Aberrant nucleo-cytoplasmic cross-talk results in donor cell mtDNA persistence in cloned embryos

The role of mtDNA in oocyte quality and embryo development

The mitochondrial genome resides in the mitochondria present in nearly all cell types. The porcine (Sus scrofa) mitochondrial genome is circa 16.7 kb in size and exists in the multimeric format in cells. Individual cell types have different numbers of mitochondrial DNA (mtDNA) copy number based on their requirements for ATP produced by oxidative phosphorylation. The oocyte has the largest number of mtDNA of any cell type. During oogenesis, the oocyte sets mtDNA copy number in order that sufficient copies are available to support subsequent developmental events. It also initiates a program of epigenetic patterning that regulates, for example, DNA methylation levels of the nuclear genome. Once fertilized, the nuclear and mitochondrial genomes establish synchrony to ensure that the embryo and fetus can complete each developmental milestone. However, altering the oocyte's mtDNA copy number by mitochondrial supplementation can affect the programming and gene expression profiles of the developing embryo and, in oocytes deficient of mtDNA, it appears to have a positive impact on the embryo development rates and gene expression profiles. Furthermore, mtDNA haplotypes, which define common maternal origins, appear to affect developmental outcomes and certain reproductive traits. Nevertheless, the manipulation of the mitochondrial content of an oocyte might have a developmental advantage.

Mitochondrial Inheritance Following Nuclear Transfer: From Cloned Animals to Patients with Mitochondrial Disease

Mitochondria are indispensable power plants of eukaryotic cells that also act as a major biochemical hub. As such, mitochondrial dysfunction, which can originate from mutations in the mitochondrial genome (mtDNA), may impair organism fitness and lead to severe diseases in humans. MtDNA is a multi-copy, highly polymorphic genome that is uniparentally transmitted through the maternal line. Several mechanisms act in the germline to counteract heteroplasmy (i.e., coexistence of two or more mtDNA variants) and prevent expansion of mtDNA mutations. However, reproductive biotechnologies such as cloning by nuclear transfer can disrupt mtDNA inheritance, resulting in new genetic combinations that may be unstable and have physiological consequences. Here, we review the current understanding of mitochondrial inheritance, with emphasis on its pattern in animals and human embryos generated by nuclear transfer.

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.

Genomic Balance: Two Genomes Establishing Synchrony to Modulate Cellular Fate and Function

It is becoming increasingly apparent that cells require cooperation between the nuclear and mitochondrial genomes to promote effective function. However, it was long thought that the mitochondrial genome was under the strict control of the nuclear genome and the mitochondrial genome had little influence on cell fate unless it was extensively mutated, as in the case of the mitochondrial DNA diseases. However, as our understanding of the roles that epigenetic regulators, including DNA methylation, and metabolism play in cell fate and function, the role of the mitochondrial genome appears to have a greater influence than previously thought. In this review, I draw on examples from tumorigenesis, stem cells, and oocyte pre- and post-fertilisation events to discuss how modulating one genome affects the other and that this results in a compromise to produce functional mature cells. I propose that, during development, both of the genomes interact with each other through intermediaries to establish genomic balance and that establishing genomic balance is a key facet in determining cell fate and viability.

The N-space Episenome unifies cellular information space-time within cognition-based evolution

Self-referential cellular homeostasis is maintained by the measured assessment of both internal status and external conditions based within an integrated cellular information field. This cellular field attachment to biologic information space-time coordinates environmental inputs by connecting the cellular senome, as the sum of the sensory experiences of the cell, with its genome and epigenome. In multicellular organisms, individual cellular information fields aggregate into a collective information architectural matrix, termed a N-space Episenome, that enables mutualized organism-wide information management. It is hypothesized that biological organization represents a dual heritable system constituted by both its biological materiality and a conjoining N-space Episenome. It is further proposed that morphogenesis derives from reciprocations between these inter-related facets to yield coordinated multicellular growth and development. The N-space Episenome is conceived as a whole cell informational projection that is heritable, transferable via cell division and essential for the synchronous integration of the diverse self-referential cells that constitute holobionts.

