Production of transmitochondrial mouse cell lines by cybrid rescue of rhodamine-6G pre-treated L-cells

Efficient Elimination of mtDNA from Mammalian Cells with 2',3'-Dideoxycytidine

Mammalian cell lines devoid of mitochondrial DNA (mtDNA) are indispensable in studies aimed at elucidating the contribution of mtDNA to various cellular processes or interactions between nuclear and mitochondrial genomes. However, the repertoire of tools for generating such cells (also known as rho-0 or ρ0 cells) remains limited, and approaches remain time- and labor-intensive, ultimately limiting their availability. Ethidium bromide (EtBr), which is most commonly used to induce mtDNA loss in mammalian cells, is cytostatic and mutagenic as it affects both nuclear and mitochondrial genomes. Therefore, there is growing interest in new tools for generating ρ0 cell lines. Here, we examined the utility of 2',3'-dideoxycytidine (ddC, zalcitabine) alone or in combination with EtBr for generating ρ0 cell lines of mouse and human origin as well as inducing the ρ0 state in mouse/human somatic cell hybrids. We report that ddC is superior to EtBr in both immortalized mouse fibroblasts and human 143B cells. Also, unlike EtBr, ddC exhibits no cytostatic effects at the highest concentration tested (200 μM), making it more suitable for general use. We conclude that ddC is a promising new tool for generating mammalian ρ0 cell lines.

A CO2-Responsive Imidazole-Functionalized Fluorescent Material Mediates Cancer Chemotherapy

We present a breakthrough in the synthesis and development of functional gas-responsive materials as highly potent anticancer agents suitable for applications in cancer treatment. Herein, we successfully synthesised a stimuli-responsive multifunctional material (I-R6G) consisting of a carbon dioxide (CO₂)-sensitive imidazole moiety and spirolactam-containing conjugated rhodamine 6G (R6G) molecule. The resulting I-R6G is highly hydrophobic and non- or weakly fluorescent. Simple CO₂ bubbling treatment induces hydrophobic I-R6G to completely dissolve in water and subsequently form self-assembled nanoparticles, which exhibit unique optical absorption and fluorescence behaviours in water and extremely low haemolytic ability against sheep red blood cells. Reversibility testing indicated that I-R6G undergoes reversible CO₂/nitrogen (N₂)-dependent stimulation in water, as its structural and physical properties can be reversibly and stably switched by alternating cycles of CO₂ and N₂ bubbling. Importantly, in vitro cellular assays clearly demonstrated that the CO₂-protonated imidazole moiety promotes rapid internalisation of CO₂-treated I-R6G into cancer cells, which subsequently induces massive levels of necrotic cell death. In contrast, CO₂-treated I-R6G was not internalised and did not affect the viability of normal cells. Therefore, this newly created system may provide an innovative and efficient route to remarkably improve the selectivity, safety and efficacy of cancer treatment.

Aberrant RNA processing contributes to the pathogenesis of mitochondrial diseases in trans-mitochondrial mouse model carrying mitochondrial tRNALeu(UUR) with a pathogenic A2748G mutation

Mitochondrial tRNAs are indispensable for the intra-mitochondrial translation of genes related to respiratory subunits, and mutations in mitochondrial tRNA genes have been identified in various disease patients. However, the molecular mechanism underlying pathogenesis remains unclear due to the lack of animal models. Here, we established a mouse model, designated 'mito-mice tRNALeu(UUR)2748', that carries a pathogenic A2748G mutation in the tRNALeu(UUR) gene of mitochondrial DNA (mtDNA). The A2748G mutation is orthologous to the human A3302G mutation found in patients with mitochondrial diseases and diabetes. A2748G mtDNA was maternally inherited, equally distributed among tissues in individual mice, and its abundance did not change with age. At the molecular level, A2748G mutation is associated with aberrant processing of precursor mRNA containing tRNALeu(UUR) and mt-ND1, leading to a marked decrease in the steady-levels of ND1 protein and Complex I activity in tissues. Mito-mice tRNALeu(UUR)2748 with ≥50% A2748G mtDNA exhibited age-dependent metabolic defects including hyperglycemia, insulin insensitivity, and hepatic steatosis, resembling symptoms of patients carrying the A3302G mutation. This work demonstrates a valuable mouse model with an inheritable pathological A2748G mutation in mt-tRNALeu(UUR) that shows metabolic syndrome-like phenotypes at high heteroplasmy level. Furthermore, our findings provide molecular basis for understanding A3302G mutation-mediated mitochondrial disorders.

Epigenetic Regulation of the Nuclear and Mitochondrial Genomes: Involvement in Metabolism, Development, and Disease

Our understanding of the interactions between the nuclear and mitochondrial genomes is becoming increasingly important as they are extensively involved in establishing early development and developmental progression. Evidence from various biological systems indicates the interdependency between the genomes, which requires a high degree of compatibility and synchrony to ensure effective cellular function throughout development and in the resultant offspring. During development, waves of DNA demethylation, de novo methylation, and maintenance methylation act on the nuclear genome and typify oogenesis and pre- and postimplantation development. At the same time, significant changes in mitochondrial DNA copy number influence the metabolic status of the developing organism in a typically cell-type-specific manner. Collectively, at any given stage in development, these actions establish genomic balance that ensures each developmental milestone is met and that the organism's program for life is established.

Cybrid technology

The study of the mitochondrial DNA (mtDNA) has been hampered by the lack of methods to genetically manipulate the mitochondrial genome in living animal cells. This limitation has been partially alleviated by the ability to transfer mitochondria (and their mtDNAs) from one cell into another, as long as they are from the same species. This is done by isolating mtDNA-containing cytoplasts and fusing these to cells lacking mtDNA. This transmitochondrial cytoplasmic hybrid (cybrid) technology has helped the field understand the mechanism of several pathogenic mutations. In this chapter, we describe procedures to obtain transmitochondrial cybrids.

Nuclear response to divergent mitochondrial DNA genotypes modulates the interferon immune response

Mitochondrial OXPHOS generates most of the energy required for cellular function. OXPHOS biogenesis requires the coordinated expression of the nuclear and mitochondrial genomes. This represents a unique challenge that highlights the importance of nuclear-mitochondrial genetic communication to cellular function. Here we investigated the transcriptomic and functional consequences of nuclear-mitochondrial genetic divergence in vitro and in vivo. We utilized xenomitochondrial cybrid cell lines containing nuclear DNA from the common laboratory mouse Mus musculus domesticus and mitochondrial DNA (mtDNA) from Mus musculus domesticus, or exogenous mtDNA from progressively divergent mouse species Mus spretus, Mus terricolor, Mus caroli and Mus pahari. These cybrids model a wide range of nuclear-mitochondrial genetic divergence that cannot be achieved with other research models. Furthermore, we used a xenomitochondrial mouse model generated in our laboratory that harbors wild-type, C57BL/6J Mus musculus domesticus nuclear DNA and homoplasmic mtDNA from Mus terricolor. RNA sequencing analysis of xenomitochondrial cybrids revealed an activation of interferon signaling pathways even in the absence of OXPHOS dysfunction or immune challenge. In contrast, xenomitochondrial mice displayed lower baseline interferon gene expression and an impairment in the interferon-dependent innate immune response upon immune challenge with herpes simplex virus, which resulted in decreased viral control. Our work demonstrates that nuclear-mitochondrial genetic divergence caused by the introduction of exogenous mtDNA can modulate the interferon immune response both in vitro and in vivo, even when OXPHOS function is not compromised. This work may lead to future insights into the role of mitochondrial genetic variation and the immune function in humans, as patients affected by mitochondrial disease are known to be more susceptible to immune challenges.

