Dietary supplementation with acetyl-l-carnitine counteracts age-related alterations of mitochondrial biogenesis, dynamics and antioxidant defenses in brain of old rats.

We previously reported the ability of dietary supplementation with acetyl-l-carnitine (ALCAR) to prevent age-related decreases of mitochondrial biogenesis in skeletal muscle and liver of old rats. Here, we investigate the effects of ALCAR supplementation in cerebral hemispheres and cerebellum of old rats by analyzing several parameters linked to mitochondrial biogenesis, mitochondrial dynamics and antioxidant defenses. We measured the level of the coactivators PGC-1α and PGC-1ß and of the factors regulating mitochondrial biogenesis, finding an age-related decrease of PGC-1ß, whereas PGC-1α level was unvaried. Twenty eight-month old rats supplemented with ALCAR for one and two months showed increased levels of both factors. Accordingly, the expression of the two transcription factors NRF-1 and TFAM followed the same trend of PGC-1ß. The level of mtDNA, ND1 and the activity of citrate synthase, were decreased with aging and increased following ALCAR treatment. Furthermore, ALCAR counteracted the age-related increase of deleted mtDNA. We also analyzed the content of proteins involved in mitochondrial dynamics (Drp1, Fis1, OPA1 and MNF2) and found an age-dependent increase of MFN2 and of the long form of OPA1. ALCAR treatment restored the content of the two proteins to the level of the young rats. No changes with aging and ALCAR were observed for Drp1 and Fis1. ALCAR reduced total cellular levels of oxidized PRXs and counteracted the age-related decrease of PRX3 and SOD2. Overall, our findings indicate a systemic positive effect of ALCAR dietary treatment and a tissue specific regulation of mitochondrial homeostasis in brain of old rats. Moreover, it appears that ALCAR acts as a nutrient since in most cases its effects were almost completely abolished one month after treatment suspension. Dietary supplementation of old rats with this compound seems a valuable approach to prevent age-related mitochondrial dysfunction and might ultimately represent a strategy to delay age-associated negative consequences in mitochondrial homeostasis.

Increase in proteins involved in mitochondrial fission, mitophagy, proteolysis and antioxidant response in type I endometrial cancer as an adaptive response to respiratory complex I deficiency.

Pathogenic mtDNA mutations associated with alterations of respiratory complex I, mitochondrial proliferation (oncocytic-like phenotype) and increase in antioxidant response were previously reported in type I endometrial carcinoma (EC). To evaluate whether in the presence of pathogenic mtDNA mutations other mitochondrial adaptive processes are triggered by cancer cells, the expression level of proteins involved in mitochondrial dynamics, mitophagy, proteolysis and apoptosis were evaluated in type I ECs harboring pathogenic mtDNA mutations and complex I deficiency. An increase in the fission protein Drp1, in the mitophagy protein BNIP3, in the mitochondrial protease CLPP, in the antioxidant and anti-apoptotic protein ALR and in Bcl-2 as well as a decrease in the fusion protein Mfn2 were found in cancer compared to matched non malignant tissue. Moreover, the level of these proteins was measured in type I EC, in hyperplastic (the premalignant form) and in non malignant tissues to verify whether the altered expression of these proteins is a common feature of endometrial cancer and of hyperplastic tissue. This analysis confirmed in type I EC samples, but not in hyperplasia, an alteration of the expression level of these proteins. These results suggest that in this cancer mitochondrial fission, antioxidant and anti-apoptotic response may be activated, as well as the discharge of damaged mitochondrial proteins as adaptation processes to mitochondrial dysfunction.

Mitochondrial changes in endometrial carcinoma: possible role in tumor diagnosis and prognosis (review).

Endometrial carcinoma (EC) is a solid neoplasia for which a role for mitochondria in cancer progression is currently emerging and yet represents a diagnostic and prognostic challenge. EC is one of the most frequently occurring gynecological malignancies in the Western world whose incidence has increased significantly during the last decades. Here, we review the literature data on mitochondrial changes reported in EC, namely, mitochondrial DNA (mtDNA) mutations, increase in mitochondrial biogenesis and discuss whether they may be used as new cancer biomarkers for early detection and prognosis of this cancer.

Current experience in testing mitochondrial nutrients in disorders featuring oxidative stress and mitochondrial dysfunction: rational design of chemoprevention trials.