Reducing the cytoplasmic volume during hand-made cloning adversely affects the developmental competence and quality, and alters relative abundance of mRNA transcripts and epigenetic status of buffalo (Bubalus bubalis) embryos

Hand-made cloning (HMC) is a method of choice for somatic cell nuclear transfer (SCNT). There is 20% to 50% of cytoplasm lost during manual enucleation of oocytes with HMC. To compensate, two enucleated demicytoplasts, instead of one, are fused with each donor cell, which leads to cytoplasm pooling from two different demicytoplasts. In this study, effects of using one, instead of two demicytoplasts (controls) was examined, for production of embryos using HMC. Use of one demicytoplast decreased blastocyst development (12.7 ± 1.98% compared with 47.6 ± 3.49%, P < 0.001), total cell number (TCN, 167.6 ± 14.66 compared with 335.9 ± 58.96, P < 0.01), apoptotic index (2.11 ± 0.38 compared with 3.43±0.38, P < 0.05) but did not significantly alter inner cell mass:trophectoderm cell number ratio (0.17 ± 0.01 compared with 0.19 ± 0.02) and the global content of H3K9ac and H3K27me3 of blastocysts, compared to controls. There were gene expression alterations in pluripotency- (SOX2 and NANOG but not OCT4), epigenetic- (DNMT1 but not DNMT3a and HDAC1), apoptosis- (CASPASE3 but not BCL-2 and BAX), trophectoderm- (CDX2), development- (G6PD but not GLUT1) and cell cycle check point control-related related genes (P53) compared with controls. Transfer of cloned blastocysts from one demicytoplast (n = 8) to recipients resulted in a live calf birth that after 12 days died whereas, with transfer of control blastocysts (n = 14) there was birth of a healthy calf. In conclusion, use of one, instead of two demicytoplasts for HMC, compromises in vitro developmental competence, and alters expression of several important genes affecting embryo development.

Transmission of Dysfunctional Mitochondrial DNA and Its Implications for Mammalian Reproduction

Mitochondrial DNA (mtDNA) encodes proteins for the electron transport chain which produces the vast majority of cellular energy. MtDNA has its own replication and transcription machinery that relies on nuclear-encoded transcription and replication factors. MtDNA is inherited in a non-Mendelian fashion as maternal-only mtDNA is passed onto the next generation. Mutation to mtDNA can cause mitochondrial dysfunction, which affects energy production and tissue and organ function. In somatic cell nuclear transfer (SCNT), there is an issue with the mixing of two populations of mtDNA, namely from the donor cell and recipient oocyte. This review focuses on the transmission of mtDNA in SCNT embryos and offspring. The transmission of donor cell mtDNA can be prevented by depleting the donor cell of its mtDNA using mtDNA depletion agents prior to SCNT. As a result, SCNT embryos harbour oocyte-only mtDNA. Moreover, culturing SCNT embryos derived from mtDNA depleted cells in media supplemented with a nuclear reprograming agent can increase the levels of expression of genes related to embryo development when compared with non-depleted cell-derived embryos. Furthermore, we have reviewed how mitochondrial supplementation in oocytes can have beneficial effects for SCNT embryos by increasing mtDNA copy number and the levels of expression of genes involved in energy production and decreasing the levels of expression of genes involved in embryonic cell death. Notably, there are beneficial effects of mtDNA supplementation over the use of nuclear reprograming agents in terms of regulating gene expression in embryos. Taken together, manipulating mtDNA in donor cells and/or oocytes prior to SCNT could enhance embryo production efficiency.

Additional mitochondrial DNA influences the interactions between the nuclear and mitochondrial genomes in a bovine embryo model of nuclear transfer

We generated cattle embryos using mitochondrial supplementation and somatic cell nuclear transfer (SCNT), named miNT, to determine how additional mitochondrial DNA (mtDNA) modulates the nuclear genome. To eliminate any confounding effects from somatic cell mtDNA in intraspecies SCNT, donor cell mtDNA was depleted prior to embryo production. Additional oocyte mtDNA did not affect embryo development rates but increased mtDNA copy number in blastocyst stage embryos. Moreover, miNT-derived blastocysts had different gene expression profiles when compared with SCNT-derived blastocysts. Additional mtDNA increased expression levels of genes involved in oxidative phosphorylation, cell cycle and DNA repair. Supplementing the embryo culture media with a histone deacetylase inhibitor, Trichostatin A (TSA), had no beneficial effects on the development of miNT-derived embryos, unlike SCNT-derived embryos. When compared with SCNT-derived blastocysts cultured in the presence of TSA, additional mtDNA alone had beneficial effects as the activity of glycolysis may increase and embryonic cell death may decrease. However, these beneficial effects were not found with additional mtDNA and TSA together, suggesting that additional mtDNA alone enhances reprogramming. In conclusion, additional mtDNA increased mtDNA copy number and expression levels of genes involved in energy production and embryo development in blastocyst stage embryos emphasising the importance of nuclear-mitochondrial interactions.