Mitochondrial DNA Haplotypes Influence Energy Metabolism across Chicken Transmitochondrial Cybrids

The association between mitochondrial DNA haplotype and productive performances has been widely reported in chicken breeds. However, there has not been physiological evidence of this seen previously. In this study, chicken transmitochondrial cells were generated using the nucleus of the DF-1 cell line and mitochondria of primary cell lines derived from two native chicken breeds, Tibetan chicken and Shouguang chicken. Generally, Tibetan chicken primary cells showed a stronger metabolic capacity than Shouguang chicken primary cells. However, the Tibetan chicken cybrids had a dramatic drop in relative mtDNA copies and oxygen consumption. Higher rates of oxygen consumption (OCR) and expression levels of mitochondrial biogenesis and fusion genes were observed in Shouguang chicken cybrids, potentially reflecting that the mitochondrial DNA haplotype of Shouguang chicken had better coordination with the DF-1 nucleus than others. Meanwhile, mitonuclear incompatibility occurred in Tibetan chicken cybrids. The results demonstrate functional differences among mitochondrial DNA haplotypes and may shed light on the interaction between the mitochondria and nucleus in Gallus gallus domesticus.

Hypertension-associated mitochondrial DNA 4401A>G mutation caused the aberrant processing of tRNAMet, all 8 tRNAs and ND6 mRNA in the light-strand transcript

Mitochondrial tRNA processing defects were associated with human diseases but their pathophysiology remains elusively. The hypertension-associated m.4401A>G mutation resided at a spacer between mitochondrial tRNAMet and tRNAGln genes. An in vitro processing experiment revealed that the m.4401A>G mutation caused 59% and 69% decreases in the 5' end processing efficiency of tRNAGln and tRNAMet precursors, catalyzed by RNase P, respectively. Using human umbilical vein endothelial cells-derived cybrids, we demonstrated that the m.4401A>G mutation caused the decreases of all 8 tRNAs and ND6 and increases of longer and uncleaved precursors from the Light-strand transcript. Conversely, the m.4401A>G mutation yielded the reduced levels of tRNAMet level but did not change the levels of other 13 tRNAs, 12 mRNAs including ND1, 12S rRNA and 16S rRNA from the Heavy-strand transcript. These implicated the asymmetrical processing mechanisms of H-strand and L-strand polycistronic transcripts. The tRNA processing defects play the determined roles in the impairing mitochondrial translation, respiratory deficiency, diminishing membrane potential, increasing production of reactive oxygen species and altering autophagy. Furthermore, the m.4401A>G mutation altered the angiogenesis, evidenced by aberrant wound regeneration and weaken tube formation in mutant cybrids. Our findings provide new insights into the pathophysiology of hypertension arising from mitochondrial tRNA processing defects.

A coronary artery disease-associated tRNAThr mutation altered mitochondrial function, apoptosis and angiogenesis

The tissue specificity of mitochondrial tRNA mutations remains largely elusive. In this study, we demonstrated the deleterious effects of tRNAThr 15927G>A mutation that contributed to pathogenesis of coronary artery disease. The m.15927G>A mutation abolished the highly conserved base-pairing (28C-42G) of anticodon stem of tRNAThr. Using molecular dynamics simulations, we showed that the m.15927G>A mutation caused unstable tRNAThr structure, supported by decreased melting temperature and slower electrophoretic mobility of mutated tRNA. Using cybrids constructed by transferring mitochondria from a Chinese family carrying the m.15927G>A mutation and a control into mitochondrial DNA (mtDNA)-less human umbilical vein endothelial cells, we demonstrated that the m.15927G>A mutation caused significantly decreased efficiency in aminoacylation and steady-state levels of tRNAThr. The aberrant tRNAThr metabolism yielded variable decreases in mtDNA-encoded polypeptides, respiratory deficiency, diminished membrane potential and increased the production of reactive oxygen species. The m.15927G>A mutation promoted the apoptosis, evidenced by elevated release of cytochrome c into cytosol and increased levels of apoptosis-activated proteins: caspases 3, 7, 9 and PARP. Moreover, the lower wound healing cells and perturbed tube formation were observed in mutant cybrids, indicating altered angiogenesis. Our findings provide new insights into the pathophysiology of coronary artery disease, which is manifested by tRNAThr mutation-induced alterations.

Mitochondria and Female Germline Stem Cells-A Mitochondrial DNA Perspective

Mitochondria and mitochondrial DNA have important roles to play in development. In primordial germ cells, they progress from small numbers to populate the maturing oocyte with high numbers to support post-fertilization events. These processes take place under the control of significant changes in DNA methylation and other epigenetic modifiers, as well as changes to the DNA methylation status of the nuclear-encoded mitochondrial DNA replication factors. Consequently, the differentiating germ cell requires significant synchrony between the two genomes in order to ensure that they are fit for purpose. In this review, I examine these processes in the context of female germline stem cells that are isolated from the ovary and those derived from embryonic stem cells and reprogrammed somatic cells. Although our knowledge is limited in this respect, I provide predictions based on other cellular systems of what is expected and provide insight into how these cells could be used in clinical medicine.

hnRNPA2 mediated acetylation reduces telomere length in response to mitochondrial dysfunction

Telomeres protect against chromosomal damage. Accelerated telomere loss has been associated with premature aging syndromes such as Werner's syndrome and Dyskeratosis Congenita, while, progressive telomere loss activates a DNA damage response leading to chromosomal instability, typically observed in cancer cells and senescent cells. Therefore, identifying mechanisms of telomere length maintenance is critical for understanding human pathologies. In this paper we demonstrate that mitochondrial dysfunction plays a causal role in telomere shortening. Furthermore, hnRNPA2, a mitochondrial stress responsive lysine acetyltransferase (KAT) acetylates telomere histone H4at lysine 8 of (H4K8) and this acetylation is associated with telomere attrition. Cells containing dysfunctional mitochondria have higher telomere H4K8 acetylation and shorter telomeres independent of cell proliferation rates. Ectopic expression of KAT mutant hnRNPA2 rescued telomere length possibly due to impaired H4K8 acetylation coupled with inability to activate telomerase expression. The phenotypic outcome of telomere shortening in immortalized cells included chromosomal instability (end-fusions) and telomerase activation, typical of an oncogenic transformation; while in non-telomerase expressing fibroblasts, mitochondrial dysfunction induced-telomere attrition resulted in senescence. Our findings provide a mechanistic association between dysfunctional mitochondria and telomere loss and therefore describe a novel epigenetic signal for telomere length maintenance.