An extensive number of pathologies are associated with mitochondrial dysfunction (MDF) and oxidative stress (OS). Thus, mitochondrial cofactors termed "mitochondrial nutrients" (MN), such as α-lipoic acid (ALA), Coenzyme Q10 (CoQ10), and l-carnitine (CARN) (or its derivatives) have been tested in a number of clinical trials, and this review is focused on the use of MN-based clinical trials. The papers reporting on MN-based clinical trials were retrieved in MedLine up to July 2014, and evaluated for the following endpoints: (a) treated diseases; (b) dosages, number of enrolled patients and duration of treatment; (c) trial success for each MN or MN combinations as reported by authors. The reports satisfying the above endpoints included total numbers of trials and frequencies of randomized, controlled studies, i.e., 81 trials testing ALA, 107 reports testing CoQ10, and 74 reports testing CARN, while only 7 reports were retrieved testing double MN associations, while no report was found testing a triple MN combination. A total of 28 reports tested MN associations with "classical" antioxidants, such as antioxidant nutrients or drugs. Combinations of MN showed better outcomes than individual MN, suggesting forthcoming clinical studies. The criteria in study design and monitoring MN-based clinical trials are discussed.

Oxidative stress and mitochondrial dysfunction across broad-ranging pathologies: toward mitochondria-targeted clinical strategies.

Beyond the disorders recognized as mitochondrial diseases, abnormalities in function and/or ultrastructure of mitochondria have been reported in several unrelated pathologies. These encompass ageing, malformations, and a number of genetic or acquired diseases, as diabetes and cardiologic, haematologic, organ-specific (e.g., eye or liver), neurologic and psychiatric, autoimmune, and dermatologic disorders. The mechanistic grounds for mitochondrial dysfunction (MDF) along with the occurrence of oxidative stress (OS) have been investigated within the pathogenesis of individual disorders or in groups of interrelated disorders. We attempt to review broad-ranging pathologies that involve mitochondrial-specific deficiencies or rely on cytosol-derived prooxidant states or on autoimmune-induced mitochondrial damage. The established knowledge in these subjects warrants studies aimed at elucidating several open questions that are highlighted in the present review. The relevance of OS and MDF in different pathologies may establish the grounds for chemoprevention trials aimed at compensating OS/MDF by means of antioxidants and mitochondrial nutrients.

Analysis of the mitochondrial proteome of cybrid cells harbouring a truncative mitochondrial DNA mutation in respiratory complex I.

Transmitochondrial cytoplasmic hybrids (cybrids) are well established model systems to reveal the effects of mitochondrial DNA (mtDNA) mutations on cell metabolism excluding the interferences of a different nuclear background. The m.3571insC mutation in the MTND1 gene of respiratory complex I (CI) is commonly detected in oncocytic tumors, in which it causes a severe CI dysfunction leading to an energetic impairment when present above 83% mutant load. To assess whether the energetic deficit may alter the mitochondrial proteome, OS-78 and OS-93 cybrid cell lines bearing two different degrees of the m.3571insC mutation (78% and 92.8%, respectively) and control cybrids bearing wild-type mtDNA (CC) were analyzed. Two-dimensional electrophoresis and mass spectrometry revealed significant alterations only in cybrids above the threshold (OS-93). All differentially expressed proteins are decreased. In particular, the levels of the pyruvate dehydrogenase E1 chain B subunit (E1ß), of lipoamide dehydrogenase (E3), the enzyme component of pyruvate and 2-oxoglutarate dehydrogenase complexes, and of lactate dehydrogenase B (LDHB) were reduced. Moreover, a significant decrease of the pyruvate dehydrogenase complex activity was found when OS-93 cybrid cells were grown in galactose medium, a metabolic condition that forces cells to use respiration. These results demonstrate that the energetic impairment caused by the almost homoplasmic m.3571insC mutation perturbs cellular metabolism leading to a decreased steady state level of components of very important mitochondrial NAD-dependent dehydrogenases.

Age- and calorie restriction-related changes in rat brain mitochondrial DNA and TFAM binding.