The role of mitochondria in the female germline: Implications to fertility and inheritance of mitochondrial diseases

Mitochondria play a fundamental role during development of the female germline. They are fragmented, round, and small. Despite these characteristics suggesting that they are inactive, there is accumulating evidence that mitochondrial dysfunctions are a major cause of infertility and generation of aneuploidies in humans. In addition, mitochondria and their own genomes (mitochondrial DNA-mtDNA) may become damaged with time, which might be one reason why aging leads to infertility. As a result, mitochondria have been proposed as an important target for evaluating oocyte and embryo quality, and developing treatments for female infertility. On the other hand, mutations in mtDNA may cause mitochondrial dysfunctions, leading to severe diseases that affect 1 in 4,300 people. Moreover, very low levels of mutated mtDNA seem to be present in every person worldwide. These may increase with time and associate with late-onset degenerative diseases such as Parkinson disease, Alzheimer disease, and common cancers. Mutations in mtDNA are transmitted down the maternal lineage, following a poorly understood pattern of inheritance. Recent findings have indicated existence in the female germline of a purifying filter against deleterious mtDNA variants. Although the underlying mechanism of this filter is largely unknown, it has been suggested to rely on autophagic degradation of dysfunctional mitochondria or selective replication/transmission of non-deleterious variants. Thus, understanding the mechanisms regulating mitochondrial inheritance is important both to improve diagnosis and develop therapeutic tools for preventing transmission of mtDNA-encoded diseases.

Study of mitochondrial respiratory defects on reprogramming to human induced pluripotent stem cells

Reprogramming of somatic cells into a pluripotent state is known to be accompanied by extensive restructuring of mitochondria and switch in metabolic requirements. Here we utilized Leber's hereditary optic neuropathy (LHON) as a mitochondrial disease model to study the effects of homoplasmic mtDNA mutations and subsequent oxidative phosphorylation (OXPHOS) defects in reprogramming. We obtained fibroblasts from a total of 6 LHON patients and control subjects, and showed a significant defect in complex I respiration in LHON fibroblasts by high-resolution respiratory analysis. Using episomal vector reprogramming, our results indicated that human induced pluripotent stem cell (hiPSC) generation is feasible in LHON fibroblasts. In particular, LHON-specific OXPHOS defects in fibroblasts only caused a mild reduction and did not significantly affect reprogramming efficiency, suggesting that hiPSC reprogramming can tolerate a certain degree of OXPHOS defects. Our results highlighted the induction of genes involved in mitochondrial biogenesis (TFAM, NRF1), mitochondrial fusion (MFN1, MFN2) and glycine production (GCAT) during reprogramming. However, LHON-associated OXPHOS defects did not alter the kinetics or expression levels of these genes during reprogramming. Together, our study provides new insights into the effects of mtDNA mutation and OXPHOS defects in reprogramming and genes associated with various aspects of mitochondrial biology.