Mitochondrial replacement in an iPSC model of Leber's hereditary optic neuropathy

Cybrid technology was used to replace Leber hereditary optic neuropathy (LHON) causing mitochondrial DNA (mtDNA) mutations from patient-specific fibroblasts with wildtype mtDNA, and mutation-free induced pluripotent stem cells (iPSCs) were generated subsequently. Retinal ganglion cell (RGC) differentiation demonstrates increased cell death in LHON-RGCs and can be rescued in cybrid corrected RGCs.

Mitochondrial haplotypes influence metabolic traits across bovine inter- and intra-species cybrids

In bovine species, mitochondrial DNA polymorphisms and their correlation to productive or reproductive performances have been widely reported across breeds and individuals. However, experimental evidence of this correlation has never been provided. In order to identify differences among bovine mtDNA haplotypes, transmitochondrial cybrids were generated, with the nucleus from MAC-T cell line, derived from a Holstein dairy cow (Bos taurus) and mitochondria from either primary cell line derived from a domestic Chinese native beef Luxi cattle breed or central Asian domestic yak (Bos grunniens). Yak primary cells illustrated a stronger metabolic capacity than that of Luxi. However, all yak cybrid parameters illustrated a drop in relative yak mtDNA compared to Luxi mtDNA, in line with a mitonuclear imbalance in yak interspecies cybrid. Luxi has 250 divergent variations relative to the mitogenome of Holsteins. In cybrids there were generally higher rates of oxygen consumption (OCR) and extracellular acidification (ECAR), and lower mRNA expression levels of nuclear-encoded mitochondrial genes, potentially reflecting active energy metabolism and cellular stress resistance. The results demonstrate that functional differences exist between bovine cybrid cells. While cybrid viability was similar between Holstein and Luxi breeds, the mitonuclear mismatch caused a marked metabolic dysfunction in cattle:yak cybrid species.

Generating Rho-0 Cells Using Mesenchymal Stem Cell Lines

Introduction

The generation of Rho-0 cells requires the use of an immortalization process, or tumor cell selection, followed by culture in the presence of ethidium bromide (EtBr), incurring the drawbacks its use entails. The purpose of this work was to generate Rho-0 cells using human mesenchymal stem cells (hMSCs) with reagents having the ability to remove mitochondrial DNA (mtDNA) more safely than by using EtBr.

Methodology

Two immortalized hMSC lines (3a6 and KP) were used; 143B.TK-Rho-0 cells were used as reference control. For generation of Rho-0 hMSCs, cells were cultured in medium supplemented with each tested reagent. Total DNA was isolated and mtDNA content was measured by real-time polymerase chain reaction (PCR). Phenotypic characterization and gene expression assays were performed to determine whether 3a6 Rho-0 hMSCs maintain the same stem properties as untreated 3a6 hMSCs. To evaluate whether 3a6 Rho-0 hMSCs had a phenotype similar to that of 143B.TK-Rho-0 cells, in terms of reactive oxygen species (ROS) production, apoptotic levels and mitochondrial membrane potential (Δψm) were measured by flow cytometry and mitochondrial respiration was evaluated using a SeaHorse XFp Extracellular Flux Analyzer. The differentiation capacity of 3a6 and 3a6 Rho-0 hMSCs was evaluated using real-time PCR, comparing the relative expression of genes involved in osteogenesis, adipogenesis and chondrogenesis.

Results

The results showed the capacity of the 3a6 cell line to deplete its mtDNA and to survive in culture with uridine. Of all tested drugs, Stavudine (dt4) was the most effective in producing 3a6-Rho cells. The data indicate that hMSC Rho-0 cells continue to express the characteristic MSC cell surface receptor pattern. Phenotypic characterization showed that 3a6 Rho-0 cells resembled 143B.TK-Rho-0 cells, indicating that hMSC Rho-0 cells are Rho-0 cells. While the adipogenic capability was higher in 3a6 Rho-0 cells than in 3a6 cells, the osteogenic and chondrogenic capacities were lower.

Conclusion

Among the drugs and conditions tested, the use of d4t was the best option for producing Rho-0 cells from hMSCs. Rho-0 cells are useful for studying the role of mitochondria in hMSC differentiation.

Modulation of ROS levels in fibroblasts by altering mitochondria regulates the process of wound healing

Mitochondria are the major source of reactive oxygen species (ROS) in fibroblasts which are thought to be crucial regulators of wound healing with a potential to affect the expression of nuclear genes involved in this process. ROS generated by mitochondria are involved in all stages of tissue repair process but the regulation of ROS-generating system in fibroblasts still remains poorly understood. The purpose of this study was to better understand molecular mechanisms of how the regulation of ROS levels generated by mitochondria may influence the process of wound repair. Cybrid model system of mtDNA variations was used to study the functional consequences of altered ROS levels on wound healing responses in a uniform nuclear background of cultured ρ(0) fibroblasts. Mitochondrial ROS in cybrids were modulated by antioxidants that quench ROS to examine their ability to close the wound. Real-time PCR arrays were used to investigate whether ROS generated by specific mtDNA variants have the ability to alter expression of some key nuclear-encoded genes central to the wound healing response and oxidative stress. Our data suggest levels of mitochondrial ROS affect expression of some nuclear encoded genes central to wound healing response and oxidative stress and modulation of mitochondrial ROS by antioxidants positively affects in vitro process of wound closure. Thus, regulation of mitochondrial ROS-generating system in fibroblasts can be used as effective natural redox-based strategy to help treat non-healing wounds.

Progressive increase in mtDNA 3243A>G heteroplasmy causes abrupt transcriptional reprogramming

Variation in the intracellular percentage of normal and mutant mitochondrial DNAs (mtDNA) (heteroplasmy) can be associated with phenotypic heterogeneity in mtDNA diseases. Individuals that inherit the common disease-causing mtDNA tRNA(Leu(UUR)) 3243A>G mutation and harbor ∼10-30% 3243G mutant mtDNAs manifest diabetes and occasionally autism; individuals with ∼50-90% mutant mtDNAs manifest encephalomyopathies; and individuals with ∼90-100% mutant mtDNAs face perinatal lethality. To determine the basis of these abrupt phenotypic changes, we generated somatic cell cybrids harboring increasing levels of the 3243G mutant and analyzed the associated cellular phenotypes and nuclear DNA (nDNA) and mtDNA transcriptional profiles by RNA sequencing. Small increases in mutant mtDNAs caused relatively modest defects in oxidative capacity but resulted in sharp transitions in cellular phenotype and gene expression. Cybrids harboring 20-30% 3243G mtDNAs had reduced mtDNA mRNA levels, rounded mitochondria, and small cell size. Cybrids with 50-90% 3243G mtDNAs manifest induction of glycolytic genes, mitochondrial elongation, increased mtDNA mRNA levels, and alterations in expression of signal transduction, epigenomic regulatory, and neurodegenerative disease-associated genes. Finally, cybrids with 100% 3243G experienced reduced mtDNA transcripts, rounded mitochondria, and concomitant changes in nuclear gene expression. Thus, striking phase changes occurred in nDNA and mtDNA gene expression in response to the modest changes of the mtDNA 3243G mutant levels. Hence, a major factor in the phenotypic variation in heteroplasmic mtDNA mutations is the limited number of states that the nucleus can acquire in response to progressive changes in mitochondrial retrograde signaling.

