a background to understand the pathological impact of genetic and acquired dysfunctions of mitochondrial oxidative phosphorylation.