Manipulating the Mitochondrial Genome To Enhance Cattle Embryo Development

The mixing of mitochondrial DNA (mtDNA) from the donor cell and the recipient oocyte in embryos and offspring derived from somatic cell nuclear transfer (SCNT) compromises genetic integrity and affects embryo development. We set out to generate SCNT embryos that inherited their mtDNA from the recipient oocyte only, as is the case following natural conception. While SCNT blastocysts produced from Holstein (Bos taurus) fibroblasts were depleted of their mtDNA, and oocytes derived from Angus (Bos taurus) cattle possessed oocyte mtDNA only, the coexistence of donor cell and oocyte mtDNA resulted in blastocysts derived from nondepleted cells. Moreover, the use of the reprogramming agent, Trichostatin A (TSA), further improved the development of embryos derived from depleted cells. RNA-seq analysis highlighted 35 differentially expressed genes from the comparison between blastocysts generated from nondepleted cells and blastocysts from depleted cells, both in the presence of TSA. The only differences between these two sets of embryos were the presence of donor cell mtDNA, and a significantly higher mtDNA copy number for embryos derived from nondepleted cells. Furthermore, the use of TSA on embryos derived from depleted cells positively modulated the expression of CLDN8, TMEM38A, and FREM1, which affect embryonic development. In conclusion, SCNT embryos produced by mtDNA depleted donor cells have the same potential to develop to the blastocyst stage without the presumed damaging effect resulting from the mixture of donor and recipient mtDNA.

The role of mitochondrial DNA copy number, variants, and haplotypes in farm animal developmental outcome

The vast majority of cellular energy is generated through the process of oxidative phosphorylation, which takes place in the electron transport chain in the mitochondria. The electron transport chain is encoded by 2 genomes, the chromosomal and the mitochondrial genomes. Mitochondrial DNA is associated with a number of traits, which include tolerance to heat, growth and physical performance, meat and milk quality, and fertility. Mitochondrial genomes can be clustered into groups known as mtDNA haplotypes. Mitochondrial DNA haplotypes are a potential genetic source for manipulating phenotypes in farm animals. The use of assisted reproductive technologies, such as nuclear transfer, allows favorable chromosomal genetic traits to be mixed and matched with sought after mtDNA haplotype traits. As a result super breeds can be generated.

Revisiting Mitochondrial Function and Metabolism in Pluripotent Stem Cells: Where Do We Stand in Neurological Diseases?

Pluripotent stem cells (PSCs) are powerful cellular tools that can generate all the different cell types of the body, and thus overcome the often limited access to human disease tissues; this becomes highly relevant when aiming to investigate cellular (dys)function in diseases affecting the central nervous system. Recent studies have demonstrated that PSC and differentiated cells show altered mitochondrial function and metabolic profiles and production of reactive oxygen species. This raises an emerging paradigm about the role of mitochondria in stem cell biology and urges the need to identify mitochondrial pathways involved in these processes. In this respect, this review focuses on the metabolic profile of PSC and how mitochondrial function can influence the reprogramming and differentiation processes. Indeed, both embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) favor the glycolytic pathway as a major source of energy production over oxidative phosphorylation. PSC mitochondria are characterized by a spherical shape, low copy number of mitochondrial DNA, and a hyperpolarized state. Indeed, mitochondria appear to have a crucial role in reprogramming iPSC, in the maintenance of a pluripotent state, and in differentiation. Moreover, an increase in mitochondrial oxidative phosphorylation has to occur for differentiation to succeed. Therefore, in vitro differentiation of neural stem cells (NSCs) into neurons can be compromised if those mechanisms are impaired. Future research should shed light on how mitochondrial impairment occurring in pre differentiation neural stages (e.g., in NSC or premature neurons) may contribute for the etiopathogenesis of neurodevelopmental and neurological disorders.

Mitochondrial DNA copy number and replication in reprogramming and differentiation

Until recently, it was thought that the role of the mitochondrial genome was confined to encoding key proteins that generate ATP through the process of oxidative phosphorylation in the electron transfer chain. However, with increasing new evidence, it is apparent that the mitochondrial genome has a major role to play in a number of diseases and phenotypes. For example, mitochondrial variants and copy number have been implicated in the processes of fertilisation outcome and development and the onset of tumorigenesis. On the other hand, mitochondrial DNA (mtDNA) haplotypes have been implicated in a variety of diseases and most likely account for the adaptation that our ancestors achieved in order that they were fit for their environments. The mechanisms, which enable the mitochondrial genome to either protect or promote the disease phenotype, require further elucidation. However, there appears to be significant 'crosstalk' between the chromosomal and mitochondrial genomes that enable this to take place. One such mechanism is the regulation of DNA methylation by mitochondrial DNA, which is often perturbed in reprogrammed cells that have undergone dedifferentiation and affects mitochondrial DNA copy number. Furthermore, it appears that the mitochondrial genome interacts with the chromosomal genome to regulate the transcription of key genes at certain stages during development. Additionally, the mitochondrial genome can accumulate a series of mtDNA variants, which can lead to diseases such as cancer. It is likely that a combination of certain mitochondrial variants and aberrant patterns of mtDNA copy number could indeed account for many diseases that have previously been unaccounted for. This review focuses on the role that the mitochondrial genome plays especially during early stages of development and in cancer.