Integrated transcriptome analysis across mitochondrial disease etiologies and tissues improves understanding of common cellular adaptations to respiratory chain dysfunction

Mitochondrial diseases are heterogeneous, multi-systemic disorders for which mechanistic understanding is limited. To investigate common downstream effects of primary respiratory chain dysfunction on global gene expression and pathway regulation, we reanalyzed transcriptome datasets from all publicly available studies of respiratory chain dysfunction resulting from genetic disorders, acute pathophysiologic processes, or environmental toxins. A general overview is provided of the bioinformatic processing of transcriptome data to uncover biological insights into in vivo and in vitro adaptations to mitochondrial dysfunction, with specific examples discussed from a variety of independent cell, animal, and human tissue studies. To facilitate future community efforts to cohesively mine these data, all reanalyzed transcriptome datasets were deposited into a publicly accessible central web archive. Our own integrated meta-analysis of these data identified several commonly dysregulated genes across diverse mitochondrial disease etiologies, models, and tissue types. Overall, transcriptome analyses provide a useful means to survey cellular adaptation to mitochondrial diseases.

The effects of nuclear reprogramming on mitochondrial DNA replication

Undifferentiated mouse embryonic stem cells (ESCs) possess low numbers of mitochondrial DNA (mtDNA), which encodes key subunits associated with the generation of ATP through oxidative phosphorylation (OXPHOS). As ESCs differentiate, mtDNA copy number is regulated by the nuclear-encoded mtDNA replication factors, which initiate a major replication event on Day 6 of differentiation. Here, we examined mtDNA replication events in somatic cells reprogrammed to pluripotency, namely somatic cell-ES (SC-ES), somatic cell nuclear transfer ES (NT-ES) and induced pluripotent stem (iPS) cells, all at low-passage. MtDNA copy number in undifferentiated iPS cells was similar to ESCs whilst SC-ES and NT-ES cells had significantly increased levels, which correlated positively and negatively with Nanog and Sox2 expression, respectively. During pluripotency and differentiation, the expression of the mtDNA-specific replication factors, PolgA and Peo1, were differentially expressed in iPS and SC-ES cells when compared to ESCs. Throughout differentiation, reprogrammed somatic cells were unable to accumulate mtDNA copy number, characteristic of ESCs, especially on Day 6. In addition, iPS and SC-ES cells were also unable to regulate ATP content in a manner similar to differentiating ESCs prior to Day 14. The treatment of reprogrammed somatic cells with an inhibitor of de novo DNA methylation, 5-Azacytidine, prior to differentiation enabled iPS cells, but not SC-ES and NT-ES cells, to accumulate mtDNA copies per cell in a manner similar to ESCs. These data demonstrate that the reprogramming process disrupts the regulation of mtDNA replication during pluripotency but this can be re-established through the use of epigenetic modifiers.

Mouse mtDNA mutant model of Leber hereditary optic neuropathy

An animal model of Leber hereditary optic neuropathy (LHON) was produced by introducing the human optic atrophy mtDNA ND6 P25L mutation into the mouse. Mice with this mutation exhibited reduction in retinal function by elecroretinogram (ERG), age-related decline in central smaller caliber optic nerve fibers with sparing of larger peripheral fibers, neuronal accumulation of abnormal mitochondria, axonal swelling, and demyelination. Mitochondrial analysis revealed partial complex I and respiration defects and increased reactive oxygen species (ROS) production, whereas synaptosome analysis revealed decreased complex I activity and increased ROS but no diminution of ATP production. Thus, LHON pathophysiology may result from oxidative stress.

Transferring isolated mitochondria into tissue culture cells

We have developed a new method for introducing large numbers of isolated mitochondria into tissue culture cells. Direct microinjection of mitochondria into typical mammalian cells has been found to be impractical due to the large size of mitochondria relative to microinjection needles. To circumvent this problem, we inject isolated mitochondria through appropriately sized microinjection needles into rodent oocytes or single-cell embryos, which are much larger than tissue culture cells, and then withdraw a ‘mitocytoplast’ cell fragment containing the injected mitochondria using a modified holding needle. These mitocytoplasts are then fused to recipient cells through viral-mediated membrane fusion and the injected mitochondria are transferred into the cytoplasm of the tissue culture cell. Since mouse oocytes contain large numbers of mouse mitochondria that repopulate recipient mouse cells along with the injected mitochondria, we used either gerbil single-cell embryos or rat oocytes to package injected mouse mitochondria. We found that the gerbil mitochondrial DNA (mtDNA) is not maintained in recipient rho0 mouse cells and that rat mtDNA initially replicated but was soon completely replaced by the injected mouse mtDNA, and so with both procedures mouse cells homoplasmic for the mouse mtDNA in the injected mitochondria were obtained.

Changes in mitochondrial DNA alter expression of nuclear encoded genes associated with tumorigenesis

We previously reported the presence of a mtDNA mutation hotspot in UV-induced premalignant and malignant skin tumors in hairless mice. We have modeled this change (9821insA) in murine cybrid cells and demonstrated that this alteration in mtDNA associated with mtBALB haplotype can alter the biochemical characteristics of cybrids and subsequently can contribute to significant changes in their behavioral capabilities. This study shows that changes in mtDNA can produce differences in expression levels of specific nuclear-encoded genes, which are capable of triggering the phenotypes such as seen in malignant cells. From a potential list of differentially expressed genes discovered by microarray analysis, we selected MMP-9 and Col1a1 for further studies. Real-time PCR confirmed up-regulation of MMP-9 and down-regulation of Col1a1 in cybrids harboring the mtDNA associated with the skin tumors. These cybrids also showed significantly increased migration and invasion abilities compared to wild type. The non-specific MMP inhibitor, GM6001, was able to inhibit migratory and invasive abilities of the 9821insA cybrids confirming a critical role of MMPs in cellular motility. Nuclear factor-κB (NF-κB) is a key transcription factor for production of MMPs. An inhibitor of NF-κB activation, Bay 11-7082, was able to inhibit the expression of MMP-9 and ultimately decrease migration and invasion of mutant cybrids containing 9821insA. These studies confirm a role of NF-κB in the regulation of MMP-9 expression and through this regulation modulates the migratory and invasive capabilities of cybrids with mutant mtDNA. Enhanced migration and invasion abilities caused by up-regulated MMP-9 may contribute to the tumorigenic phenotypic characteristics of mutant cybrids.

