Elimination of mitochondrial elements and improved viability in hybrid cells

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.

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.

Supramolecular Organization of Respiratory Complexes

Since the discovery of the existence of superassemblies between mitochondrial respiratory complexes, such superassemblies have been the object of a passionate debate. It is accepted that respiratory supercomplexes are structures that occur in vivo, although which superstructures are naturally occurring and what could be their functional role remain open questions. The main difficulty is to make compatible the existence of superassemblies with the corpus of data that drove the field to abandon the early understanding of the physical arrangement of the mitochondrial respiratory chain as a compact physical entity (the solid model). This review provides a nonexhaustive overview of the evolution of our understanding of the structural organization of the electron transport chain from the original idea of a compact organization to a view of freely moving complexes connected by electron carriers. Today supercomplexes are viewed not as a revival of the old solid model but rather as a refined revision of the fluid model, which incorporates a new layer of structural and functional complexity.

Why are most organelle genomes transmitted maternally?

Why the DNA-containing organelles, chloroplasts, and mitochondria, are inherited maternally is a long standing and unsolved question. However, recent years have seen a paradigm shift, in that the absoluteness of uniparental inheritance is increasingly questioned. Here, we review the field and propose a unifying model for organelle inheritance. We argue that the predominance of the maternal mode is a result of higher mutational load in the paternal gamete. Uniparental inheritance evolved from relaxed organelle inheritance patterns because it avoids the spread of selfish cytoplasmic elements. However, on evolutionary timescales, uniparentally inherited organelles are susceptible to mutational meltdown (Muller's ratchet). To prevent this, fall-back to relaxed inheritance patterns occurs, allowing low levels of sexual organelle recombination. Since sexual organelle recombination is insufficient to mitigate the effects of selfish cytoplasmic elements, various mechanisms for uniparental inheritance then evolve again independently. Organelle inheritance must therefore be seen as an evolutionary unstable trait, with a strong general bias to the uniparental, maternal, mode.

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.

Exclusion of male mitochondria and plastids during syngamy in barley as a basis for maternal inheritance

It is known from genetic analyses that maternal inheritance of cytoplasmic organelles is the rule among plants and animals. Although recognized as one of several possible mechanisms for strictly maternal cytoplasmic inheritance, exclusion of sperm cytoplasm at the time of gametic fusion has remained poorly documented for the flowering plants. In the present investigation, enucleated, cytoplasmic bodies approximately the size of intact, prefusion sperm cells have been observed within degenerated synergids and adjacent to recently fertilized egg cells. A complete series of ultrathin sections (68 sections) through such a cytoplasmic body revealed 59 mitochondria, 3 plastids, 7 dictyosomes, and a large vacuole with no limiting membrane. This structure is interpreted as the entire male cytoplasm that was left outside the egg during fusion between egg and sperm. The observation of only one cytoplasmic body per embryo sac may indicate a preliminary fusion between sperm cells or, more likely, the existence of a fundamentally different mechanism of fertilization between the second sperm and the central cell.

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.

Regulation of IGF-I receptor signaling in diabetic cardiac muscle: dysregulation of cytosolic and mitochondria HSP60