Segregation of Naturally Occurring Mitochondrial DNA Variants in a Mini-Pig Model

The maternally inherited mitochondrial genome (mtDNA) is present in multimeric form within cells and harbors sequence variants (heteroplasmy). While a single mtDNA variant at high load can cause disease, naturally occurring variants likely persist at low levels across generations of healthy populations. To determine how naturally occurring variants are segregated and transmitted, we generated a mini-pig model, which originates from the same maternal ancestor. Following next-generation sequencing, we identified a series of low-level mtDNA variants in blood samples from the female founder and her daughters. Four variants, ranging from 3% to 20%, were selected for validation by high-resolution melting analysis in 12 tissues from 31 animals across three generations. All four variants were maintained in the offspring, but variant load fluctuated significantly across the generations in several tissues, with sex-specific differences in heart and liver. Moreover, variant load was persistently reduced in high-respiratory organs (heart, brain, diaphragm, and muscle), which correlated significantly with higher mtDNA copy number. However, oocytes showed increased heterogeneity in variant load, which correlated with increased mtDNA copy number during in vitro maturation. Altogether, these outcomes show that naturally occurring mtDNA variants segregate and are maintained in a tissue-specific manner across generations. This segregation likely involves the maintenance of selective mtDNA variants during organogenesis, which can be differentially regulated in oocytes and preimplantation embryos during maturation.

Developmental competence of 8?16-cell stage bison embryos produced by interspecies somatic cell nuclear transfer

Altered communication between nuclear and cytoplasmic components has been linked to impaired development in interspecies somatic cell nuclear transfer (iSCNT) embryos as a result of genetic divergence between the two species. This study investigated the developmental potential and mitochondrial function of cattle (Bos taurus), plains bison (Bison bison bison) and wood bison (Bison bison athabascae) embryos produced by iSCNT using domestic cattle oocytes as cytoplasts. Embryos in all groups were analysed for development, accumulation of ATP, apoptosis and gene expression of nuclear- and mitochondrial-encoded genes at the 8-16-cell stage. The results of this study showed no significant differences in the proportion of developed embryos at the 2-, 4- and 8-16-cell stages between groups. However, significantly higher ATP levels were observed in cattle SCNT embryos compared with bison iSCNT embryos. Significantly more condensed and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling (TUNEL)-positive nuclei were found in plains bison iSCNT embryos. No significant differences in the expression levels of nuclear respiratory factor 2 (NRF2) or mitochondrial subunit 2 of cytochrome c oxidase (mt-COX2) were found in any of the groups. However, mitochondrial transcription factor A (TFAM) expression significantly differed between groups. The results of this study provide insights into the potential causes that might lead to embryonic arrest in bison iSCNT embryos, including mitochondrial dysfunction, increased apoptosis and abnormal gene expression.

The carp-goldfish nucleocytoplasmic hybrid has mitochondria from the carp as the nuclear donor species

It is widely accepted that mitochondria and its DNA (mtDNA) exhibit strict maternal inheritance, with sperm contributing no or non-detectable mitochondria to the next generation. In fish, nuclear transfer (NT) through the combination of a donor nucleus and an enucleated oocyte can produce fertile nucleocytoplasmic hybrids (NCHs) even between different genera and subfamilies. One of the best studied fish NCHs is CyCa produced by transplanting the nuclei plus cytoplasm from the common carp (Cyprinus carpio var. wuyuanensis) into the oocytes of the wild goldfish (Carassius auratus), which has been propagated by self-mating for three generations. These NCH fish thus provide a unique model to study the origin of mitochondria. Here we report the complete mtDNA sequence of the CyCa hybrid and its parental species carp and goldfish as nuclear donor and cytoplasm host, respectively. Interestingly, the mtDNA of NCH fish CyCa is 99.69% identical to the nuclear donor species carp, and 89.25% identical to the oocyte host species goldfish. Furthermore, an amino acid sequence comparison of 13 mitochondrial proteins reveals that CyCa is 99.68% identical to the carp and 87.68% identical to the goldfish. On an mtDNA-based phylogenetic tree, CyCa is clustered with the carp but separated from the goldfish. A real-time PCR analysis revealed the presence of carp mtDNA but the absence of goldfish mtDNA. These results demonstrate--for the first time to our knowledge--that the mtDNA of a NCH such as CyCa fish may originate from its nuclear donor rather than its oocyte host.