Mesenchymal stem cells transfer mitochondria to the cells with virtually no mitochondrial function but not with pathogenic mtDNA mutations

It has been reported that human mesenchymal stem cells (MSCs) can transfer mitochondria to the cells with severely compromised mitochondrial function. We tested whether the reported intercellular mitochondrial transfer could be replicated in different types of cells or under different experimental conditions, and tried to elucidate possible mechanism. Using biochemical selection methods, we found exponentially growing cells in restrictive media (uridine⁻ and bromodeoxyuridine [BrdU]⁺) during the coculture of MSCs (uridine-independent and BrdU-sensitive) and 143B-derived cells with severe mitochondrial dysfunction induced by either long-term ethidium bromide treatment or short-term rhodamine 6G (R6G) treatment (uridine-dependent but BrdU-resistant). The exponentially growing cells had nuclear DNA fingerprint patterns identical to 143B, and a sequence of mitochondrial DNA (mtDNA) identical to the MSCs. Since R6G causes rapid and irreversible damage to mitochondria without the removal of mtDNA, the mitochondrial function appears to be restored through a direct transfer of mitochondria rather than mtDNA alone. Conditioned media, which were prepared by treating mtDNA-less 143B ρ⁰ cells under uridine-free condition, induced increased chemotaxis in MSC, which was also supported by transcriptome analysis. Cytochalasin B, an inhibitor of chemotaxis and cytoskeletal assembly, blocked mitochondrial transfer phenomenon in the above condition. However, we could not find any evidence of mitochondrial transfer to the cells harboring human pathogenic mtDNA mutations (A3243G mutation or 4,977 bp deletion). Thus, the mitochondrial transfer is limited to the condition of a near total absence of mitochondrial function. Elucidation of the mechanism of mitochondrial transfer will help us create a potential cell therapy-based mitochondrial restoration or mitochondrial gene therapy for human diseases caused by mitochondrial dysfunction.

Mitochondrial and metabolic-based protective strategies in Huntington's disease: the case of creatine and coenzyme Q

Huntington's disease (HD) is a neurodegenerative genetic disorder caused by an expansion of CAG repeats in the HD gene encoding for huntingtin (Htt), resulting in progressive death of striatal neurons, with clinical symptoms of chorea, dementia and dramatic weight loss. Metabolic and mitochondrial dysfunction caused by the expanded polyglutamine sequence have been described along with other mechanisms of neurodegeneration previously described in human tissues and animal models of HD. In this review, we focus on mitochondrial and metabolic disturbances affecting both the central nervous system and peripheral cells, including mitochondrial DNA damage, mitochondrial complexes defects, loss of calcium homeostasis and transcriptional deregulation. Glucose abnormalities have also been described in peripheral tissues of HD patients and in HD animal and cellular models. Moreover, there are no effective neuroprotective treatments available in HD. Thus, we briefly discuss the role of creatine and coenzyme Q10 that target mitochondrial dysfunction and impaired bioenergetics and have been previously used in HD clinical trials.

Identification of an mtDNA mutation hot spot in UV-induced mouse skin tumors producing altered cellular biochemistry

There is increasing awareness of the role of mtDNA alterations in the development of cancer, as mtDNA point mutations are found at high frequency in a variety of human tumors. To determine the biological effects of mtDNA mutations in UV-induced skin tumors, hairless mice were irradiated to produce tumors, and the tumor mtDNAs were screened for single-nucleotide changes using temperature gradient capillary electrophoresis (TGCE), followed by direct sequencing. A mutation hot spot (9821insA) in the mitochondrially encoded tRNA arginine (mt-Tr) locus (tRNA(Arg)) was discovered in approximately one-third of premalignant and malignant skin tumors. To determine the functional relevance of this particular mutation in vitro, cybrid cell lines containing different mt-Tr (tRNA(Arg)) alleles were generated. The resulting cybrid cell lines contained the same nuclear genotype and differed only in their mtDNAs. The biochemical analysis of the cybrids revealed that the mutant haplotype is associated with diminished levels of complex I protein (CI), resulting in lower levels of baseline oxygen consumption and lower cellular adenosine triphosphate (ATP) production. We hypothesize that this specific mtDNA mutation alters cellular biochemistry, supporting the development of keratinocyte neoplasia.

Inflammation, mitochondria, and the inhibition of adult neurogenesis

The process of neurogenesis continues throughout life, with thousands of new neurons generated every day in the mammalian brain. Impairment of hippocampal neurogenesis has been suggested to be involved in neurodegenerative conditions, including the cognitive decline associated with aging, Alzheimer's disease, Parkinson's disease, and ionizing radiation. These neurodegenerative conditions are all characterized by proinflammatory changes and increased numbers of activated microglia. Activated microglia produce a variety of proinflammatory factors, including interleukin-6, tumor necrosis factor-α, reactive oxygen species, and nitric oxide, all of which are antineurogenic. These same factors have also been shown to suppress mitochondrial function, but the role of mitochondria in neurogenesis remains barely investigated. This brief review summarizes the findings of several studies that support a role for mitochondrial impairment as part of the mechanism of the reduction of neurogenesis associated with inflammation.

Maternally inherited susceptibility to cancer

Tumor microenvironment promotes mtDNA mutations. A number of these mutations will affect cell metabolism and increase cell survival. These mutations are positively selected and contribute to other tumor features, such as extracellular matrix remodeling and angiogenic processes, thus favoring metastases. Like somatic mutations, although with less marked effects, some mtDNA population polymorphisms will affect OXPHOS function, cell metabolism, and homeostasis. Thus, they could behave as inherited susceptibility factors for cancer. However, in addition to epidemiological evidence, other more direct clues are required. The cybrid approach can help to clarify this issue.

Mitochondrial energetics and therapeutics

Mitochondrial dysfunction has been linked to a wide range of degenerative and metabolic diseases, cancer, and aging. All these clinical manifestations arise from the central role of bioenergetics in cell biology. Although genetic therapies are maturing as the rules of bioenergetic genetics are clarified, metabolic therapies have been ineffectual. This failure results from our limited appreciation of the role of bioenergetics as the interface between the environment and the cell. A systems approach, which, ironically, was first successfully applied over 80 years ago with the introduction of the ketogenic diet, is required. Analysis of the many ways that a shift from carbohydrate glycolytic metabolism to fatty acid and ketone oxidative metabolism may modulate metabolism, signal transduction pathways, and the epigenome gives us an appreciation of the ketogenic diet and the potential for bioenergetic therapeutics.