Insulin deficiency downregulates HSP60 and IGF-I receptor signaling and disrupts intracellular signaling homeostasis in diabetic cardiac muscle. Our previous studies had shown that IGF-I receptor signaling can be modulated by the abundance of HSP60. Since HSP60 localizes to the cytoplasmic compartment and mitochondria, this study was carried out to determine the distribution of cytosolic and mitochondria HSP60 in diabetic myocardium and to explore whether cytosolic HSP60 can modulate IGF-I receptor signaling in cardiac muscle cells. In streptozotocin-induced diabetes, both the cytosolic and mitochondrial fractions of HSP60 were decreased in the myocardium. Incubating primary cardiomyocytes with insulin leads to increased abundance of HSP60 in the cytosolic and mitochondria compartments. To determine whether cytosolic HSP60 can modulate IGF-I receptor signaling, we used rhodamine 6G to deplete functional mitochondria in cardiomyocytes. In the mitochondria-depleted cells, overexpression of HSP60 with adenoviral vector increased the abundance of IGF-I receptor, enhanced IGF-I-activated receptor phosphorylation, and augmented IGF-I activation of Akt and ERK. Thus overexpressing HSP60 in the cytosolic compartment enhanced IGF-I receptor signaling through upregulation of IGF-I receptor protein. However, IGF-I receptor signaling was significantly reduced in the mitochondria-depleted cells, which suggested that maintaining normal IGF-I receptor signaling in cardiomyocytes required functioning mitochondria. The effect of cytosolic HSP60 involved suppression of ubiquitin conjugation to IGF-I receptor in cardiomyocytes. These data suggest two different mechanisms that can regulate IGF-I signaling, one via cytosolic HSP60 suppression of IGF-I receptor ubiquitination and the other via mitochondria modulation. These findings provide new insight into the regulation of IGF-I signaling in diabetic cardiomyopathy.

Generation of trans-mitochondrial mice carrying homoplasmic mtDNAs with a missense mutation in a structural gene using ES cells

Generation of various kinds of trans-mitochondrial mice, mito-mice, each carrying mtDNAs with a different pathogenic mutation, is required for precise investigation of the pathogenesis of mitochondrial diseases. This study used two respiration-deficient mouse cell lines as donors of mtDNAs with possible pathogenic mutations. One cell line expressed 45-50% respiratory activity due to mouse mtDNAs with a T6589C missense mutation in the COI gene (T6589C mtDNA) and the other expressed 40% respiratory activity due to rat (Rattus norvegicus) mtDNAs in mouse cells. By cytoplasmic transfer of these mtDNAs to mouse ES cells, we isolated respiration-deficient ES cells. We obtained chimeric mice and generated their F(6) progeny carrying mouse T6589C mtDNAs by its female germ line transmission. They were respiration-deficient and thus could be used as models of mitochondrial diseases caused by point mutations in mtDNA structural genes. However, chimeric mice and mito-mice carrying rat mtDNAs were not obtained, suggesting that significant respiration defects or some deficits induced by rat mtDNAs in mouse ES cells prevented their differentiation to generate mice carrying rat mtDNAs.

Application of ES cells for generation of respiration-deficient mice carrying mtDNA with a large-scale deletion

In a previous study, we used mouse zygotes as recipients of mtDNA with a large-scale deletion mutation (DeltamtDNA) and generated respiration-deficient mice (mito-mice) carrying DeltamtDNA. In this study, we used mouse ES cells as recipients of DeltamtDNA, and generated mito-mice with DeltamtDNA only when the ES cells carried 17% DeltamtDNA. No chimera mice or their F(1) progenies were obtained from ES cells carrying more than 61% DeltamtDNA. These observations suggest that respiratory defects of ES cells inhibit their normal differentiation into chimera mice and mito-mice, and that ES cells are more effective than zygotes for generation of mito-mice carrying mtDNAs without significant pathogenic mutations.

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.

The thankless task of playing genetics with mammalian mitochondrial DNA: a 30-year review

The advances obtained through the genetic tools available in yeast for studying the oxidative phosphorylation (OXPHOS) biogenesis and in particular the role of the mtDNA encoded genes, strongly contrast with the very limited benefits that similar approaches have generated for the study of mammalian mtDNA. Here we review the use of the genetic manipulation in mammalian mtDNA, its difficulty and the main types of mutants accumulated in the past 30 years and the information derived from them. We also point out the need for a substantial improvement in this field in order to obtain new tools for functional genetic studies and for the generation of animal models of mtDNA-linked diseases.

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.

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.