The control of mtDNA replication during differentiation and development

BACKGROUND

Mitochondrial DNA (mtDNA) is important for energy production as it encodes some of the key genes of electron transfer chain, where the majority of cellular energy is generated through oxidative phosphorylation (OXPHOS). MtDNA replication is mediated by nuclear DNA-encoded proteins or enzymes, which translocate to the mitochondria, and is strictly regulated throughout development. It starts with approximately 200 copies in each primordial germ cell and these copies undergo expansion and restriction events at various stages of development.

SCOPE OF REVIEW

I describe the patterns of mtDNA replication at key stages of development. I explain that it is essential to regulate mtDNA copy number and to establish the mtDNA set point in order that the mature, specialised cell acquires the appropriate numbers of mtDNA copy to generate sufficient adenosine triphosphate (ATP) through OXPHOS to undertake its specialised function. I discuss how these processes are dependent on the controlled expression of the nuclear-encoded mtDNA-specific replication factors and that this can be modulated by mtDNA haplotypes. I discuss how these events are altered by certain assisted reproductive technologies, some of which have been proposed to prevent the transmission of mutant mtDNA and others to overcome infertility. Furthermore, some of these technologies are predisposed to transmitting two or more populations of mtDNA, which can be extremely harmful.

MAJOR CONCLUSIONS

The failure to regulate mtDNA replication and mtDNA transmission during development is disadvantageous.

GENERAL SIGNIFICANCE

Manipulation of oocytes and embryos can lead to significant implications for the maternal-only transmission of mtDNA. This article is part of a Special Issue entitled Frontiers of mitochondrial research.

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.

Mitochondrial DNA copy number is regulated in a tissue specific manner by DNA methylation of the nuclear-encoded DNA polymerase gamma A

DNA methylation is an essential mechanism controlling gene expression during differentiation and development. We investigated the epigenetic regulation of the nuclear-encoded, mitochondrial DNA (mtDNA) polymerase γ catalytic subunit (PolgA) by examining the methylation status of a CpG island within exon 2 of PolgA. Bisulphite sequencing identified low methylation levels (<10%) within exon 2 of mouse oocytes, blastocysts and embryonic stem cells (ESCs), while somatic tissues contained significantly higher levels (>40%). In contrast, induced pluripotent stem (iPS) cells and somatic nuclear transfer ESCs were hypermethylated (>20%), indicating abnormal epigenetic reprogramming. Real time PCR analysis of 5-methylcytosine (5mC) and 5-hydroxymethylcytosine (5hmC) immunoprecipitated DNA suggests active DNA methylation and demethylation within exon 2 of PolgA. Moreover, neural differentiation of ESCs promoted de novo methylation and demethylation at the exon 2 locus. Regression analysis demonstrates that cell-specific PolgA expression levels were negatively correlated with DNA methylation within exon 2 and mtDNA copy number. Finally, using chromatin immunoprecipitation (ChIP) against RNA polymerase II (RNApII) phosphorylated on serine 2, we show increased DNA methylation levels are associated with reduced RNApII transcriptional elongation. This is the first study linking nuclear DNA epigenetic regulation with mtDNA regulation during differentiation and cell specialization.

Improvement of the nuclear transfer efficiency by using the same genetic background of recipient oocytes as the somatic donor cells in goats

