Mitochondrial protein import: two membranes, three translocases

The Role of Genetic Mutations in Mitochondrial-Driven Cancer Growth in Selected Tumors: Breast and Gynecological Malignancies

There is an increasing understanding of the molecular and cytogenetic background of various tumors that helps us better conceptualize the pathogenesis of specific diseases. Additionally, in many cases, these molecular and cytogenetic alterations have diagnostic, prognostic, and/or therapeutic applications that are heavily used in clinical practice. Given that there is always room for improvement in cancer treatments and in cancer patient management, it is important to discover new therapeutic targets for affected individuals. In this review, we discuss mitochondrial changes in breast and gynecological (endometrial and ovarian) cancers. In addition, we review how the frequently altered genes in these diseases (BRCA1/2, HER2, PTEN, PIK3CA, CTNNB1, RAS, CTNNB1, FGFR, TP53, ARID1A, and TERT) affect the mitochondria, highlighting the possible associated individual therapeutic targets. With this approach, drugs targeting mitochondrial glucose or fatty acid metabolism, reactive oxygen species production, mitochondrial biogenesis, mtDNA transcription, mitophagy, or cell death pathways could provide further tailored treatment.

The Role of Hsp70s in the Development and Pathogenicity of Plasmodium falciparum

The main agent of human malaria, the protozoa, Plasmodium falciparum is known to infect liver cells, subsequently invading the host erythrocyte, leading to the manifestation of clinical outcomes of the disease. As part of its survival in the human host, P. falciparum employs several heat shock protein (Hsp) families whose primary purpose is to ensure cytoprotection through their molecular chaperone role. The parasite expresses six Hsp70s that localise to various subcellular organelles of the parasite, with one, PfHsp70-x, being exported to the infected human erythrocyte. The role of these Hsp70s in the survival and pathogenicity of malaria has received immense research attention. Several studies have reported on their structure-function features, network partnerships, and elucidation of their potential substrates. Apart from their role in cytoprotection and pathogenicity, Hsp70s are implicated in antimalarial drug resistance. As such, they are deemed potential antimalarial drug candidates, especially suited for co-targeting in combination therapies. In addition, Hsp70 is implicated in host immune modulation. The current report highlights the various structure-function features of these proteins, their roles in the development of malaria, current and prospective efforts being employed towards targeting them in malaria intervention efforts.

Depletion of TMEM65 leads to oxidative stress, apoptosis, induction of mitochondrial unfolded protein response, and upregulation of mitochondrial protein import receptor TOMM22

Mutation in the transmembrane protein 65 gene (TMEM65) results in mitochondrial dysfunction and a severe mitochondrial encephalomyopathy phenotype. However, neither the function of TMEM65 nor the cellular responses to its depletion have been fully elucidated. Hence, we knocked down TMEM65 in human cultured cells and analyzed the resulting cellular responses. Depletion of TMEM65 led to a mild increase in ROS generation and upregulation of the mRNA levels of oxidative stress suppressors, such as NFE2L2 and SESN3, indicating that TMEM65 knockdown induced an oxidative stress response. A mild induction of apoptosis was also observed upon depletion of TMEM65. Depletion of TMEM65 upregulated protein levels of the mitochondrial chaperone HSPD1 and mitochondrial protease LONP1, indicating that mitochondrial unfolded protein response (UPR^mt) was induced in response to TMEM65 depletion. Additionally, we found that the mitochondrial protein import receptor TOMM22 and HSPA9 (mitochondrial Hsp70), were also upregulated in TMEM65-depleted cells. Notably, the depletion of TMEM65 did not lead to upregulation of TOMM22 in an ATF5-dependent manner, although upregulation of LONP1 reportedly occurs in an ATF5-dependent manner. Taken together, our findings suggest that depletion of TMEM65 causes mild oxidative stress and apoptosis, induces UPR^mt, and upregulates protein expression of mitochondrial protein import receptor TOMM22 in an ATF5-independent manner.

MGARP is ultrastructurally located in the inner faces of mitochondrial membranes

Mitochondria, the centers of energy production, are highly organized with inner membranes, cristae and outer membranes. The mitochondrial architecture determines their functions in all cellular processes. Changes in the mitochondrial ultrastructure are tightly related to a wide variety of diseases. MGARP, a mitochondria-localized protein, was predicted by bioinformatics and confirmed by cellular and biochemical methods to be located in mitochondria, but there is no direct and clear evidence for its precise location. This report demonstrates the precise ultrastructural location of MGARP within mitochondria by the ascorbate peroxidase 2 (APEX2) system in combination with electron microscopy (EM). EM revealed that more MGARP is located in the inner/cristae membranes, with its C-terminus at the inner faces of the intramembrane spaces, than in the outer membranes. MGARP overexpression caused both mitochondrial remodeling and cristae shaping, leading to the collapse of the mitochondrial network. The mitochondrial morphologies in MGARP-overexpressing cells were diverse; the cells became round or short, and their cristae were deformed and became discontinuous or circular. An engineered MGARP mutant deficient in its transmembrane domain no longer localized to the mitochondria and lost its effects on mitochondrial structure, confirming that the localization of MGARP in the mitochondria depends on its structural integrity. Collectively, our findings define the location of MGARP within the mitochondria, which is associated with its functional implications for the architecture and organization of mitochondria.

HIF-1α protects against oxidative stress by directly targeting mitochondria

The transcription factor hypoxia inducible factor-1α (HIF-1α) mediates adaptive responses to oxidative stress by nuclear translocation and regulation of gene expression. Mitochondrial changes are critical for the adaptive response to oxidative stress. However, the transcriptional and non-transcriptional mechanisms by which HIF-1α regulates mitochondria in response to oxidative stress are poorly understood. Here, we examined the subcellular localization of HIF-1α in human cells and identified a small fraction of HIF-1α that translocated to the mitochondria after exposure to hypoxia or H₂O₂ treatment. Moreover, the livers of mice with CCl₄-induced fibrosis showed a progressive increase in HIF-1α association with the mitochondria, indicating the clinical relevance of this finding. To probe the function of this HIF-1α population, we ectopically expressed a mitochondrial-targeted form of HIF-1α (mito-HIF-1α). Expression of mito-HIF-1α was sufficient to attenuate apoptosis induced by exposure to hypoxia or H₂O₂-induced oxidative stress. Moreover, mito-HIF-1α expression reduced the production of reactive oxygen species, the collapse of mitochondrial membrane potential, and the expression of mitochondrial DNA-encoded mRNA in response to hypoxia or H₂O₂ treatment independently of nuclear pathways. These data suggested that mitochondrial HIF-1α protects against oxidative stress induced-apoptosis independently of its well-known role as a transcription factor.

•

HIF-1α is recruited to mitochondria in response to oxidative stress.

•

Mitochondrial HIF-1α protects against oxidative stress induced apoptosis.

•

HIF-1α in mitochondria reduces ROS levels and reverses mitochondrial damage.

•

Mitochondrial HIF-1α reduces mtDNA encoded mRNA levels.

•

Mitochondrial HIF-1α may involve in liver fibrosis.