Cytochrome c oxidase assembly in primates is sensitive to small evolutionary variations in amino acid sequence

Respiring mitochondria require many interactions between nuclear and mitochondrial genomes. Although mitochondrial DNA (mtDNA) from the gorilla and the chimpanzee are able to restore oxidative phosphorylation in a human cell, mtDNAs from more distant primate species are functionally incompatible with human nuclear genes. Using microcell-mediated chromosome and mitochondria transfer, we introduced and maintained a functional orangutan mtDNA in a human nuclear background. However, partial oxidative phosphorylation function was restored only in the presence of most orangutan chromosomes, suggesting that human oxidative phosphorylation-related nuclear-coded genes are not able to replace many orangutan ones. The respiratory capacity of these hybrids was decreased by 65%-80%, and cytochrome c oxidase (COX) activity was decreased by 85%-95%. The function of other respiratory complexes was not significantly altered. The translation of mtDNA-coded COX subunits was normal, but their steady-state levels were approximately 10% of normal ones. Nuclear-coded COX subunits were loosely associated with mitochondrial membranes, a characteristic of COX assembly-defective mutants. Our results suggest that many human nuclear-coded genes not only cannot replace the orangutan counterparts, but also exert a specific interference at the level of COX assembly. This cellular model underscores the precision of COX assembly in mammals and sheds light on the nature of nuclear-mtDNA coevolutionary constraints.

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.

Generation of nuclear hybrids overcoming the natural barrier of incompatibility: transfer of nuclei from Lentinula edodes into protoplasts of Coriolus versicolor

Heterokaryotic nuclear hybrids overcoming the natural barriers of incompatibility have been studied in basidiomycetes. To produce these nuclear hybrids between incompatible mushrooms, which have several potent pharmacological effects, nuclear transfer was performed between Lentinula edodes and Coriolus versicolor. Nuclei from serine auxotrophs of Lentinula edodes, LE207 (Ser-) were transferred into the protoplasts of arginine auxotrophs of Coriolus versicolor, CV17 (Arg-), using 30% polyethylene glycol 4000 in 10 mM CaCl2-glycine solution (pH 8.0). Nuclear transfer progenies were selected by nutritional complementation on minimal media supplemented with 0.6 M sucrose. The progenies were classified based on colony morphology to L. edodes-like, C. versicolor-like and non-parental type. Most of the progenies grew slower than either parent. The number of nuclei per cell was similar but the DNA content varied between progenies. The isozyme patterns of nuclear hybrids resembled either of the parent profiles or showed a mixed profile.

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.

Expanding the functional human mitochondrial DNA database by the establishment of primate xenomitochondrial cybrids

The nuclear and mitochondrial genomes coevolve to optimize approximately 100 different interactions necessary for an efficient ATP-generating system. This coevolution led to a species-specific compatibility between these genomes. We introduced mitochondrial DNA (mtDNA) from different primates into mtDNA-less human cells and selected for growth of cells with a functional oxidative phosphorylation system. mtDNA from common chimpanzee, pigmy chimpanzee, and gorilla were able to restore oxidative phosphorylation in the context of a human nuclear background, whereas mtDNA from orangutan, and species representative of Old-World monkeys, New-World monkeys, and lemurs were not. Oxygen consumption, a sensitive index of respiratory function, showed that mtDNA from chimpanzee, pigmy chimpanzee, and gorilla replaced the human mtDNA and restored respiration to essentially normal levels. Mitochondrial protein synthesis was also unaltered in successful "xenomitochondrial cybrids." The abrupt failure of mtDNA from primate species that diverged from humans as recently as 8-18 million years ago to functionally replace human mtDNA suggests the presence of one or a few mutations affecting critical nuclear-mitochondrial genome interactions between these species. These cellular systems provide a demonstration of intergenus mtDNA transfer, expand more than 20-fold the number of mtDNA polymorphisms that can be analyzed in a human nuclear background, and provide a novel model for the study of nuclear-mitochondrial interactions.