Aging markedly affects mitochondrial biogenesis and functions particularly in tissues highly dependent on the organelle's bioenergetics capability such as the brain's frontal cortex. Calorie restriction (CR) diet is, so far, the only intervention able to delay or prevent the onset of several age-related alterations in different organisms. We determined the contents of mitochondrial transcription factor A (TFAM), mitochondrial DNA (mtDNA), and the 4.8-kb mtDNA deletion in the frontal cortex from young (6-month-old) and aged (26-month-old), ad libitum-fed (AL) and calorie-restricted (CR), rats. We found a 70 % increase in TFAM amount, a 25 % loss in mtDNA content, and a 35 % increase in the 4.8-kb deletion content in the aged AL animals with respect to the young rats. TFAM-specific binding to six mtDNA regions was analyzed by mtDNA immunoprecipitation and semiquantitative polymerase chain reaction (PCR), showing a marked age-related decrease. Quantitative real-time PCR at two subregions involved in mtDNA replication demonstrated, in aged AL rats, a remarkable decrease (60-70 %) of TFAM-bound mtDNA. The decreased TFAM binding is a novel finding that may explain the mtDNA loss in spite of the compensatory TFAM increased amount. In aged CR rats, TFAM amount increased and mtDNA content decreased with respect to young rats' values, but the extent of the changes was smaller than in aged AL rats. Attenuation of the age-related effects due to the diet in the CR animals was further evidenced by the unchanged content of the 4.8-kb deletion with respect to that of young animals and by the partial prevention of the age-related decrease in TFAM binding to mtDNA.

Mitochondrial DNA content and mass increase in progression from normal to hyperplastic to cancer endometrium.

BACKGROUND: An increase in mitochondrial DNA (mtDNA) content and mitochondrial biogenesis associated with the activation of PGC-1α signalling pathway was previously reported in type I endometrial cancer. The aim of this study has been to evaluate if mtDNA content and the citrate synthase (CS) activity, an enzyme marker of mitochondrial mass, increase in progression from control endometrium to hyperplasia to type I endometrial carcinoma. RESULTS: Given that no statistically significant change in mtDNA content and CS activity in endometrium taken from different phases of the menstrual cycle or in menopause was found, these samples were used as control. Our research shows, for the first time, that mtDNA content and citrate synthase activity increase in hyperplastic endometrium compared to control tissues, even if their levels remain lower compared to cancer tissue. In particular, mtDNA content increases seem to precede increases in CS activity. No statistically significant change in mtDNA content and in CS activity was found in relation to different histopathological conditions such as grade, myometrial invasion and stage. CONCLUSION: MtDNA content and citrate synthase activity increases in pre-malignant lesions could be a potential molecular marker for progression from hyperplasia to carcinoma.

Acetyl-L-carnitine activates the peroxisome proliferator-activated receptor-γ coactivators PGC-1α/PGC-1ß-dependent signaling cascade of mitochondrial biogenesis and decreases the oxidized peroxiredoxins content in old rat liver.

The behavior of the peroxisome proliferator-activated receptor-γ coactivators PGC-1α/PGC-ß-dependent mitochondrial biogenesis signaling pathway, as well as the level of some antioxidant enzymes and proteins involved in mitochondrial dynamics in the liver of old rats before and after 2 months of acetyl-L-carnitine (ALCAR) supplementation, was tested. The results reveal that ALCAR treatment is able to reverse the age-associated decline of PGC-1α, PGC-1ß, nuclear respiratory factor 1 (NRF-1), mitochondrial transcription factor A (TFAM), nicotinamide adenine dinucleotide (NADH) dehydrogenase subunit 1 (ND1), and cytochrome c oxidase subunit IV (COX IV) protein levels, of mitochondrial DNA (mtDNA) content, and of citrate synthase activity. Moreover, it partially reverses the mitochondrial superoxide dismutase 2 (SOD2) decline and reduces the cellular content of oxidized peroxiredoxins. These data demonstrate that ALCAR treatment is able to promote in the old rat liver a new mitochondrial population that can contribute to the cellular oxidative stress reduction. Furthermore, a remarkable decline of Drp1 and of Mfn2 proteins is reported here for the first time, suggesting a reduced mitochondrial dynamics in aging liver with no effect of ALCAR treatment.

Rat liver mitochondrial proteome: changes associated with aging and acetyl-L-carnitine treatment.

Oxidative stress has a central role in aging and in several age-linked diseases such as neurodegenerative diseases, diabetes and cancer. Mitochondria, as the main cellular source and target of reactive oxygen species (ROS) in aging, are recognized as very important players in the above reported diseases. Impaired mitochondrial oxidative phosphorylation has been reported in several aging tissues. Defective mitochondria are not only responsible of bioenergetically less efficient cells but also increase ROS production further contributing to tissues oxidative stress. Acetyl-L-carnitine (ALCAR) is a biomolecule able to limit age-linked mitochondrial decay in brain, liver, heart and skeletal muscles by increasing mitochondrial efficiency. Here the global changes induced by aging and by ALCAR supplementation to old rat on the mitochondrial proteome of rat liver has been analyzed by means of the two-dimensional polyacrylamide gel electrophoresis. Mass spectrometry has been used to identify the differentially expressed proteins. A significant age-related change occurred in 31 proteins involved in several metabolisms. ALCAR supplementation altered the levels of 26 proteins. In particular, ALCAR reversed the age-related alterations of 10 mitochondrial proteins relative to mitochondrial cristae morphology, to the oxidative phosphorylation and antioxidant systems, to urea cycle, to purine biosynthesis.