Novel insights into SLC25A46-related pathologies in a genetic mouse model

The mitochondrial protein SLC25A46 has been recently identified as a novel pathogenic cause in a wide spectrum of neurological diseases, including inherited optic atrophy, Charcot-Marie-Tooth type 2, Leigh syndrome, progressive myoclonic ataxia and lethal congenital pontocerebellar hypoplasia. SLC25A46 is an outer membrane protein, member of the Solute Carrier 25 (SLC25) family of nuclear genes encoding mitochondrial carriers, with a role in mitochondrial dynamics and cristae maintenance. Here we identified a loss-of-function mutation in the Slc25a46 gene that causes lethal neuropathology in mice. Mutant mice manifest the main clinical features identified in patients, including ataxia, optic atrophy and cerebellar hypoplasia, which were completely rescued by expression of the human ortholog. Histopathological analysis revealed previously unseen lesions, most notably disrupted cytoarchitecture in the cerebellum and retina and prominent abnormalities in the neuromuscular junction. A distinct lymphoid phenotype was also evident. Our mutant mice provide a valid model for understanding the mechanistic basis of the complex SLC25A46-mediated pathologies, as well as for screening potential therapeutic interventions.

Neurodegenerative diseases encompass a wide range of clinical conditions remaining incurable while the genetic and molecular basis of most of these conditions remains unknown due to their heterogeneity and the lack of animal models. Mutations in nuclear genes encoding mitochondrial proteins have recently emerged as novel causalities in diseases affecting the nervous system. SLC25A46, a novel mitochondrial protein, has recently been identified as a pathogenic target in a wide spectrum of rare genetic neurological diseases, including optic atrophy, Charcot-Marie-Tooth type 2, Leigh syndrome, progressive myoclonic ataxia and lethal congenital pontocerebellar hypoplasia. Following a genetic approach, we identified a novel neurological mouse model caused by a functional mutation in the Slc25a46 gene. Our SLC25A46 mutant mice constitute the first genetic animal model for this disease and represent an ideal tool for elucidating the underlying molecular mechanisms. The aim of this study was to characterize the complex phenotype displayed by the mutant mice in order to understand the mechanistic basis of the SLC25A46-mediated pathologies.

ABCB10 depletion reduces unfolded protein response in mitochondria

Mitochondria have many functions, including ATP generation. The electron transport chain (ETC) and the coupled ATP synthase generate ATP by consuming oxygen. Reactive oxygen species (ROS) are also produced by ETC, and ROS damage deoxyribonucleic acids, membrane lipids and proteins. Recent analysis indicate that mitochondrial unfolded protein response (UPR^mt), which enhances expression of mitochondrial chaperones and proteases to remove damaged proteins, is activated when damaged proteins accumulate in the mitochondria. In Caenorhabditis elegans, HAF-1, a putative ortholog of human ABCB10, plays an essential role in signal transduction from mitochondria to nuclei to enhance UPR^mt. Therefore, it is possible that ABCB10 has a role similar to that of HAF-1. However, it has not been reported whether ABCB10 is a factor in the signal transduction pathway to enhance UPR^mt. In this study, ABCB10 was depleted in HepG2 cells using small interfering RNA (siRNA), and the effect was examined. ABCB10 depletion upregulated ROS and the expression of ROS-detoxifying enzymes (SOD2, GSTA1, and GSTA2), and SESN3, a protein induced by ROS to protect the cell from oxidative stress. In addition, ABCB10 depletion significantly decreased expression of UPR^mt-related mitochondrial chaperones (HSPD1 and DNAJA3), and a mitochondrial protease (LONP1). However, the putative activity of ABCB10 to export peptides from mitochondria was not lost by ABCB10 depletion. Altogether, these data suggest that ABCB10 is involved in UPR^mt signaling pathway similar to that of HAF-1, although ABCB10 probably does not participate in peptide export from mitochondria.

Mitochondrial Cristae: Where Beauty Meets Functionality

Mitochondrial cristae are dynamic bioenergetic compartments whose shape changes under different physiological conditions. Recent discoveries have unveiled the relation between cristae shape and oxidative phosphorylation (OXPHOS) function, suggesting that membrane morphology modulates the organization and function of the OXPHOS system, with a direct impact on cellular metabolism. As a corollary, cristae-shaping proteins have emerged as potential modulators of mitochondrial bioenergetics, a concept confirmed by genetic experiments in mouse models of respiratory chain deficiency. Here, we review our knowledge of mitochondrial ultrastructural organization and how it impacts mitochondrial metabolism.

TMEM65 is a mitochondrial inner-membrane protein

It has been reported that the expression of TMEM65 is regulated by steroid receptor RNA activator (SRA). To date, however, the localization and function of TMEM65 remained unknown. We analyzed the intracellular localization of TMEM65. Immunoblot and immunostaining analysis revealed mitochondrial localization of TMEM65. Alkali extraction analysis and digitonin extraction test using isolated mitochondria revealed that TMEM65 is an integral membrane protein that localizes to the inner-membrane of mitochondria. Analysis using deletion mutants of TMEM65 suggested that the N-terminal region (1–20) of this protein is sufficient for mitochondrial targeting and that this mitochondrial targeting signal (MTS) is cleaved between the amino acid positions 35 and 64, which contain a putative recognition site of matrix processing protease (MPP). Together, these results suggest that TMEM65 is imported into the mitochondria, integrated into mitochondrial inner-membrane, and processed into its mature form by an MPP.

Apolipoprotein e sets the stage: response to injury triggers neuropathology

Apolipoprotein (apo) E4 is the major genetic risk factor for Alzheimer's disease and is associated with poor clinical outcome following traumatic brain injury and other neuropathological disorders. Protein instability and an isoform-specific apoE property called domain interaction are responsible for these neuropathological effects. ApoE4 is the most neurotoxic isoform and can induce neuropathology through various cellular pathways. Neuronal damage or stress induces apoE synthesis as part of the repair response; however, when apoE4 is expressed in neurons, its unique conformation makes it susceptible to proteolysis, resulting in the generation of neurotoxic fragments. These fragments cause pathological mitochondrial dysfunction and cytoskeletal alterations. Here, we review data supporting the hypothesis that apoE4 (> apoE3 > apoE2) has direct neurotoxic effects and highlight studies showing that blocking domain interaction reverses these detrimental effects.

Biogenesis of mitochondrial carrier proteins: molecular mechanisms of import into mitochondria

Mitochondrial metabolite carriers are hydrophobic proteins which catalyze the flux of several charged or hydrophilic substrates across the inner membrane of mitochondria. These proteins, like most mitochondrial proteins, are nuclear encoded and after their synthesis in the cytosol are transported into the inner mitochondrial membrane. Most metabolite carriers, differently from other nuclear encoded mitochondrial proteins, are synthesized without a cleavable presequence and contain several, poorly characterized, internal targeting signals. However, an interesting aspect is the presence of a positively charged N-terminal presequence in a limited number of mitochondrial metabolite carriers. Over the last few years the molecular mechanisms of import of metabolite carrier proteins into mitochondria have been thoroughly investigated. This review summarizes the present knowledge and discusses recent advances on the import and sorting of mitochondrial metabolite carriers.