Transmitochondrial embryonic stem cells containing pathogenic mtDNA mutations are compromised in neuronal differentiation

Objectives:  Defects of the mitochondrial genome (mtDNA) cause a series of rare, mainly neurological disorders. In addition, they have been implicated in more common forms of movement disorders, dementia and the ageing process. In order to try to model neuronal dysfunction associated with mitochondrial disease, we have attempted to establish a series of transmitochondrial mouse embryonic stem cells harbouring pathogenic mtDNA mutations.

Materials and methods: Transmitochondrial embryonic stem cell cybrids were generated by fusion of cytoplasts carrying a variety of mtDNA mutations, into embryonic stem cells that had been pretreated with rhodamine 6G, to prevent transmission of endogenous mtDNA. Cybrids were differentiated into neurons and assessed for efficiency of differentiation and electrophysiological function.

Results:  Neuronal differentiation could occur, as indicated by expression of neuronal markers. Differentiation was impaired in embryonic stem cells carrying mtDNA mutations that caused severe biochemical deficiency. Electrophysiological tests showed evidence of synaptic activity in differentiated neurons carrying non‐pathogenic mtDNA mutations or in those that caused a mild defect of respiratory activity. Again, however, neurons carrying mtDNA mutations that resulted in severe biochemical deficiency had marked reduction in post‐synaptic events.

Conclusions:  Differentiated neurons carrying severely pathogenic mtDNA defects can provide a useful model for understanding how such mutations can cause neuronal dysfunction.

Generation of mtDNA-exchanged cybrids for determination of the effects of mtDNA mutations on tumor phenotypes

It has been proposed that mutations of mitochondrial DNA (mtDNA) and resultant mitochondrial dysfunction induce various phenotypes, such as mitochondrial diseases, aging, and tumorigenesis. However, it is difficult to conclude whether mtDNA mutations are truly responsible for these phenotypes due to the regulation of the mitochondrial functions by both mtDNA and nuclear DNA. The mtDNA-exchange techniques are very effective to exclude the influence of nuclear DNA mutations on expression of these phenotypes. Using these techniques, we recently showed that specific mtDNA mutations can regulate tumor cell metastasis. In this chapter, we describe the methods to establish the mtDNA-exchanged cell lines (cybrids). Applying this technique will reveal how mtDNA mutations are related to various biological phenomena.

Overexpression of human mitochondrial valyl tRNA synthetase can partially restore levels of cognate mt-tRNAVal carrying the pathogenic C25U mutation

Phenotypic diversity associated with pathogenic mutations of the human mitochondrial genome (mtDNA) has often been explained by unequal segregation of the mutated and wild-type genomes (heteroplasmy). However, this simple hypothesis cannot explain the tissue specificity of disorders caused by homoplasmic mtDNA mutations. We have previously associated a homoplasmic point mutation (1624C>T) in MTTV with a profound metabolic disorder that resulted in the neonatal deaths of numerous siblings. Affected tissues harboured a marked biochemical defect in components of the mitochondrial respiratory chain, presumably due to the extremely low (<1%) steady-state levels of mt-tRNA^Val. In primary myoblasts and transmitochondrial cybrids established from the proband (index case) and offspring, the marked respiratory deficiency was lost and steady-state levels of the mutated mt-tRNA^Val were greater than in the biopsy material, but were still an order of magnitude lower than in control myoblasts. We present evidence that the generalized decrease in steady-state mt-tRNA^Val observed in the homoplasmic 1624C>T-cell lines is caused by a rapid degradation of the deacylated form of the abnormal mt-tRNA^Val. By both establishing the identity of the human mitochondrial valyl-tRNA synthetase then inducing its overexpression in transmitochondrial cell lines, we have been able to partially restore steady-state levels of the mutated mt-tRNA^Val, consistent with an increased stability of the charged mt-tRNA. These data indicate that variations in the levels of VARS2L between tissue types and patients could underlie the difference in clinical presentation between individuals homoplasmic for the 1624C>T mutation.

A mouse model of mitochondrial disease reveals germline selection against severe mtDNA mutations

The majority of mitochondrial DNA (mtDNA) mutations that cause human disease are mild to moderately deleterious, yet many random mtDNA mutations would be expected to be severe. To determine the fate of the more severe mtDNA mutations, we introduced mtDNAs containing two mutations that affect oxidative phosphorylation into the female mouse germ line. The severe ND6 mutation was selectively eliminated during oogenesis within four generations, whereas the milder COI mutation was retained throughout multiple generations even though the offspring consistently developed mitochondrial myopathy and cardiomyopathy. Thus, severe mtDNA mutations appear to be selectively eliminated from the female germ line, thereby minimizing their impact on population fitness.

Cybrid models of mtDNA disease and transmission, from cells to mice

Oxidative phosphorylation (OXPHOS) is the only mammalian biochemical pathway dependent on the coordinated assembly of protein subunits encoded by both nuclear and mitochondrial DNA (mtDNA) genes. Cytoplasmic hybrid cells, cybrids, are created by introducing mtDNAs of interest into cells depleted of endogenous mtDNAs, and have been a central tool in unraveling effects of disease-linked mtDNA mutations. In this way, the nuclear genetic complement is held constant so that observed effects on OXPHOS can be linked to the introduced mtDNA. Cybrid studies have confirmed such linkage for many defined, disease-associated mutations. In general, a threshold principle is evident where OXPHOS defects are expressed when the proportion of mutant mtDNA in a heteroplasmic cell is high. Cybrids have also been used where mtDNA mutations are not known, but are suspected, and have produced some support for mtDNA involvement in more common neurodegenerative diseases. Mouse modeling of mtDNA transmission and disease has recently taken advantage of cybrid approaches. By using cultured cells as intermediate carriers of mtDNAs, ES cell cybrids have been produced in several laboratories by pretreatment of the cells with rhodamine 6G before cytoplast fusion. Both homoplasmic and heteroplasmic mice have been produced, allowing modeling of mtDNA transmission through the mouse germ line. We also briefly review and compare other transgenic approaches to modeling mtDNA dynamics, including mitochondrial injection into oocytes or zygotes, and embryonic karyoplast transfer. When breakthrough technology for mtDNA transformation arrives, cybrids will remain valuable for allowing exchange of engineered mtDNAs between cells.