Mitochondrial DNA backgrounds might modulate diabetes complications rather than T2DM as a whole.

Mitochondrial dysfunction has been implicated in rare and common forms of type 2 diabetes (T2DM). Additionally, rare mitochondrial DNA (mtDNA) mutations have been shown to be causal for T2DM pathogenesis. So far, many studies have investigated the possibility that mtDNA variation might affect the risk of T2DM, however, when found, haplogroup association has been rarely replicated, even in related populations, possibly due to an inadequate level of haplogroup resolution. Effects of mtDNA variation on diabetes complications have also been proposed. However, additional studies evaluating the mitochondrial role on both T2DM and related complications are badly needed. To test the hypothesis of a mitochondrial genome effect on diabetes and its complications, we genotyped the mtDNAs of 466 T2DM patients and 438 controls from a regional population of central Italy (Marche). Based on the most updated mtDNA phylogeny, all 904 samples were classified into 57 different mitochondrial sub-haplogroups, thus reaching an unprecedented level of resolution. We then evaluated whether the susceptibility of developing T2DM or its complications differed among the identified haplogroups, considering also the potential effects of phenotypical and clinical variables. MtDNA backgrounds, even when based on a refined haplogroup classification, do not appear to play a role in developing T2DM despite a possible protective effect for the common European haplogroup H1, which harbors the G3010A transition in the MTRNR2 gene. In contrast, our data indicate that different mitochondrial haplogroups are significantly associated with an increased risk of specific diabetes complications: H (the most frequent European haplogroup) with retinopathy, H3 with neuropathy, U3 with nephropathy, and V with renal failure.

Placing mitochondrial DNA mutations within the progression model of type I endometrial carcinoma.

Mitochondrial DNA (mtDNA) mutations have been described in almost all types of cancer. However, their exact role and timing of occurrence during tumor development and progression are still a matter of debate. A Vogelstein-like model of progression is well established for endometrial carcinoma (EC), however, mtDNA has been scarcely investigated in these tumors despite the fact that mitochondrial biogenesis increase has been shown to be a hallmark of type I EC. Here, we screened a panel of 23 type I EC tissues and matched typical hyperplasia for mutations in mtDNA and in four oncosupressors/oncogenes, namely PTEN, KRAS, CTNNB1 and TP53. Overall, mtDNA mutations were identified in 69% of cases, while mutational events in nuclear genes occurred in 56% of the cases, indicating that mtDNA mutations may precede the genetic instability of these genes canonically involved in progression from hyperplasia to tumor. Protein expression analysis revealed an increase in mitochondrial biogenesis and activation of oxidative stress response mechanisms in tumor tissues, but not in hyperplasia, in correlation with the occurrence of pathogenic mtDNA mutations. Our results point out an involvement of mtDNA mutations in EC progression and explain the increase in mitochondrial biogenesis of type I EC. Last, since mtDNA mutations occur after hyperplasia, their potential role in contributing to genetic instability may be envisioned.

Acetyl-L-carnitine supplementation to old rats partially reverts the age-related mitochondrial decay of soleus muscle by activating peroxisome proliferator-activated receptor gamma coactivator-1alpha-dependent mitochondrial biogenesis.

The age-related decay of mitochondrial function is a major contributor to the aging process. We tested the effects of 2-month-daily acetyl-L-carnitine (ALCAR) supplementation on mitochondrial biogenesis in the soleus muscle of aged rats. This muscle is heavily dependent on oxidative metabolism. Mitochondrial (mt) DNA content, citrate synthase activity, transcript levels of some nuclear- and mitochondrial-coded genes (cytochrome c oxidase subunit IV [COX-IV], 16S rRNA, COX-I) and of some factors involved in the mitochondrial biogenesis signaling pathway (peroxisome proliferator-activated receptor gamma [PPARgamma] coactivator-1alpha [PGC-1alpha], mitochondrial transcription factor A mitochondrial [TFAM], mitochondrial transcription factor 2B [TFB2]), as well as the protein content of PGC-1alpha were determined. The results suggest that the ALCAR treatment in old rats activates PGC-1alpha-dependent mitochondrial biogenesis, thus partially reverting the age-related mitochondrial decay.