Mitochondrial Markers for Cancer: Relevance to Diagnosis, Therapy, and Prognosis and General Understanding of Malignant Disease Mechanisms

Cancer cells display changes that aid them to escape from cell death, sustain their proliferative powers, and shift their metabolism toward glycolytic energy production. Mitochondria are key organelles in many metabolic and biosynthetic pathways, and the adaptation of mitochondrial function has been recognized as crucial to the changes that occur in cancer cells. This paper zooms in on the pathologic evaluation of mitochondrial markers for diagnosing and staging of human cancer and determining the patients’ prognoses.

Mitochondrial biogenesis fails in secondary biliary cirrhosis in rats leading to mitochondrial DNA depletion and deletions

Chronic cholestasis is characterized by mitochondrial dysfunction, associated with loss of mitochondrial membrane potential, decreased activities of respiratory chain complexes, and ATP production. Our aim was to determine the molecular mechanisms that link long-term cholestasis to mitochondrial dysfunction. We studied a model of chronic cholestasis induced by bile duct ligation in rats. Key sensors and regulators of the energetic state and mitochondrial biogenesis, mitochondrial DNA (mtDNA)-to-nuclear DNA (nDNA) ratio (mtDNA/nDNA) relative copy number, mtDNA deletions, and indexes of apoptosis (BAX, BCL-2, and cleaved caspase 3) and cell proliferation (PCNA) were evaluated. Our results show that long-term cholestasis is associated with absence of activation of key sensors of the energetic state, evidenced by decreased SIRT1 and pyruvate dehydrogenase kinase levels and lack of AMPK activation. Key mitochondrial biogenesis regulators (PGC-1α and GABP-α) decreased and NRF-1 was not transcriptionally active. Mitochondrial transcription factor A (TFAM) protein levels increased transiently in liver mitochondria at 2 wk after bile duct ligation, but they dramatically decreased at 4 wk. Reduced TFAM levels at this stage were mirrored by a marked decrease (65%) in mtDNA/nDNA relative copy number. The blockade of mitochondrial biogenesis should not be ascribed to activation of apoptosis or inhibition of cell proliferation. Impaired mitochondrial turnover and loss of the DNA stabilizing effect of TFAM are likely the causative event involved in the genetic instability evidenced by accumulation of mtDNA deletions. In conclusion, the lack of stimulation of mitochondrial biogenesis leads to mtDNA severe depletion and deletions in long-term cholestasis. Hence, long-term cholestasis should be considered a secondary mitochondrial hepatopathy.

Why chloroplasts and mitochondria contain genomes

Chloroplasts and mitochondria originated as bacterial symbionts. The larger, host cells acquired genetic information from their prokaryotic guests by lateral gene transfer. The prokaryotically-derived genes of the eukaryotic cell nucleus now function to encode the great majority of chloroplast and mitochondrial proteins, as well as many proteins of the nucleus and cytosol. Genes are copied and moved between cellular compartments with relative ease, and there is no established obstacle to successful import of any protein precursor from the cytosol. Yet chloroplasts and mitochondria have not abdicated all genes and gene expression to the nucleus and to cytosolic translation. What, then, do chloroplast- and mitochondrially-encoded proteins have in common that confers a selective advantage on the cytoplasmic location of their genes? The proposal advanced here is that co-location of chloroplast and mitochondrial genes with their gene products is required for rapid and direct regulatory coupling. Redox control of gene expression is suggested as the common feature of those chloroplast and mitochondrial proteins that are encoded in situ. Recent evidence is consistent with this hypothesis, and its underlying assumptions and predictions are described.

The mitochondrial import gene tomm22 is specifically required for hepatocyte survival and provides a liver regeneration model

Understanding liver development should lead to greater insights into liver diseases and improve therapeutic strategies. In a forward genetic screen for genes regulating liver development in zebrafish, we identified a mutant--oliver--that exhibits liver-specific defects. In oliver mutants, the liver is specified, bile ducts form and hepatocytes differentiate. However, the hepatocytes die shortly after their differentiation, and thus the resulting mutant liver consists mainly of biliary tissue. We identified a mutation in the gene encoding translocase of the outer mitochondrial membrane 22 (Tomm22) as responsible for this phenotype. Mutations in tomm genes have been associated with mitochondrial dysfunction, but most studies on the effect of defective mitochondrial protein translocation have been carried out in cultured cells or unicellular organisms. Therefore, the tomm22 mutant represents an important vertebrate genetic model to study mitochondrial biology and hepatic mitochondrial diseases. We further found that the temporary knockdown of Tomm22 levels by morpholino antisense oligonucleotides causes a specific hepatocyte degeneration phenotype that is reversible: new hepatocytes repopulate the liver as Tomm22 recovers to wild-type levels. The specificity and reversibility of hepatocyte ablation after temporary knockdown of Tomm22 provides an additional model to study liver regeneration, under conditions where most hepatocytes have died. We used this regeneration model to analyze the signaling commonalities between hepatocyte development and regeneration.

Chaperone-Assisted Folding of Newly Synthesized Proteins in the Cytosol

The way in which a newly synthesized polypeptide chain folds into its unique three-dimensional structure remains one of the fundamental questions in molecular biology. Protein folding in the cell is a problematic process and, in many cases, requires the assistance of a network of molecular chaperones to support productive protein foldingin vivo. During protein biosynthesis, ribosome-associated chaperones guide the folding of the nascent polypeptide emerging from the ribosomal tunnel. In this review we summarize the basic principles of the protein-folding process and the involved chaperones, and focus on the role of ribosome-associated chaperones. Our discussion emphasizes the bacterial Trigger Factor, which is the best studied chaperone of this type. Recent advances have determined the atomic structure of the Trigger Factor, providing new, exciting insights into the role of ribosome-associated chaperones in co-translational protein folding.

Oxidative folding competes with mitochondrial import of the small Tim proteins

All small Tim proteins of the mitochondrial intermembrane space contain two conserved CX(3)C motifs, which form two intramolecular disulfide bonds essential for function, but only the cysteine-reduced, but not oxidized, proteins can be imported into mitochondria. We have shown that Tim10 can be oxidized by glutathione under cytosolic concentrations. However, it was unknown whether oxidative folding of other small Tims can occur under similar conditions and whether oxidative folding competes kinetically with mitochondrial import. In the present study, the effect of glutathione on the cysteine-redox state of Tim9 was investigated, and the standard redox potential of Tim9 was determined to be approx. -0.31 V at pH 7.4 and 25 degrees C with both the wild-type and Tim9F43W mutant proteins, using reverse-phase HPLC and fluorescence approaches. The results show that reduced Tim9 can be oxidized by glutathione under cytosolic concentrations. Next, we studied the rate of mitochondrial import and oxidative folding of Tim9 under identical conditions. The rate of import was approx. 3-fold slower than that of oxidative folding of Tim9, resulting in approx. 20% of the precursor protein being imported into an excess amount of mitochondria. A similar correlation between import and oxidative folding was obtained for Tim10. Therefore we conclude that oxidative folding and mitochondrial import are kinetically competitive processes. The efficiency of mitochondrial import of the small Tim proteins is controlled, at least partially in vitro, by the rate of oxidative folding, suggesting that a cofactor is required to stabilize the cysteine residues of the precursors from oxidation in vivo.