Isolation of mitochondrial DNA-less mouse cell lines and their application for trapping mouse synaptosomal mitochondrial DNA with deletion mutations

For isolation of mouse mtDNA-less (rho0) cell lines, we searched for various antimitochondrial drugs that were expected to decrease the mtDNA content and found that treatment with ditercalinium, an antitumor bis-intercalating agent, was extremely effective for completely excluding mtDNA in all the mouse cell lines we tested. The resulting rho0 mouse cells were successfully used for trapping the mtDNA of living nerve cells into dividing cultured cells by fusion of the rho0 cells with mouse brain synaptosomes, which represent synaptic endings isolated from nerve cells. With neuronal mtDNA obtained, all of the cybrid clones restored mitochondrial translation activity similarly regardless of whether the mtDNA was derived from young or aged mice, thus at least suggesting that defects in mitochondrial genomes are not involved in the age-associated mitochondrial dysfunction observed in the brain of aged mice. Furthermore, we could trap a very small amount of a common 5823-base pair deletion mutant mtDNA (DeltamtDNA5823) that was detectable by polymerase chain reaction in the cybrid clones. As the amount of mutant mtDNA with large scale deletions was expected to increase during prolonged cultivation of the cybrids, these cells should be available for establishment of mice containing the deletion mutant mtDNA.

Nuclear inheritance of a gene affecting mitochondrial gene expression

Due to a deficiency in mitochondrial protein synthesis, Chinese hamster lung (CHL) cell mutant Gal- 32 does not grow in galactose or fructose. This report examines the nuclear or cytoplasmic inheritance of this single, recessive mutation. In a control experiment, fusion of Gal+TGSTK- cells with Gal- 32TGRTK+ cells resulted in tetraploid hybrids (as verified by karyotyping) that were selected in hypoxanthine/aminopterin/thymidine medium. The majority (2/3) of the control hybrids grew in galactose as expected since Gal+ is dominant over Gal-. Fusion of Rhodamine 6-G treated Gal+TGSTK- cells with Gal- 32TGRTK+ cells resulted in Rhodamine 6-G-tetraploid hybrids that were selected in hypoxanthine/aminopterin/thymidine medium. The majority (7/12) of the Rhodamine 6-G-hybrids grew in galactose as expected for a nuclearly encoded gene considering that Rhodamine 6-G interferes with transmission of mtDNA but not nuclear DNA. Therefore, these results are compelling in their demonstration of the nuclear origin of the Gal- 32 mutation.

Establishment and characterization of permanent human endothelial cell clones

Though vasculitic diseases have been claimed to be associated with anti-endothelial cells antibodies (AECA), there is a widespread awareness of the limitations of the tests currently in use. Our objective was therefore to establish clones, in the hope that some of them would express disease-specific membrane autoantigens. Two EC lines and 7 clones were established by fusing human umbilical vein EC with epithelial A549/8 cells, and cloning by limiting dilution. An additional clone was derived from the EA.hy 926 cell line. All clones carried EC markers, such as thrombomoduline (TM) and platelet-EC adhesion molecule 1 but differed from each other, depending on whether they expressed HLA class II antigen, LFA-1, thrombospondin receptor or von Willebrand factor (vWf) antigen. Clones were also characterized by their ability to release tissue plasminogen activator, interleukin 6, TM and vWf. This panel is meant to distinguish reactivities of AECA.

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

Treatment of mouse LMTK- cells with the toxic mitochondrial dye rhodamine 6G (R-6G) at 2.5 micrograms/ml for 7 days prevented cell growth while maintaining viability, with less than 10(-6) cells recovering to form colonies. Pre-treatment of LMTK- cells with R-6G was followed by fusion with enucleated mouse 501-1 cells harboring a homoplasmic point mutation in the mitochondrial DNA (mtDNA) 16S rRNA gene conferring chloramphenicol resistance (CAPR). Cybrids and any surviving unfused LMTK- cells were selected in BrdU with or without CAP and their mtDNAs screened for the presence of the CAPR marker. Approximately 1 colony per 2 x 10(5) LMTK- cells appeared in the fusion plates selected both with and without CAP. Most clones investigated were confirmed to be cybrids by showing the presence of the generally homoplasmic CAPR mutation, whether or not CAP selection was used. Hence, R-6G pre-treatment permits construction of transmitochondrial cybrid cell lines carrying a variety of mtDNAs, without the need for rho 0 cell lines.