Mitochondrial dysfunction in some oxidative stress-related genetic diseases: Ataxia-Telangiectasia, Down Syndrome, Fanconi Anaemia and Werner Syndrome.

Oxidative stress is a phenotypic hallmark in several genetic disorders characterized by cancer predisposition and/or propensity to premature ageing. Here we review the published evidence for the involvement of oxidative stress in the phenotypes of Ataxia-Telangiectasia (A-T), Down Syndrome (DS), Fanconi Anaemia (FA), and Werner Syndrome (WS), from the viewpoint of mitochondrial dysfunction. Mitochondria are recognized as both the cell compartment where energetic metabolism occurs and as the first and most susceptible target of reactive oxygen species (ROS) formation. Thus, a critical evaluation of the basic mechanisms leading to an in vivo pro-oxidant state relies on elucidating the features of mitochondrial impairment in each disorder. The evidence for different mitochondrial dysfunctions reported in A-T, DS, and FA is reviewed. In the case of WS, clear-cut evidence linking human WS phenotype to mitochondrial abnormalities is lacking so far in the literature. Nevertheless, evidence relating mitochondrial dysfunctions to normal ageing suggests that WS, as a progeroid syndrome, is likely to feature mitochondrial abnormalities. Hence, ad hoc research focused on elucidating the nature of mitochondrial dysfunction in WS pathogenesis is required. Based on the recognized, or reasonably suspected, role of mitochondrial abnormalities in the pathogenesis of these disorders, studies of chemoprevention with mitochondria-targeted supplements are warranted.

Muscle unloading potentiates the effects of acetyl-L-carnitine on the slow oxidative muscle phenotype.

The effect of acetyl-L-carnitine (ALCAR) supplementation to 3-month-old rats in normal-loading and unloading conditions has been here investigated by a combined morphological, biochemical and transcriptional approach to test whether ALCAR might cause a remodeling of the metabolic/contractile phenotype of soleus muscle. Morphological assessment demonstrated an increase of type I oxidative fiber content and cross-sectional area in ALCAR-treated animals both in normal-loading and in unloading conditions. ALCAR prevented loss of mitochondrial mass in unloaded animals whereas no ALCAR-dependent increase of mitochondrial mass occurred in normal-loaded muscle. Validated microarray analysis delineated an ALCAR-induced maintenance of a slow-oxidative expression program only in unloaded soleus muscle. Indeed, the muscle adjustment of the expression profile of factors underlying mitochondrial oxidative metabolism, protein turnover, fiber type differentiation and an adaptation of voltage-gated ion channel expression was distinguishable with respect to the loading status. This selectivity may suggest a key role of muscle loading status in the manifestation of ALCAR effects. The results extend to a broader level of biological informations the previous notion on ALCAR positive effect in rat soleus muscle during unloading and point to a role of ALCAR for the maintenance of its slow-oxidative fiber character.

MTERF factors: a multifunction protein family.

The MTERF family is a large protein family, identified in metazoans and plants, which consists of four subfamilies, MTERF1, 2, 3 and 4. Mitochondrial localisation was predicted for the vast majority of MTERF family members and demonstrated for the characterised MTERF proteins. The main structural feature of MTERF proteins is the presence of a modular architecture, based on repetitions of a 30-residue module, the mTERF motif, containing leucine zipper-like heptads. The MTERF family includes transcription termination factors: human mTERF, sea urchin mtDBP and Drosophila DmTTF. In addition to terminating transcription, they are involved in transcription initiation and in the control of mtDNA replication. This multiplicity of functions seems to flank differences in the gene organisation of mitochondrial genomes. MTERF2 and MTERF3 play antithetical roles in controlling mitochondrial transcription: that is, mammalian and Drosophila MTERF3 act as negative regulators, whereas mammalian MTERF2 functions as a positive regulator. Both proteins contact mtDNA in the promoter region, perhaps establishing interactions, either mutual or with other factors. Regulation of MTERF gene expression in human and Drosophila depends on nuclear transcription factors NRF-2 and DREF, respectively, and proceeds through pathways which appear to discriminate between factors positively or negatively acting in mitochondrial transcription. In this emerging scenario, it appears that MTERF proteins act to coordinate mitochondrial transcription.