The mitochondrial genome in human adaptive radiation and disease: on the road to therapeutics and performance enhancement

The human mitochondrial genome consists of approximately 1500 genes, 37 encoded by the maternally inherited mitochondrial DNA (mtDNA) and the remainder encoded in the nuclear DNA (nDNA). The mtDNA is present in thousands of copies per cell and encodes proteins that are essential components of the mitochondrial energy generation pathway, oxidative phosphorylation (OXPHOS). OXPHOS generates heat to maintain our body temperature and ATP to do work. The mitochondria also produce much of the cellular reactive oxygen species (ROS) and can initiate apoptosis through activation of the mitochondrial permeability transition pore (mtPTP) in response to energy deficiency and oxidative damage. Mitochondrial ROS mutates the mtDNA and mtDNA mutations have been associated with a wide range of age-related diseases including neurodegenerative diseases, cardiomyopathy, diabetes and various cancers. The cellular accumulation of mtDNA mutations may also be the aging clock. Ancient mtDNA variants have also been adaptive and may influence individual health today. Mutations in nDNA-encoded mitochondrial genes can also disrupt OXPHOS, alter mtDNA replication, and affect mitochondrial division. In an effort to treat mitochondrial disease, both metabolic and genetic interventions have been attempted. Metabolic interventions have been directed at increasing energy output, reducing ROS production and stabilizing the mtPTP. Genetic therapies have attempted introduction of nucleic acids into the mitochondrion, nDNA-mitochondrial genes into the nucleus, and mtDNA-encoded genes into the nucleus. These therapeutic approaches might also be used to enhance performance, but we must be careful that catering to short term individual interests might undermine our capacity to adapt and survive.

Respiratory complex III is required to maintain complex I in mammalian mitochondria

A puzzling observation in patients with oxidative phosphorylation (OXPHOS) deficiencies is the presence of combined enzyme complex defects associated with a genetic alteration in only one protein-coding gene. In particular, mutations in the mtDNA encoded cytochrome b gene are associated either with combined complex I+III deficiency or with only complex III deficiency. We have reproduced the combined complex I+III defect in mouse and human cultured cell models harboring cytochrome b mutations. In both, complex III assembly is impeded and causes a severe reduction in the amount of complex I, not observed when complex III activity was pharmacologically inhibited. Metabolic labeling in mouse cells revealed that complex I was assembled, although its stability was severely hampered. Conversely, complex III stability was not influenced by the absence of complex I. This structural dependence among complexes I and III was confirmed in a muscle biopsy of a patient harboring a nonsense cytochrome b mutation.

A chemical enucleation method for the transfer of mitochondrial DNA to rho(o) cells

The study of pathogenic mitochondrial DNA mutations has, in most cases, relied on the production of transmitochondrial cybrids. Although the procedure to produce such cybrids is well established, it is laborious and cumbersome. Moreover, the mechanical enucleation procedure is inefficient and different techniques have to be used depending on the adherence properties of the cell. To circumvent these difficulties, we developed a chemical enucleation method that can have wide applicability for the production of transmitochondrial cybrids. The method is based on the use of actinomycin D to render the nuclear genome transcription/replication inactive and unable to recover after treatment. Such treated cells are fused to cells devoid of mitochondrial DNA and selected for the presence of a functional oxidative phosphorylation system. Our results showed that 95% of the clones recovered by this procedure are true transmitochondrial cybrids. This method greatly facilitates the production of transmitochondrial cybrids, thereby increasing the number of mtDNA mutations and the recipient cell types that can be studied by this system.

Inducible expression of a dominant negative DNA polymerase-gamma depletes mitochondrial DNA and produces a rho0 phenotype

We report the inducible, stable expression of a dominant negative form of mitochondria-specific DNA polymerase-gamma to eliminate mitochondrial DNA (mtDNA) from human cells in culture. HEK293 cells were transfected with a plasmid encoding inactive DNA polymerase-gamma harboring a D1135A substitution (POLGdn). The cells rapidly lost mtDNA (t1/2 = 2-3 days) when expression of the transgene was induced. Concurrent reduction of mitochondrial encoded mRNA and protein, decreased cellular growth rate, and compromised respiration and mitochondrial membrane potential were observed. mtDNA depletion was reversible, as demonstrated by restoration of mtDNA copy number to normal within 10 days when the expression of POLGdn was suppressed following a 3-day induction period. Long term (20 days) expression of POLGdn completely eliminated mtDNA from the cells, resulting in rho0 cells that were respiration-deficient, lacked electron transport complex activities, and were auxotrophic for pyruvate and uridine. Fusion of the rho0 cells with human platelets yielded clonal cybrid cell lines that were populated exclusively with donor-derived mtDNA. Respiratory function, mitochondrial membrane potential, and electron transport activities were restored to normal in the cybrid cells. Inducible expression of a dominant negative DNA polymerase-gamma can yield mtDNA-deficient cell lines, which can be used to study the impact of specific mtDNA mutations on cellular physiology, and to investigate mitochondrial genome function and regulation.

Mouse models for mitochondrial disease

Mutations in mitochondrial genes encoded by both mitochondrial DNA (mtDNA) and nuclear DNA (nDNA) genes have been implicated in a wide range of neuromuscular diseases. MtDNA base substitution and rearrangement mutations generally inactivate one or more tRNA or rRNA genes and can cause myopathy, cardiomyopathy, cataracts, growth retardation, diabetes, etc. nDNA mutations can cause Leigh syndrome, cardiomyopathy, and nephropathy, due to defects in oxidative phosphorylation (OXPHOS) enzyme complexes; cartilage-hair hypoplasia (CHH) and mtDNA depletion syndrome, through defects in mitochondrial nucleic acid metabolism; and ophthalmoplegia with multiple mtDNA deletions, caused by adenine nucleotide translocator-1 (ANT1) mutations. Mouse models have been prepared that recapitulate a number of these diseases. The mtDNA 16S rRNA chloramphenicol (CAP) resistance mutation was introduced into the mouse female germline and caused cataracts and rod and cone abnormalities in chimeras and neonatal lethal myopathy and cardiomyopathy in mutant animals. A mtDNA deletion was introduced into the mouse germline and caused myopathy, cardiomyopathy, and nephropathy. Conditional inactivation of the nDNA mitochondrial transcription factor (Tfam) gene in the heart resulted in neonatal lethal cardiomyopathy, while its inactivation in the pancreatic beta-cells caused diabetes. The ATP/ADP ratio was implicated in mitochondrial diabetes through transgenic modification of the beta-cell ATP-sensitive K(+) channel (K(ATP)). Mutational inactivation of the mouse Ant1 gene resulted in myopathy, cardiomyopathy, and multiple mtDNA deletions in association with elevated reactive oxygen species (ROS) production. Inactivation of uncoupler proteins (Ucp) 1-3 revealed that mitochondrial Delta Psi regulated ROS production. The role of mitochondrial ROS toxicity in disease and aging was confirmed by inactivating glutathione peroxidase (GPx1), resulting in growth retardation, and by total and partial inactivation of Mn superoxide dismutase (MnSOD; Sod2), resulting in neonatal lethal dilated cardiomyopathy and accelerated apoptosis in aging, respectively. The importance of mitochondrial ROS in degenerative diseases and aging was confirmed by treating Sod2 -/- mice and C. elegans with catalytic antioxidant drugs.