We have compared the effect of the genetic background of recipient oocytes on the in vitro and in vivo development of nuclear transfer reconstructed embryos in goats. Adult fibroblast cells from Boer goats were used as donor cells, and recipient oocytes were obtained from Boer goats and Boer cross-breeds (Boer♂×Huanghuai♀). Nuclear transfer reconstructed embryos were cultured in vitro, or transferred into recipient goats. The mitochondrial origin of 2 cloned Boer goats was investigated by analysing the D-loop region based on polymorphisms via DNA sequencing. There was no significant difference in the fusion rate and cleavage rate of reconstructed embryos (P>0.05), when using Boer and cross-breeding goat oocytes as recipient cytoplast respectively. However, in vitro morula development of reconstructed embryos from Boer oocytes was significantly higher than that of cross-breeding embryos (34.1% versus 19.1%, P<0.05). There was no significant difference in the rate of pregnancy and foetus loss between the 2 breeds. However, the live-birth rate was significantly higher with Boer goat oocyte recipients than the cross-breeds (3.1% versus 0.8%, P<0.05). Mitochondrial analysis showed that the 2 cloned goats were similar to their respective oocyte donor goats, and significantly different from the nucleus donor. In conclusion, genetic background of recipient oocytes affected in vitro and in vivo development of reconstructed embryos, with the homologous background of cytoplast and nuclear donor benefiting development of reconstructed embryos. The mitochondrial origin of the 2 cloned Boer goats came from recipient oocytes, not donors.

High levels of mitochondrial heteroplasmy modify the development of ovine-bovine interspecies nuclear transferred embryos

To investigate the effect of mitochondrial heteroplasmy on embryo development, cloned embryos produced using bovine oocytes as the recipient cytoplasm and ovine granulosa cells as the donor nuclei were complemented with 2pL mitochondrial suspension isolated from ovine (BOOMT embryos) or bovine (BOBMT embryos) granulosa cells; cloned embryos without mitochondrial injection served as the control group (BO embryos). Reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and sodium bisulfite genomic sequencing were used to analyse mRNA and methylation levels of pluripotency genes (OCT4, SOX2) and mitochondrial genes (TFAM, POLRMT) in the early developmental stages of cloned embryos. The number of mitochondrial DNA copies in 2pL ovine-derived and bovine-derived mitochondrial suspensions was 960±110 and 1000±120, respectively. The blastocyst formation rates were similar in BOBMT and BO embryos (P>0.05), but significantly higher than in BOOMT embryos (P<0.01). Expression of OCT4 and SOX2, as detected by RT-qPCR, decreased significantly in BOOMT embryos (P<0.05), whereas the expression of TFAM and POLRMT increased significantly, compared with expression in BOOMT and BO embryos (P<0.05). In addition, methylation levels of OCT4 and SOX2 were significantly greater (P<0.05), whereas those of TFAM and POLRMT were significantly lower (P<0.01), in BOOMT embryos compared with BOBMT and BO embryos. Together, the results of the present study suggest that the degree of mitochondrial heteroplasmy may affect embryonic development.

Mitochondrial physiology and gene expression analyses reveal metabolic and translational dysregulation in oocyte-induced somatic nuclear reprogramming

While reprogramming a foreign nucleus after somatic cell nuclear transfer (SCNT), the enucleated oocyte (ooplasm) must signal that biomass and cellular requirements changed compared to the nucleus donor cell. Using cells expressing nuclear-encoded but mitochondria-targeted EGFP, a strategy was developed to directly distinguish maternal and embryonic products, testing ooplasm demands on transcriptional and post-transcriptional activity during reprogramming. Specifically, we compared transcript and protein levels for EGFP and other products in pre-implantation SCNT embryos, side-by-side to fertilized controls (embryos produced from the same oocyte pool, by intracytoplasmic injection of sperm containing the EGFP transgene). We observed that while EGFP transcript abundance is not different, protein levels are significantly lower in SCNT compared to fertilized blastocysts. This was not observed for Gapdh and Actb, whose protein reflected mRNA. This transcript-protein relationship indicates that the somatic nucleus can keep up with ooplasm transcript demands, whilst transcription and translation mismatch occurs after SCNT for certain mRNAs. We further detected metabolic disturbances after SCNT, suggesting a place among forces regulating post-transcriptional changes during reprogramming. Our observations ascribe oocyte-induced reprogramming with previously unsuspected regulatory dimensions, in that presence of functional proteins may no longer be inferred from mRNA, but rather depend on post-transcriptional regulation possibly modulated through metabolism.