Organellar segregation, rearrangement and recombination in protoplast fusion-derived Brassica oleracea calli

Cauliflower protoplasts were fused to determine the effect of protoplast source and pretreatment on organellar segregation in fusion products. Mitochondrial and chloroplast type were determined for over 250 calli from eight fusions between iodoacetate-treated or γ-irradiated leaf or hypocotyl protoplasts with fertile or Ogura cytoplasms. Organelles in fusion-derived calli were identified with five mitochondrial probes and one chloroplast probe. Mitochondrial and chloroplast segregation were independent but biased. Most calli had B. oleracea chloroplasts, but more calli had Ogura mitochondria than B. oleracea ones. Neither protoplast source nor pretreatment alone affected organelle segregation. However, iodoacetate treatment of hypocotyl protoplasts reduced their mitochondrial contribution to the fusion products although it did not affect chloroplast segregation. Over half of the calli had mitochondrial genomes distinct from those of either fusion partner; many of these contained the complete mitochondrial genome of one partner along with some mitochondrial DNA from the other. Out of 258 calli, 83 showed evidence of mitochondrial recombination, most commonly by formation of a novel 11-kb PstI fragment near the atp9 region.

On the tumorigenicity of mitochondrial DNA-depleted avian cells

We have examined the tumorigenic potential of mitochondrial DNA-depleted (mtDNA-) cells derived from the tumorigenic chicken cell line DU24. The mtDNA- cells were unable to proliferate in the wing web of day-old chicks. Cytoplasmic hybrids resulting from crosses between the mtDNA- whole cells and cytoplasts from enucleated parental cells (mtDNA+) recover both mtDNA and tumorigenicity. These results are in accordance with those obtained in prior experiments where mtDNA was shown to modulate the anchorage-independent phenotype of transformed avian cells.

Intravital fluorescence microscopy of endothelial cells on vascular grafts

The ability to evaluate the extent of initial endothelial cell coverage on a vascular graft subsequent to an endothelial cell seeding technique would be desirable to substantiate the durability of the endothelial cell lining. To this end, we have evaluated the use of intravital fluorescence microscopy to assess human endothelial cell interaction with vascular grafts. Five fluorescent stains, mithramycin, Hoechst 33342, sulfofluorescein diacetate, Nile red, and rhodamine 123 were evaluated for their ability to fluorescently label human endothelial cells. The staining capability of each dye was also evaluated with respect to accuracy in determining seeded cell number and cell spreading. Of the stains evaluated, rhodamine 123 produced the most desirable characteristics. We observed excellent cell visualization after a 30-min incubation. Unlike the other four stains, rhodamine 123 exhibited a bright orange fluorescence emission at a 510-nm excitation wavelength while the underlying dacron or expanded polytetrafluoroethylene demonstrated minimal autofluorescence. Rhodamine 123 also exhibited no inhibitory effect on cell attachment to plastic or subsequent cell growth in culture. Intravital fluorescence microscopy could be easily utilized in the operating room to visualize part or all of an endothelial cell-seeded graft prior to implantation and would permit a quantitative as well as qualitative evaluation of the seeding process. Since intravital fluorescence imaging does not require tissue fixation, the same surface as that evaluated for seeding efficiency can be directly implanted.

Cytoplasmic mediation of malignancy

The relative roles of nucleus and cytoplasm in the induction and maintenance of the malignant state were studied. Cytoplasmic hybrid (cybrid) clones, derived from the fusion of cytoplasts from malignantly transformed cells to normal whole cells, produced tumors in 17% of the animals injected with them. Nuclear/cytoplasmic hybrid (reconstituted cell) clones, derived by fusion of cytoplasts from malignant cells with karyoplasts of normal cells, produced tumors in 97% of the animals injected. A unique aspect of this study is the fact that all of the cells utilized, both normal and malignantly transformed, were derived from an original cloned cell.