Maternal germ-line transmission of mutant mtDNAs from embryonic stem cell-derived chimeric mice

We report a method for introducing mtDNA mutations into the mouse female germ line by means of embryonic stem (ES) cell cybrids. Mitochondria were recovered from the brain of a NZB mouse by fusion of synaptosomes to a mtDNA-deficient (rho degrees ) cell line. These cybrids were enucleated and the cytoplasts were electrofused to rhodamine-6G (R-6G)-treated female ES cells. The resulting ES cell cybrids permitted transmission of the NZB mtDNAs through the mouse maternal lineage for three generations. Similarly, mtDNAs from a partially respiratory-deficient chloramphenicol-resistant (CAP(R)) cell line also were introduced into female chimeric mice and were transmitted to the progeny. CAP(R) chimeric mice developed a variety of ocular abnormalities, including congenital cataracts, decreased retinal function, and hamaratomas of the optic nerve. The germ-line transmission of the CAP(R) mutation resulted in animals with growth retardation, myopathy, dilated cardiomyopathy, and perinatal or in utero lethality. Skeletal and heart muscle mitochondria of the CAP(R) mice were enlarged and atypical with inclusions. This mouse ES cell-cybrid approach now provides the means to generate a wide variety of mouse models of mitochondrial disease.

Functional constraints of nuclear-mitochondrial DNA interactions in xenomitochondrial rodent cell lines

The co-evolution of nuclear and mitochondrial genomes in vertebrates led to more than 100 specific interactions that are crucial for an optimized ATP generation. These interactions have been examined by introducing rat mtDNA into mouse cells devoid of mitochondrial DNA (mtDNA). When mtDNA-less cells derived from the common mouse (Mus musculus domesticus) were fused to cytoplasts prepared from Mus musculus, Mus spretus, or rat (Rattus norvegicus), a comparable number of respiring clones could be obtained. Mouse xenomitochondrial cybrids harboring rat mtDNA had a slower growth rate in medium containing galactose as the carbon source, suggesting a defect in oxidative phosphorylation. These clones respired approximately 50% less than the parental mouse cells or xenomitochondrial cybrids harboring Mus spretus mtDNA. The activities of respiratory complexes I and IV were approximately 50% lower, but mitochondrial protein synthesis was unaffected. The defects in complexes I and IV were associated with decreased steady-state levels of respective subunits suggesting problems in assembly. We also showed that the presence of 10% mouse mtDNA co-existing with rat mtDNA was sufficient to restore respiration to normal levels. Our results suggest that evolutionary distance alone is not a precise predictor of nuclear-mitochondrial interactions as previously suggested for primates.

Mitochondrial defects in cardiomyopathy and neuromuscular disease

Over the past 11 years, a considerable body of evidence has accumulated implicating defects in the mitochondrial energy-generating pathway, oxidative phosphorylation, in a wide variety of degenerative diseases including myopathy and cardiomyopathy. Most classes of pathogenic mitochondrial DNA mutations affect the heart, in association with a variety of other clinical manifestations that can include skeletal muscle, the central nervous system (including eye), the endocrine system, and the renal system. To better understand the pathophysiologic basis of mitochondrial diseases and their role in myopathy and cardiomyopathy, several mouse models of mitochondrial disease have been prepared. Mitochondrial DNA mutations from cultured cells have been introduced into mice; nuclear DNA genes involved in mitochondrial energy production and reactive oxygen species detoxification have been genetically inactivated, which resulted in mice with hypertrophic and dilated cardiomyopathy, respectively. Physiologic characterization of these mice has confirmed the importance of decreased mitochondrial energy production, increased mitochondrial reactive oxygen species production, and the mitochondrial initiation of apoptosis in mitochondrial disease. With these insights, new therapeutic approaches for neuromuscular and cardiac disease have been suggested.

Long-term analysis of differentiation in human myoblasts repopulated with mitochondria harboring mtDNA mutations

Short-term analysis of myogenesis in respiration-deficient myoblasts demonstrated that respiratory chain dysfunction impairs muscle differentiation. To investigate long-term consequences of a deficiency in oxidative phosphorylation on myogenesis, we quantitated myoblast fusion and expression of sarcomeric myosin in respiration-deficient myogenic cybrids. We produced viable myoblasts harboring exclusively mtDNA with large-scale deletions by treating wild-type myoblasts with rhodamine 6G and fusing them with cytoplasts homoplasmic for two different mutated mtDNAs. Recovery of growth in transmitochondrial myoblasts demonstrated that respiratory chain function is not required for recovery of rhodamine 6G-treated cells. Both transmitochondrial respiration-deficient cultures exhibited impaired myoblast fusion. Expression of sarcomeric myosin was also delayed in deficient myoblasts. However, 4 weeks after induction of differentiation, one cell line was able to quantitatively recover its capacity to form postmitotic muscle cells. This indicates that while oxidative phosphorylation is an important source of ATP for muscle development, myoblast differentiation can be supported entirely by glycolysis.

A novel system for assigning the mode of inheritance in mitochondrial disorders using cybrids and rhodamine 6G

When normal human cultured skin fibroblasts were treated with the fluorescent dye rhodamine 6G (R6G), there was a drastic reduction in numbers of intact mitochondria and electron transport chain enzyme activities, despite the fact that mitochondrial DNA (mtDNA) was still present in treated cells. We used this observation to develop a novel system for generating cybrids. When cultured skin fibroblast cells from a patient with the mitochondrial encephalopathy and ragged-red fibers (MERRF) syndrome harboring the A8344G mtDNA mutation and which showed a severe reduction in cytochrome c oxidase activity were treated with R6G and fused to enucleated HeLaCOT cells, the resulting cybrid clones showed recovery of cytochrome c oxidase activity, and were shown to have mtDNA derived solely from the HeLaCOT cell line. R6G has significant advantages over ethidium bromide in removing the mitochondrial elements from cultured cells, and the results reported here demonstrate that this strategy can be used to determine the origin of the genetic defect in patients with electron transport chain abnormalities.

Transfer of chloramphenicol-resistant mitochondrial DNA into the chimeric mouse

The mitochondrial DNA (mtDNA) chloramphenicol (CAP)-resistance (CAPR) mutation has been introduced into the tissues of adult mice via female embryonic stem (ES) cells. The endogenous CAP-sensitive (CAPS) mtDNAs were eliminated by treatment of the ES cells with the lipophilic dye Rhodamine-6-G (R-6-G). The ES cells were then fused to enucleated cell cytoplasts prepared from the CAPR mouse cell line 501-1. This procedure converted the ES cell mtDNA from 100% wild-type to 100% mutant. The CAPR ES cells were then injected into blastocysts and viable chimeric mice were isolated. Molecular testing for the CAPR mutant mtDNAs revealed that the percentage of mutant mtDNAs varied from zero to approximately 50% in the tissues analyzed. The highest percentage of mutant mtDNA was found in the kidney in three of the chimeric animals tested. These data suggest that, with improved efficiency, it may be possible to transmit exogenous mtDNA mutants through the mouse germ-line.