Nuclear respiratory factor 2 induces the expression of many but not all human proteins acting in mitochondrial DNA transcription and replication.

In mammals, NRF-2 (nuclear respiratory factor 2), also named GA-binding protein, is an Ets family transcription factor that controls many genes involved in cell cycle progression and protein synthesis as well as in mitochondrial biogenesis. In this paper, we analyzed the role of NRF-2 in the regulation of human genes involved in mitochondrial DNA transcription and replication. By a combination of bioinformatic and biochemical approaches, we found that the factor binds in vitro and in vivo to the proximal promoter region of the genes coding for the transcription termination factor mTERF, the RNA polymerase POLRMT, the B subunit of the DNA polymerase-gamma, the DNA helicase TWINKLE, and the single-stranded DNA-binding protein mtSSB. The role of NRF-2 in modulating the expression of those genes was further established by RNA interference and overexpression strategies. On the contrary, we found that NRF-2 does not control the genes for the subunit A of DNA polymerase-gamma and for the transcription repressor MTERF3; we suggest that these genes are under regulatory mechanisms that do not involve NRF proteins. Since NRFs are known to positively control the expression of transcription-activating proteins, the novelty emerging from our data is that proteins playing antithetical roles in mitochondrial DNA transcription, namely activators and repressors, are under different regulatory pathways. Finally, we developed a more stringent consensus with respect to the general consensus of NRF-2/GA-binding protein when searching for NRF-2 binding sites in the promoter of mitochondrial proteins.

The PGC-1alpha-dependent pathway of mitochondrial biogenesis is upregulated in type I endometrial cancer.

PGC-1alpha-dependent pathway of mitochondrial biogenesis was investigated for the first time in type I endometrial cancer and in normal endometrium. In cancer endometrial tissue the citrate synthase activity, the mitochondrial DNA content and the TFAM level were found doubled compared to control endometrial tissue. Moreover, a 1.6- and 1.8-fold increase, respectively, of NRF-1 and PGG-1alpha expression was found. This study demonstrates, for the first time, that the increased mitochondrial biogenesis in type I endometrial cancer is associated to the upregulation of PGC-1alpha signalling pathway.

Methods for studying mitochondrial transcription termination with isolated components.

Characterization of the basic transcription machinery of mammalian mitochondrial DNA has been greatly supported by the availability of pure recombinant mitochondrial RNA polymerase (mtRNAP) and accessory factors, which allowed to develop a reconstituted in vitro transcription system. This chapter outlines a general strategy that makes use of a minimal promoter-independent transcription assay to study mitochondrial transcription termination in animal systems. We used such a system to investigate the transcription termination properties of the sea urchin factor mtDBP, however, it is applicable to the study of transcription termination in a variety of organisms, provided that the pure mtRNAP and the transcription termination factor are available.The assay here described contains the recombinant proteins mtRNAP and mtDBP, both expressed in insect cells, and a template consisting of a 3'-tailed DNA construct bearing the sequence bound by mtDBP. Transcription by the RNA polymerase produces run-off and terminated molecules, the size of the latter being consistent with RNA chain arrest in correspondence of the mtDBP-DNA complex. Transcription termination is protein-dependent as addition of increasing amounts of mtDBP to the assay causes a decrease in the intensity of the run-off and the gradual appearance of short-terminated molecules. Furthermore, we report a method, based on pulse-chase experiments, which allows us to distinguish between the true termination and the pausing events.

The MTERF family proteins: mitochondrial transcription regulators and beyond.

The MTERF family is a wide protein family, identified in Metazoa and plants, which consists of 4 subfamilies named MTERF1-4. Proteins belonging to this family are localized in mitochondria and show a modular architecture based on repetitions of a 30 amino acid module, the mTERF motif, containing leucine zipper-like heptads. The MTERF family includes the characterized transcription termination factors human mTERF, sea urchin mtDBP and Drosophila DmTTF. In vitro and in vivo studies show that these factors play different roles which are not restricted to transcription termination, but concern also transcription initiation and the control of mtDNA replication. The multiplicity of functions could be related to the differences in the gene organization of the mitochondrial genomes. Studies on the function of human and Drosophila MTERF3 factor showed that the protein acts as negative regulator of mitochondrial transcription, possibly in cooperation with other still unknown factors. The complete elucidation of the role of the MTERF family members will contribute to the unraveling of the molecular mechanisms of mtDNA transcription and replication.