Characterization of mouse nuclear and mitochondrial mutants with increased resistance to cytochrome b inhibitors

A series of mouse lines with increased resistance to respiratory inhibitors which block electron transport through the protonmotive cytochrome b of complex III have been isolated in this laboratory. We describe here the isolation of a mutant with increased resistance to HQNO (2-n-heptyl-4-hydroxyquinoline-N-oxide) whose phenotype is due to a nuclear mutation. At the cellular level, there is a severe reduction in respiration with the residual oxygen consumption being resistant to inhibitors of both ubiquinol-cytochrome c oxidoreductase and cytochrome oxidase. At the mitochondrial level, there was a severe derangement in NADH oxidase activity. Electron transport through the succinate oxidase span of the respiratory chain and its coupling to oxidative phosphorylation are also reduced in this nuclear mutant but not to the same extent. It is concluded that the primary defect in the mutant lies within a nuclear gene encoding a component of complex I (NADH-ubiquinol oxidoreductase). In addition, further biochemical characterization of the mitochondrially inherited inhibitor-resistant mutants has demonstrated that they also show significant reductions in the efficiency of energy transduction and in the rate of cytochrome b electron transport.

Mitochondrial genomes in intraspecies mammalian cell hybrids display codominant or dominant/recessive behavior

A unique type of nonstochastic mitochondrial DNA (mtDNA) segregation was found in mammalian cells. In human cell hybrids isolated from the fusion of HeLa cells with 23, GM639, A549, or 293 cells, HeLa mtDNA was always lost from the hybrids, whereas both parental mtDNAs were maintained in hybrids of HeLa X 143BTK-. Similar phenomena were observed in mouse cell hybrids isolated by the fusion of cells with different mtDNA types. Types 1, 2, and 3, can be distinguished from each other by restriction fragment-length polymorphisms. The mouse cell hybrids between cells with type 1 and type 2 mtDNA always lost type 2 mtDNA, whereas the hybrids between cells with type 2 and type 3 mtDNA retained both types stably. These observations suggest that either a codominant or a dominant/recessive relationship may be present in intraspecies mitochondrial genomes of human and mouse cells. When the mitochondrial genomes in cell hybrids are codominant, stochastic segregation occurs while nonstochastic segregation occurs when they are in the dominant/recessive relationship. These concepts may help elucidate organelle heredity in animals.

Does pretreatment by rhodamine 6-G affect the mitochondrial composition of fusion-derived Nicotiana cybrids?

Rhodamine-6G(R6G), a lipophilic dye which degrades mammalian mitochondria, was shown to arrest the division of Nicotiana protoplasts. When albino recipient-protoplasts were treated with R6G and fused with X-irradiated (green) donor- protoplasts, only green cybrid plants were obtained. The mtDNA of the cybrids was analyzed by Southern-blot hybridization. We found that cybrids which resulted from N. rustica (donor) protoplasts, fused with R6G-treated albino protoplasts, had only parental-type mtDNA. When another donor, with N. undulata mtDNA, was used, most of the resulting cybrids contained non-parental mtDNA. Only one cybrid (out of 12) had N. undulata -type (donor) mtDNA.

The presence of intact mitochondrial DNA in HeLa cell nuclei

Restriction analysis of DNA labelled with [32P]dCTP in an in vitro replication system with isolated nuclei from early S phase cells showed preferential labelling of restriction fragments derived from mitochondrial DNA (mtDNA) by a replication machinery distinct from that responsible for bulk nuclear DNA replication. Use of restriction nucleases with one recognition site in mtDNA gave rise to 16.5 kbp long fragments corresponding to full-length linearized mtDNA, indicating the presence of intact mtDNA in the isolated cell nuclei. Incorporation of dNTPs into mtDNA was not restricted to the S phase of the cell cycle. We were unable to increase the labelling of mtDNA by the addition of purified mitochondria or mtDNA to the nuclear replication system. These and other results presented is evidenced that the presence of mtDNA in the isolated nuclei was not due to uptake during preparation, thus indicating its presence in the cell nucleus in vivo.