Cigarette smoke decreases mitochondrial porin expression and steroidogenesis

Steroidogenic acute regulatory protein (StAR) facilitates the movement of cholesterol from the outer to inner mitochondrial membrane for steroidogenesis. Here, we investigated the effect of cigarette smoke (CS) on steroidogenesis using adrenal mitochondria isolated from mice chronically exposed to CS. Steroidogenesis was decreased approximately 78% in CS-exposed mitochondria, as measured by synthesis of the steroid hormone precursor pregnenolone. This effect was accompanied by decreased mitochondrial import of (35)S-StAR. Further characterization of the imported (35)S-StAR by native gradient PAGE revealed the presence of a high molecular weight complex in both control and CS-exposed groups. Following density gradient fractionation of (35)S-StAR that had been extracted from control mitochondria, precursor StAR could be found in fractions 2-6 and smaller-sized StAR complexes in fractions 6-13. In the CS-exposed group, the appearance of precursor shifted from fraction 1-6 and the smaller complexes in fractions 6-9 disappeared. Mass spectrometric analysis revealed that the (35)S-StAR-associated protein complex was composed of several resident matrix proteins as well as the OMM resident, VDAC. VDAC expression was greatly reduced by CS, and blockage of VDAC with Koenig's polyanion decreased pregnenolone synthesis in isolated mitochondria. Taken together, these results suggest that VDAC may participate in steroidogenesis by promoting StAR interaction with the OMM and that CS may inhibit steroidogenesis by reducing VDAC-StAR interactions.

Protein synthesis in sperm: dialog between mitochondria and cytoplasm

Ejaculated sperm are capable of using mRNAs transcripts for protein translation during the final maturation steps before fertilization. In a capacitation-dependent process, nuclear-encoded mRNAs are translated by mitochondrial-type ribosomes while the cytoplasmic translation machinery is not involved. Our findings suggest that new proteins are synthesized to replace degraded proteins while swimming and waiting in the female reproductive tract before fertilization, or produced due to the specific needs of the capacitating spermatozoa. In addition, a growing number of articles have reported evidence for the correlation of nuclear-encoded mRNA and protein synthesis in somatic mitochondria. It is known that all of the proteins necessary for the replication, transcription and translation of the genes encoded in mtDNA are now encoded in the nuclear genome. This genetic investment is far out of proportion to the number of proteins involved, as there have been multiple movements and duplications of genes. However, the evolutionary retention (or secondary uptake) of the mitochondrial machinery for translation of nuclear-encoded mRNAs may shed light on this paradox.

Over-expression of the catalytic core of mitochondrial DNA (mtDNA) polymerase in the nervous system of Drosophila melanogaster reduces median life span by inducing mtDNA depletion

DNA polymerase gamma (pol gamma) is the sole DNA polymerase devoted to mitochondrial DNA (mtDNA) replication. We have characterized the molecular and physiological effects of over-expression of the catalytic subunit of pol gamma, pol gamma-alpha, in the nervous system of Drosophila melanogaster using the upstream activation sequence (UAS)/yeast transcriptional activator by binding to UAS (GAL4) system. Tissue-specific over-expression of pol gamma-alpha was confirmed by immunoblot analysis, whereas the very low levels of endogenous protein are undetectable in UAS or GAL4 control lines. The transgenic flies over-expressing pol gamma-alpha in the nervous system showed a moderate increase in pupal lethality, and a significant decrease in the median life span of adult flies. Moreover, these flies displayed a decrease in the rate of synthesis of mtDNA, which is accompanied by a significant mtDNA depletion, and a corresponding decrease in the levels of mitochondrial transcription factor A (mtTFA). Biochemical analysis showed an oxidative phosphorylation (OXPHOS) defect in transgenic flies, which were more susceptible to oxidative stress. Although we did not detect apoptosis in the nervous system of adult transgenic flies, brains of larvae over-expressing pol gamma-alpha showed evidence of increased cell death that correlates with the observed phenotypes. Our data establish an animal model that mimics some of the features of human mtDNA depletion syndromes.

Mitochondrial import of Omi: The definitive role of the putative transmembrane region and multiple processing sites in the amino-terminal segment

The mitochondrial serine protease Omi/HtrA2 has a proapoptotic role in mammalian cells. However, neither the topology nor the processing of Omi in mitochondria is clearly understood. To determine the topology of Omi in the mitochondrial IMS, EGFP fusions were expressed with the entire N-terminal segment of full-length Omi (FL-Omi) (133-EGFP), and that without the transmembrane region (DeltaTM-EGFP) in the cells. Immunocytochemical staining and alkaline extraction experiments revealed that the TM determines the topology of Omi in the IMS and anchors the pro form into the inner membrane. As a result, the protease and the PDZ domains are exposed to the IMS. Mature Omi largely exists in the IMS as a soluble form. The processing sites of the precursor protein were examined by in vitro import experiments. The import of the processing mutants revealed importance of Arg80, Arg91, and Arg93 residues for the processing of the N-terminal segment of FL-Omi. These results suggest that the N-terminal segment of FL-Omi contains multiple processing sites processed by matrix processing proteases.

BH3-only proteins trigger cytochrome c release, but how?

The mitochondrial apoptosis pathway has been neatly ordered. Mitochondrial apoptosis is governed by Bcl-2 family proteins, and their respective contributions determine the release of cytochrome c. It is clear that, among the Bcl-2 family, BH3-only proteins are the triggers: activation of BH3-only proteins by apoptotic stimuli initiates the process. BH3-only proteins cause cytochrome c release by activating Bax and/or Bak, and the anti-apoptotic group of Bcl-2-like proteins prevents this. However, it is curiously uncertain how BH3-only proteins activate Bax/Bak. Current models suggest that this is either through direct interaction--although this interaction is not detectable experimentally--or by the neutralisation of Bcl-2-like proteins. Here we discuss the context in which these models are placed and attempt to weigh the evidence.

Awaking TIM22, a Dynamic Ligand-gated Channel for Protein Insertion in the Mitochondrial Inner Membrane

Aqueous channels are at the core of the translocase of the outer membrane (TOM) and the translocase of the inner membrane for the transport of preproteins (TIM23), the translocases mediating the transport of proteins across the outer and inner mitochondrial membranes. Yet, the existence of a channel associated to the translocase of the inner membrane for the insertion of multitopic protein (TIM22) complex has been arguable, as its function relates to the insertion of multispanning proteins into the inner membrane. For the first time, we report conditions for detecting a channel activity associated to the TIM22 translocase in organelle, i.e. intact mitoplasts. An internal signal peptide in the intermembrane space of mitochondria is a requisite to inducing this channel, which is otherwise silent. The channel showed slightly cationic and high conductance activity of 1000 pS with a predominant half-open substate. Despite their different composition, the channels of the three mitochondrial translocases were thus remarkably similar, in agreement with their common task as pores transiently trapping proteins en route to their final destination. The opening of the TIM22 channel was a step-up process depending on the signal peptide concentration. Interestingly, low membrane potentials kept the channel fully open, providing a threshold level of the peptide is present. Our results portray TIM22 as a dynamic channel solely active in the presence of its cargo proteins. In its fully open conformation, favored by the combined action of internal signal peptide and low membrane potential, the channel could embrace the in-transit protein. As insertion progressed and initial interaction with the signal peptide faded, the channel would close, sustaining its role as a shunt that places trapped proteins into the membrane.