Rhodamine 6G inhibits the matrix-catalyzed processing of precursors of rat-liver mitochondrial proteins

Several inner membrane proteins from rat liver mitochondria have been translated for the first time in rabbit reticulocyte lysates. These include the Rieske iron-sulfur protein, cytochrome c1 and core protein I of the cytochrome bc1 complex, the alpha and beta subunits of F1 ATPase, and subunit IV of cytochrome oxidase. All were translated from free polysomes as larger-molecular-mass precursors, and were processed to their mature forms by isolated liver mitochondria or by the isolated mitochondrial matrix fraction. In vitro processing, catalyzed by the isolated matrix fraction, is inhibited by rhodamine 6G. The latter is a fluorescent probe, which accumulates specifically in mitochondria of whole cells and which is used extensively to visualize mitochondrial morphology. The concentration of rhodamine 6G required for inhibition in vitro is similar to that of o-phenanthroline. Rhodamine 6G inhibits matrix-catalyzed processing of all precursors tested, indicating that the mechanism of inhibition is common for a variety of functionally unrelated precursors. The novel action of rhodamine 6G reported here can form the basis for its inhibition of precursor processing in intact hepatoma cells [Kolarov, J. & Nelson, B.D. (1984) Eur. J. Biochem. 144, 387-392].

Mitotic segregation of mitochondrial DNAs in human cell hybrids and expression of chloramphenicol resistance

The relationship between the chloramphenicol (CAP)-resistant phenotype and the mtDNA genotype was investigated in segregating human, HeLa X HT1080, somatic cell hybrids. The parental mtDNAs were quantitated in heteroplasmic cells by using restriction fragment length polymorphisms (RFLPs) detected in Southern blots. CAP-resistant (R) X CAP-sensitive (S) hybrids selected and grown in CAP for brief periods had as little as 25% CAP-R mtDNA. With prolonged selection, the CAP-R mtDNA increased to 90-95%. Hybrids selected and passaged without CAP either retained both mtDNAs or progressively lost one mtDNA (mitotic segregation). The CAP-resistance phenotype of these hybrids changed abruptly when the proportion of CAP-R mtDNAs fluctuated around approximately 10% (threshold effect). Hybrids with greater than 25% HT1080 mtDNA had an additional characteristic. They cloned better with CAP than without. The cloning efficiency in CAP of hybrids having 90% HT1080 mtDNA was more than fivefold greater than the control.

A rodent-human hybrid containing Xq24-qter translocated to a hamster chromosome expresses the Xq27 folate-sensitive fragile site

A somatic cell hybrid containing a single human X chromosome bearing the Xq27 fragile site was lethally irradiated and re-hybridized to its HPRT- Chinese hamster parent. One of 24 colonies surviving selection for HPRT was found to have retained human G6PD but not PGK. This line, X3000-11, which shows Xq24-qter translocated to a hamster chromosome by trypsin G-banding and a single human chromatin fragment corresponding to this segment of the X by G-11 staining, expresses the fragile site on exposure to 5-fluorodeoxyuridine. Dot blots using total human DNA suggest that X3000-11 retains approximately 0.2% of the human genome. By Southern blotting, X3000-11 retains Factor IX, DXS11 and DXS42 but lacks DXYS1, DXS3 and DXS17. This hybrid is being used to construct a cosmid library in the vector pCOS2 from which a sub-library of 500-1000 clones of human origin will be isolated using in vivo recombination with cloned Alu and Kpn family repeats. Such a sub-library will greatly facilitate chromosome walking to the fragile site as well as the testing of individual clones for their ability to create a folate-sensitive fragile site by DNA transfer into permissive Chinese hamster recipient cells.

Human cell variants resistant to rhodamine 6G

Two variants have been isolated from the cultured human cell line VA2-B which are resistant in vivo to the mitochondrial-specific fluorescent dyes rhodamine 6G and rhodamine 123. Both mutants are cross-resistant to ethidium bromide but are sensitive to both colchicine and chloramphenicol. When either mutant is treated with low levels of rhodamine 6G, mitochondrial-associated fluorescence is significantly lower than in wild-type cells. Furthermore, when cell cultures are treated with high concentrations of either rhodamine 123 or 6G, and then washed free of the dye, mitochondrial-associated rhodamine fluorescence rapidly diminishes in the parental cell line. In hybrid cell fusions between resistant and sensitive cell lines, rhodamine resistance is gradually expressed, reaching maximal expression approximately 11 days after fusion. Cytoplasmic transmission of rhodamine resistance has not been clearly demonstrated in cytoplast-cell fusions, and thus resistance is probably due to a mutation of a nuclear, rather than mitochondrial DNA gene(s). These observations indicate that mitochondria of both rhodamine-resistant variants, unlike wild type, have a significantly decreased ability to bind and retain rhodamine, and thus their mitochondrial tramsmembrane electrical potential may be significantly reduced.

Mitochondrial modulation of maternally transmitted antigen: analysis of cell hybrids

Maternally transmitted antigen (Mta) is a murine cell surface class I-like antigen that is defined by specific cytotoxic lymphocyte reactivity. Mta is unique in that its expression requires cooperation between genetic elements both in the Qa/Tla region of chromosome 17 and in the cytoplasm. In view of the known cytoplasmic, and thus maternal, inheritance of mitochondria, we have directly assessed their potential involvement in Mta expression. The mitochondria-specific lethal dye rhodamine 6G (R6G) was used to control the input of mitochondria into cell hybrids. The parental lines, one of BALB/c and one of NZB origin, were known to differ in Mta and mtDNA phenotype. Our data show that most control BALB/c-NZB hybrids expressed the BALB/c Mta phenotype and likewise contained only BALB/c-type mtDNA. The NZB Mta phenotype was not coexpressed in the control hybrids. However, when the mitochondrial contribution from BALB/c was prevented by R6G treatment, the majority of the resultant hybrids expressed only the NZB Mta type and likewise contained only NZB mtDNA. The exceptional R6G-treated hybrids that continued to express the BALB/c Mta phenotype likewise contained only BALB/c mtDNA. Thus, in every case the mtDNA phenotype correlated with the Mta phenotype of the cells. Together, the data support the remarkable conclusion that mitochondria modulate the phenotypic expression of a cell surface molecule.

Segregation of mitochondrial DNA in human somatic cell hybrids

The maintenance of mtDNA has been examined in human intraspecific hybrid cells constructed from the fusion of HEB7A, a HeLa tumor cell line carrying the mitochondrially coded chloramphenical (CAP) resistance mutation, and GM 2291, a limited lifespan human diploid fibroblast which is CAP sensitive. These two cells can be distinguished by a polymorphism in a site for the restriction endonuclease, HaeIII. Independently isolated clones of hybrid cells were characterized for their growth properties (either normal limited lifespan or transformed and "immortal"). Whole cell DNA preparations were made from each hybrid, digested with HaeIII, and the resultant fragments were detected by hybridization to 32P labelled mouse mtDNA as probe. Experiments with mixtures of HEB7A and GM 2291 DNA reveal that HEB7A mtDNA can be detected when it constitutes as little as 5% of the total cell mtDNA. The results indicate that the HEB7A mtDNA is lost from most hybrids, and when it does persist it is usually a minor component of total mtDNA. The addition of CAP at the time of fusion slightly increases the quantity of HEB7A mtDNA, but not enough to confer CAP resistance. Furthermore, five limited lifespan hybrids contained no detectable HEB7A mtDNA, while three transformed hybrids contained varying quantities of HEB7A mtDNA, suggesting that retention of this tumor form of mtDNA is associated with tumor growth behavior. These results suggest that cytoplasmic genetic incompatibility occurs in intraspecific hybrids.