A cooperative action of the ATP-dependent import motor complex and the inner membrane potential drives mitochondrial preprotein import

The import of mitochondrial preproteins requires an electric potential across the inner membrane and the hydrolysis of ATP in the matrix. We assessed the contributions of the two energy sources to the translocation driving force responsible for movement of the polypeptide chain through the translocation channel and the unfolding of preprotein domains. The import-driving activity was directly analyzed by the determination of the protease resistances of saturating amounts of membrane-spanning translocation intermediates. The ability to generate a strong translocation-driving force was solely dependent on the activity of the ATP-dependent import motor complex in the matrix. For a sustained import-driving activity on the preprotein in transit, an unstructured N-terminal segment of more than 70 to 80 amino acid residues was required. The electric potential of the inner membrane was required to maintain the import-driving activity at a high level. The electrophoretic force of the potential exhibited only a limited capacity to unfold preprotein domains. We conclude that the membrane potential increases the probability of a dynamic interaction of the preprotein with the import motor. Polypeptide translocation and unfolding are mainly driven by the inward-directed translocation activity based on the functional cooperation of the import motor components.

Structure of the monomeric outer-membrane porin OmpG in the open and closed conformation

OmpG, a monomeric pore-forming protein from Escherichia coli outer membranes, was refolded from inclusion bodies and crystallized in two different conformations. The OmpG channel is a 14-stranded beta-barrel, with short periplasmic turns and seven extracellular loops. Crystals grown at neutral pH show the channel in the open state at 2.3 A resolution. In the 2.7 A structure of crystals grown at pH 5.6, the pore is blocked by loop 6, which folds across the channel. The rearrangement of loop 6 appears to be triggered by a pair of histidine residues, which repel one another at acidic pH, resulting in the breakage of neighbouring H-bonds and a lengthening of loop 6 from 10 to 17 residues. A total of 151 ordered LDAO detergent molecules were found in the 2.3 A structure, mostly on the hydrophobic outer surface of OmpG, mimicking the outer membrane lipid bilayer, with three LDAO molecules in the open pore. In the 2.7 A structure, OmpG binds one OG and one glucose molecule as sugar substrates in the closed pore.

Effect of Mutations in Tom40 on Stability of the Translocase of the Outer Mitochondrial Membrane (TOM) Complex, Assembly of Tom40, and Import of Mitochondrial Preproteins

Mitochondrial preproteins synthesized in the cytosol are imported through the mitochondrial outer membrane by the translocase of the outer mitochondrial membrane (TOM) complex. Tom40 is the major component of the complex and is essential for cell viability. We generated 21 different mutations in conserved regions of the Neurospora crassa Tom40 protein. The mutant genes were transformed into a tom40 null nucleus maintained in a sheltered heterokaryon, and 17 of the mutant genes gave rise to viable strains. All mutations reduced the efficiency of the altered Tom40 molecules to assemble into the TOM complex. Mitochondria isolated from seven of the mutant strains had defects for importing mitochondrial preproteins. Only one strain had a general import defect for all preproteins examined. Another mutation resulted in defects in the import of a matrix-destined preprotein and an outer membrane beta-barrel protein, but import of the ADP/ATP carrier to the inner membrane was unaffected. Five strains showed deficiencies in the import of beta-barrel proteins. The latter results suggest that the TOM complex distinguishes beta-barrel proteins from other classes of preprotein during import. This supports the idea that the TOM complex plays an active role in the transfer of preproteins to subsequent translocases for insertion into the correct mitochondrial subcompartment.

Copper trafficking to the mitochondrion and assembly of copper metalloenzymes

Copper is required within the mitochondrion for the function of two metalloenzymes, cytochrome c oxidase (CcO) and superoxide dismutase (Sod1). Copper metallation of these two enzymes occurs within the mitochondrial intermembrane space and is mediated by metallochaperone proteins. Cox17 is a key copper donor to two accessory proteins, Sco1 and Cox11, to form the two copper centers in the mature CcO complex. Ccs1 is the necessary metallochaperone for the copper metallation of Sod1 in the IMS as well as within the cytoplasm where the bulk of Sod1 resides. Copper ions used in the metallation of CcO and Sod1 appear to be provided by a novel copper pool within the mitochondrial matrix. This review documents copper ion shuttling within the mitochondrion and the proteins that mediate assembly of active CcO and Sod1.

Apolipoprotein E4: a causative factor and therapeutic target in neuropathology, including Alzheimer's disease

The premise of this review is that apolipoprotein (apo) E4 is much more than a contributing factor to neurodegeneration. ApoE has critical functions in redistributing lipids among CNS cells for normal lipid homeostasis, repairing injured neurons, maintaining synapto-dendritic connections, and scavenging toxins. In multiple pathways affecting neuropathology, including Alzheimer's disease, apoE acts directly or in concert with age, head injury, oxidative stress, ischemia, inflammation, and excess amyloid beta peptide production to cause neurological disorders, accelerating progression, altering prognosis, or lowering age of onset. We envision that unique structural features of apoE4 are responsible for apoE4-associated neuropathology. Although the structures of apoE2, apoE3, and apoE4 are in dynamic equilibrium, apoE4, which is detrimental in a variety of neurological disorders, is more likely to assume a pathological conformation. Importantly, apoE4 displays domain interaction (an interaction between the N- and C-terminal domains of the protein that results in a compact structure) and molten globule formation (the formation of stable, reactive intermediates with potentially pathological activities). In response to CNS stress or injury, neurons can synthesize apoE. ApoE4 uniquely undergoes neuron-specific proteolysis, resulting in bioactive toxic fragments that enter the cytosol, alter the cytoskeleton, disrupt mitochondrial energy balance, and cause cell death. Our findings suggest potential therapeutic strategies, including the use of "structure correctors" to convert apoE4 to an "apoE3-like" molecule, protease inhibitors to prevent the generation of toxic apoE4 fragments, and "mitochondrial protectors" to prevent cellular energy disruption.

Use of the Addressing Sequence of Yeast D-Lactate Dehydrogenase for Insertion of CYP11A1p into the Inner Membrane of Yeast Mitochondria

Mammalian cytochrome P450scc (CYP11A1p) is a pseudointegral protein of the inner membrane of mitochondria with the active center exposed in the matrix. Upon import of the CYP11A1p precursor into yeast mitochondria, only a minor part was incorporated into the inner mitochondrial membrane and acquired catalytic activity (Kovaleva, I. E., Novikova, L. A., Nazarov, P. A., Grivennikov, S. I., and Luzikov, V. N. (2003) Eur. J. Biochem., 270, 222-229). The present work is an attempt to increase the efficiency of this process by substitution of the inherent N-terminal presequence of CYP11A1p by the addressing signal of D-lactate dehydrogenase (D-LD) of the yeast Saccharomyces cerevisiae. D-LD is known to be inserted into the inner membrane of mitochondria through its transmembrane domain located close to the N-terminus of the polypeptide chain in such a way that the protein globule is exposed in the intermembrane space. The hybrid protein D-LD(1-72)-mCYP11A1p synthesized in yeast cells was imported into yeast mitochondria, underwent processing, and was inserted into the inner membrane on the side of the intermembrane space. In the presence of adrenodoxin and adrenodoxin reductase, the hybrid protein exhibited cholesterol side-chain cleavage activity. Thus, CYP11A1p insertion into the inner membrane of mitochondria mediated by the D-LD topogenic signal resulted in the catalytically active mCYP11A1p domain in the hybrid protein.

Functional and mutational characterization of human MIA40 acting during import into the mitochondrial intermembrane space

A first component involved in import into the mitochondrial intermembrane space, named Mia40, has been described recently in yeast. Here, we identified the human MIA40 as a novel and ubiquitously expressed component of human mitochondria. It belongs to a novel protein family whose members share six highly conserved cysteine residues constituting a -CXC-CX9C-CX9C- motif. Human MIA40 is significantly smaller than the fungal protein and lacks the N-terminal extension including a transmembrane region and mitochondrial targeting signal. It forms soluble complexes within the intermembrane space of human mitochondria. Depletion of MIA40 in human cells by RNA interference specifically affected steady-state levels of small and cysteine-containing intermembrane space proteins like DDP1 and TIM10A, suggesting that MIA40 acts along the import pathway into the intermembrane space. Studies on the in vivo redox state of human MIA40 demonstrated that it contains intramolecular disulfide bonds. Thiol-trapping assays revealed the co-existence of different oxidation states of human MIA40 within the cell. Furthermore, we show that the twin -CX9C- motif is specifically required for import and stability of MIA40 in mitochondria. Partial mutation of this motif affects stable accumulation of MIA40 in the intermembrane space, whereas mutation of all cysteine residues in this motif inhibits import in mitochondria. Taken together, we conclude that the biogenesis and function of MIA40 in the mitochondrial intermembrane space is dependent on redox processes involving conserved cysteine residues.

Protein- and energy-mediated targeting of chloroplast outer envelope membrane proteins

While the import of nuclear-encoded chloroplast proteins is relatively well studied, the targeting of proteins to the outer membrane of the chloroplast envelope is not. The insertion of most outer membrane proteins (OMP) is generally considered to occur without the utilization of energy or proteinaceous components. Recently, however, proteins have been shown to be involved in the integration of outer envelope protein 14 (OEP14), whose outer membrane insertion was previously thought to be spontaneous. Here we investigate the insertion of two proteins from Physcomitrella patens, PpOEP64-1 and PpOEP64-2 (formerly known as PpToc64-1 and PpToc64-2), into the outer membrane of chloroplasts. The association of PpOEP64-1 with chloroplasts was not affected by chloroplast pre-treatments. Its insertion into the membrane was affected, however, demonstrating the importance of measuring insertion specifically in these types of assays. We found that the insertion of PpOEP64-1, PpOEP64-2 and two other OMPs, OEP14 and digalactosyldiacylglycerol synthase 1 (DGD1), was reduced by either nucleotide depletion or proteolysis of the chloroplasts. Integration was also inhibited in the presence of an excess of an imported precursor protein. In addition, OEP14 competed with the insertion of the OEP64s and DGD1. These data demonstrate that the targeting of several OMPs involves proteins present in chloroplasts and requires nucleotides. Together with previous reports, our data suggest that OMPs in general do not insert spontaneously.

Stabilization of ubiquitous mitochondrial creatine kinase preprotein by APP family proteins

Amyloid precursor protein (APP) is involved in the pathogenesis of Alzheimer's disease (AD). However, the physiological role of APP and its family members is still unclear. To gain insights into APP function, we used a proteomic approach to identify APP interacting proteins. We report here for the first time a direct interaction between the C-terminal region of APP family proteins and ubiquitous mitochondrial creatine kinase (uMtCK). This interaction was confirmed in vitro as well as in cultured cells and in brain. Interestingly, expression of full-length and C-terminal domain of APP family proteins stabilized uMtCK preprotein in cultured cells. Our data suggest that APP may regulate cellular energy levels and mitochondrial function via a direct interaction and stabilization of uMtCK.

Biogenesis of β-barrel membrane proteins of mitochondria

beta-Barrel membrane proteins have several important functions in outer membranes of Gram-negative bacteria and in the organelles of endosymbiotic origin, mitochondria and chloroplasts. The biogenesis of beta-barrel membrane proteins was, until recently, an unresolved process. A breakthrough was achieved when a specific pathway for the insertion of beta-barrel outer-membrane proteins was identified in both mitochondria and Gram-negative bacteria. The key component of this pathway is Tob55 (also known as Sam50) in mitochondria and Omp85 in bacteria, both beta-barrel membrane proteins themselves. Tob55 is part of the hetero-oligomeric TOB (topogenesis of mitochondrial outer-membrane beta-barrel proteins) or SAM (sorting and assembly of mitochondria) complex, which is present in the mitochondrial outer membrane. Tob55 belongs to an evolutionarily conserved protein family, the members of which are present in almost all eukaryotes and in Gram-negative bacteria and chloroplasts. Thus, is it emphasized that the insertion pathway of mitochondrial beta-barrel membrane proteins was conserved during evolution of mitochondria from endosymbiotic bacterial ancestors.

A single glutamine synthetase gene produces tissue-specific subcellular localization by alternative splicing

Glutamine synthetase (GS) plays a key role in two major biochemical pathways: In liver GS catalyzes ammonia detoxification, whereas in neural tissues it also functions in recycling of the neurotransmitter glutamate. In most species the GS gene gives rise to a cytoplasmic protein in both liver and neural tissues. However, in species that utilize the ureosmotic or uricotelic system for ammonia detoxification, the enzyme is cytoplasmic in neural tissues, but mitochondrial in liver cells. Since most vertebrates have a single copy of the GS gene, it is not clear how tissue-specific subcellular localization is achieved. Here we show that in the ureosmotic elasmobranch, Squalus acanthias (spiny dogfish), two different GS transcripts are generated by tissue-specific alternative splicing. The liver transcript contains an alternative exon that is not present in the neural one. This exon leads to acquisition of an upstream in-frame start codon and formation of a mitochondrial targeting signal (MTS). Therefore, the liver product is targeted to the mitochondria while the neural one is retained in the cytoplasm. These findings present a mechanism in which alternative splicing of an MTS-encoding exon is used to generate tissue-specific subcellular localization.

Preprotein translocase of the outer mitochondrial membrane: reconstituted Tom40 forms a characteristic TOM pore

Tom40 is the central pore-forming component of the translocase of the outer mitochondrial membrane (TOM complex). Different views exist about the secondary structure and electrophysiological characteristics of Tom40 from Saccharomyces cerevisiae and Neurospora crassa. We have directly compared expressed and renatured Tom40 from both species and find a high content of beta-structure in circular dichroism measurements in agreement with refined secondary structure predictions. The electrophysiological characterization of renatured Tom40 reveals the same characteristics as the purified TOM complex or mitochondrial outer membrane vesicles, with two exceptions. The total conductance of the TOM complex and outer membrane vesicles is twofold higher than the total conductance of renatured Tom40, consistent with the presence of two TOM pores. TOM complex and outer membrane vesicles possess a strongly enhanced sensitivity to a mitochondrial presequence compared to Tom40 alone, in agreement with the presence of several presequence binding sites in the TOM complex, suggesting a role of the non-channel Tom proteins in regulating channel activity.

Protein import into mitochondria: origins and functions today (review)

Mitochondria are organelles derived from alpha-proteobacteria over the course of one to two billion years. Mitochondria from the major eukaryotic lineages display some variation in functions and coding capacity but sequence analysis demonstrates them to be derived from a single common ancestral endosymbiont. The loss of assorted functions, the transfer of genes to the nucleus, and the acquisition of various 'eukaryotic' proteins have resulted in an organelle that contains approximately 1000 different proteins, with most of these proteins imported into the organelle across one or two membranes. A single translocase in the outer membrane and two translocases in the inner membrane mediate protein import. Comparative sequence analysis and functional complementation experiments suggest some components of the import pathways to be directly derived from the eubacterial endosymbiont's own proteins, and some to have arisen 'de novo' at the earliest stages of 'mitochondrification' of the endosymbiont. A third class of components appears lineage-specific, suggesting they were incorporated into the process of protein import long after mitochondria was established as an organelle and after the divergence of the various eukaryotic lineages. Protein sorting pathways inherited from the endosymbiont have been co-opted and play roles in intraorganelle protein sorting after import. The import apparatus of animals and fungi show significant similarity to one another, but vary considerably to the plant apparatus. Increasing complexity in the eukaryotic lineage, i.e., from single celled to multi-cellular life forms, has been accompanied by an expansion in genes encoding each component, resulting in small gene families encoding many components. The functional differences in these gene families remain to be elucidated, but point to a mosaic import apparatus that can be regulated by a variety of signals.

Novel variants of human SCaMC-3, an isoform of the ATP-Mg/P(i) mitochondrial carrier, generated by alternative splicing from 3'-flanking transposable elements

CaMCs (calcium-dependent mitochondrial carriers) represent a novel subfamily of metabolite carriers of mitochondria. The ATP-Mg/P(i) co-transporter, functionally characterized more than 20 years ago, has been identified to be a CaMC member. There are three isoforms of the ATP-Mg/P(i) carrier in mammals, SCaMC-1 (short CaMC-1), -2 and -3 (or APC-1, -3 and -2 respectively), corresponding to the genes SLC25A24, SLC25A25 and SLC25A23 respectively, as well as six N-terminal variants generated by alternative splicing for SCaMC-1 and -2 isoforms. In the present study, we describe four new variants of human SCaMC-3 generated by alternative splicing. The new mRNAs use the exon 9 3'-donor site and distinct 5'-acceptor sites from repetitive elements, in regions downstream of exon 10, the last exon in all SCaMCs. Transcripts lacking exon 10 (SCaMC-3b, -3b', -3c and -3d) code for shortened proteins lacking the last transmembrane domain of 422, 456 and 435 amino acids, and were found in human tissues and HEK-293T cells. Mitochondrial targeting of overexpressed SCaMC-3 variants is incomplete. Surprisingly, the import impairment is overcome by removing the N-terminal extension of these proteins, suggesting that the hydrophilic N-terminal domain also participates in the mitochondrial import process, as shown for the CaMC members aralar and citrin [Roesch, Hynds, Varga, Tranebjaerg and Koehler (2004) Hum. Mol. Genet. 13, 2101-2111].

Two novel targeting peptide degrading proteases, PrePs, in mitochondria and chloroplasts, so similar and still different

Two novel metalloproteases from Arabidopsis thaliana, termed AtPrePI and AtPrePII, were recently identified and shown to degrade targeting peptides in mitochondria and chloroplasts using an ambiguous targeting peptide. AtPrePI and AtPrePII are classified as dually targeted proteins as they are targeted to both mitochondria and chloroplasts. Both proteases harbour an inverted metal binding motif and belong to the pitrilysin subfamily A. Here we have investigated the subsite specificity of AtPrePI and AtPrePII by studying their proteolytic activity against the mitochondrial F(1)beta pre-sequence, peptides derived from the F(1)beta pre-sequence as well as non-mitochondrial peptides and proteins. The degradation products were analysed, identified by MALDI-TOF spectrometry and superimposed on the 3D structure of the F(1)beta pre-sequence. AtPrePI and AtPrePII cleaved peptides that are in the range of 10 to 65 amino acid residues, whereas folded or longer unfolded peptides and small proteins were not degraded. Both proteases showed preference for basic amino acids in the P(1) position and small, uncharged amino acids or serine residues in the P'(1) position. Interestingly, both AtPrePI and AtPrePII cleaved almost exclusively towards the ends of the alpha-helical elements of the F(1)beta pre-sequence. However, AtPrePI showed a preference for the N-terminal amphiphilic alpha-helix and positively charged amino acid residues and degraded the F(1)beta pre-sequence into 10-16 amino acid fragments, whereas AtPrePII did not show any positional preference and degraded the F(1)beta pre-sequence into 10-23 amino acid fragments. In conclusion, despite the high sequence identity between AtPrePI and AtPrePII and similarities in cleavage specificities, cleavage site recognition differs for both proteases and is context and structure dependent.

Functions of the small proteins in the TOM complex of Neurospora crasssa

The TOM (translocase of the outer mitochondrial membrane) complex of the outer mitochondrial membrane is required for the import of proteins into the organelle. The core TOM complex contains five proteins, including three small components Tom7, Tom6, and Tom5. We have created single and double mutants of all combinations of the three small Tom proteins of Neurospora crassa. Analysis of the mutants revealed that Tom6 plays a major role in TOM complex stability, whereas Tom7 has a lesser role. Mutants lacking both Tom6 and Tom7 have an extremely labile TOM complex and are the only class of mutant to exhibit an altered growth phenotype. Although single mutants lacking N. crassa Tom5 have no apparent TOM complex abnormalities, studies of double mutants lacking Tom5 suggest that it also has a minor role in maintaining TOM complex stability. Our inability to isolate triple mutants supports the idea that the three proteins have overlapping functions. Mitochondria lacking either Tom6 or Tom7 are differentially affected in their ability to import different precursor proteins into the organelle, suggesting that they may play roles in the sorting of proteins to different mitochondrial subcompartments. Newly imported Tom40 was readily assembled into the TOM complex in mitochondria lacking any of the small Tom proteins.

Levels of reactive oxygen species and primary antioxidant enzymes in WI38 versus transformed WI38 cells following bleomcyin treatment

Bleomycin (BLM) is an anticancer drug that generates reactive oxygen species (ROS) after interacting with iron and oxygen. We hypothesized that BLM could cause a different status of oxidative stress in normal versus tumor cells due to possible altered redox status and gene expression in cells following transformation. In this study, the extent of cytotoxicity, levels of ROS, and activities of antioxidant enzymes were compared between normal WI38 cells and SV40-transformed WI38 (VA13) cells following BLM treatment. Basal activities of MnSOD and catalase were lower in VA13 cells and basal ROS levels were higher in VA13 cells. Although BLM caused greater growth inhibition and apoptosis in VA13 cells, it increased ROS levels at an earlier time point in WI38 cells. Moreover, BLM treatment (100 microg/ml) had no effect on the activities of MnSOD, CuZnSOD, and catalase, but increased the activities of glutathione peroxidase (GPX) in WI38 cells after a 48-h treatment and in VA13 cells after a 24- and 48-h treatment. Northern blot analysis indicated that the increase in GPX activities was due to increased transcript levels of GPX1 but not GPX4 in both cells. Our results indicate selective induction of the GPX1 gene by BLM and different redox responses to BLM between WI38 and VA13 cells.

Emerging treatments for soft tissue sarcoma of adults

Soft tissue sarcomas are a heterogeneous group of malignancies arising from mesenchymal tissues. Patients can present with a localised tumour (primary or local recurrence) at different sites (e.g., extremity, abdomen) or metastatic disease, which may require different treatment strategies. Is the surgical resection of a localised sarcoma enough or is it better to give an additional treatment like adjuvant and/or preoperative radiotherapy and/or chemotherapy? Which chemotherapy should be selected in the first or second line situation? Do new treatment options exist, such as targeted therapies? This review provides answers to some of these questions. To decide on consecutive treatment steps, it is important to know as many relevant factors as possible at first presentation. Therefore, the first part of this review discusses the specific characteristics and prognostic factors of importance for treatment planning. A short summary of current therapy strategies and existing standards is then given. The main body of the review summarises information on new and emerging clinical compounds for patients with soft tissue sarcoma of adults, including recent developments of targeted therapy.

Electrophysiological approaches to the study of protein translocation in mitochondria

Electrophysiological techniques have been integral to our understanding of protein translocation across various membranes, and, in particular, the mitochondrial inner and outer membranes. Descriptions of various methodologies (for example, patch clamp, planar bilayers, and tip dip, and their past and potential contributions) are detailed within. The activity of protein import channels of native mitochondrial inner and outer membranes can be studied by directly patch clamping mitochondria and mitoplasts (mitochondria stripped of their outer membrane by French pressing) from various genetically manipulated strains of yeast and mammalian tissue cultured cells. The channel activities of TOM, TIM23, and TIM22 complexes are compared with those reconstituted in proteoliposomes and with those of the recombinant proteins Tom40p, Tim23p, and Tim22p, which play major roles in protein translocation. Studies of the mechanism(s) and the role of channels in protein translocation in mitochondria are prototypes, as the same principles are likely followed in all biological membranes including the endoplasmic reticulum and chloroplasts. The ability to apply electrophysiological techniques to these channels is now allowing investigations into the role of mitochondria in diverse fields such as neurotransmitter release, long-term potentiation, and apoptosis.

The mitochondrial morphology protein Mdm10 functions in assembly of the preprotein translocase of the outer membrane

The biogenesis of mitochondrial outer membrane proteins involves the general translocase of the outer membrane (TOM complex) and the sorting and assembly machinery (SAM complex). The two known subunits of the SAM complex, Mas37 and Sam50, are required for assembly of the abundant outer membrane proteins porin and Tom40. We have identified an unexpected subunit of the SAM complex, Mdm10, which is involved in maintenance of mitochondrial morphology. Mitochondria lacking Mdm10 are selectively impaired in the final steps of the assembly pathway of Tom40, including the association of Tom40 with the receptor Tom22 and small Tom proteins, while the biogenesis of porin is not affected. Yeast mutants of TOM40, MAS37, and SAM50 also show aberrant mitochondrial morphology. We conclude that Mdm10 plays a specific role in the biogenesis of the TOM complex, indicating a connection between the mitochondrial protein assembly apparatus and the machinery for maintenance of mitochondrial morphology.

A C-terminal targeting signal controls differential compartmentalisation of Caenorhabditis elegans host cell factor (HCF) to the nucleus or mitochondria

HCF-1 (host cell factor 1) is a human protein originally identified as a component of the VP16 transcription complex. A related protein HCF-2 is also present in humans and while at least HCF-1 appears to be required for normal cell growth there is currently little information on the precise cellular role(s) of these proteins. C. elegans contains a single HCF orthologue (CeHCF) which is very closely related to human HCF-2. To contribute to an understanding of the activities of these proteins here we analyse the subcellular localisation of the CeHCF protein in live transgenic worms and in mammalian cells. We constructed a green fluorescent protein (GFP) fusion of CeHCF and studied localisation after ectopic expression under the control of a heat shock protein promoter. The CeHCF-GFP protein accumulated in the cell nuclei at every stage of development and in a wide variety of cell types. Nuclear accumulation with nucleolar sparing was evident on the larvae and adult stages, but not earlier in development in which the protein accumulated diffusely in the nucleoplasm. Surprisingly the same protein accumulated in the mitochondria of a stable HeLa cell line, suggesting a differential localisation of CeHCF in mammalian cells. Furthermore, when overexpressed in transient transfection the CeHCF accumulated in both nuclear and mitochondrial compartments. We have refined the targeting determinants of CeHCF to the last 23 amino acids at the extreme C-terminus and show that they contain interdigitated amino acids involved in both nuclear and mitochondrial targeting. This novel targeting signal is sufficient to redirect HCF-2 into mitochondria. It can also be transferred to an unrelated protein, resulting in its targeting to both the mitochondrial and nuclear compartments.

Toxicity of Familial ALS-Linked SOD1 Mutants from Selective Recruitment to Spinal Mitochondria

One cause of amyotrophic lateral sclerosis (ALS) is mutation in ubiquitously expressed copper/zinc superoxide dismutase (SOD1), but the mechanism of toxicity to motor neurons is unknown. Multiple disease-causing mutants, but not wild-type SOD1, are now demonstrated to be recruited to mitochondria, but only in affected tissues. This is independent of the copper chaperone for SOD1 and dismutase activity. Highly preferential association with spinal cord mitochondria is seen in human ALS for a mutant SOD1 that accumulates only to trace cytoplasmic levels. Despite variable proportions that are successfully imported, nearly constant amounts of SOD1 mutants and covalently damaged adducts of them accumulate as apparent import intermediates and/or are tightly aggregated or crosslinked onto integral membrane components on the cytoplasmic face of those mitochondria. These findings implicate damage from action of spinal cord-specific factors that recruit mutant SOD1 to spinal mitochondria as the basis for their selective toxicity in ALS.