High expression of Mfn1 promotes early development of bovine SCNT embryos: improvement of mitochondrial membrane potential and oxidative metabolism

Mitofusin 1 (Mfn1) is the main mediator of mitochondrial fusion and homeostasis. To determine whether increased Mfn1 expression level could promote the fusion of heteroplasmic mitochondria and development of somatic cell nuclear transfer (SCNT) embryos. Embryos were constructed using bovine oocytes as recipient cytoplasm, and Holstein cow fetal fibroblasts with different expression levels of Mfn1 gene as donor nuclei. Mitochondrial membrane potential, ATP and H(2)O(2) generation, as well as the expression level of Mfn1 were detected in different development stages. The results showed that high level of Mfn1 expression significantly improved the embryo development rates by increasing ATP level and Δψm, while reducing H(2)O(2) generation. This study suggests that overexpression of Mfn1 could promote the early development of bovine SCNT embryos via improving oxidative phosphorylation.

John JC. Interspecies somatic cell nuclear transfer is dependent on compatible mitochondrial DNA and reprogramming factors

Interspecies somatic cell nuclear transfer (iSCNT) involves the transfer of a nucleus or cell from one species into the cytoplasm of an enucleated oocyte from another. Once activated, reconstructed oocytes can be cultured in vitro to blastocyst, the final stage of preimplantation development. However, they often arrest during the early stages of preimplantation development; fail to reprogramme the somatic nucleus; and eliminate the accompanying donor cell's mitochondrial DNA (mtDNA) in favour of the recipient oocyte's genetically more divergent population. This last point has consequences for the production of ATP by the electron transfer chain, which is encoded by nuclear and mtDNA. Using a murine-porcine interspecies model, we investigated the importance of nuclear-cytoplasmic compatibility on successful development. Initially, we transferred murine fetal fibroblasts into enucleated porcine oocytes, which resulted in extremely low blastocyst rates (0.48%); and failure to replicate nuclear DNA and express Oct-4, the key marker of reprogramming. Using allele specific-PCR, we detected peak levels of murine mtDNA at 0.14±0.055% of total mtDNA at the 2-cell embryo stage and then at ever-decreasing levels to the blastocyst stage (<0.001%). Furthermore, these embryos had an overall mtDNA profile similar to porcine embryos. We then depleted porcine oocytes of their mtDNA using 10 µM 2′,3′-dideoxycytidine and transferred murine somatic cells along with murine embryonic stem cell extract, which expressed key pluripotent genes associated with reprogramming and contained mitochondria, into these oocytes. Blastocyst rates increased significantly (3.38%) compared to embryos generated from non-supplemented oocytes (P<0.01). They also had significantly more murine mtDNA at the 2-cell stage than the non-supplemented embryos, which was maintained throughout early preimplantation development. At later stages, these embryos possessed 49.99±2.97% murine mtDNA. They also exhibited an mtDNA profile similar to murine preimplantation embryos. Overall, these data demonstrate that the addition of species compatible mtDNA and reprogramming factors improves developmental outcomes for iSCNT embryos.

The four reprogramming factors and embryonic development in mice

The transcription factors (Oct4, Sox2, c-Myc, and Klf4) play an important role in the generation of induced pluripotent stem cells. These factors are expressed in metaphase II oocytes and embryonic stem cells (ESCs). The mechanisms responsible for the reprogramming of ooplasm during nuclear transfer are expected to be associated with the four factors. Here, we show that different paternal genetic backgrounds are able to influence the in vitro development of parthenogenetic and cloned embryos. Using real- time polymerase chain reaction (PCR) we found that the expression level of Oct4 in oocytes was less than that of ESCs, whereas oocytes from KM x C3H females showed the highest expression level of Sox2 than the other strains tested or in G1 ESCs. c-Myc mRNA levels in oocytes from KM mice were greater than those found in ESCs or oocytes of KM x C3H mice. These data demonstrate that the expression of the four transcription factors was different among the oocytes, which may be a contributing factor for the different efficiencies of parthenogenesis and the development of cloned embryos in vitro.

Generation of mtDNA homoplasmic cloned lambs

Generally in mammals, individual animals contain only maternally inherited mitochondrial DNA (mtDNA), as paternal (sperm)-derived mitochondria are usually eliminated during early development. Somatic cell nuclear transfer (SCNT) bypasses the normal routes of mtDNA inheritance and introduces not only a different nuclear genome into the recipient cytoplast (in general an enucleated oocyte) but also somatic mitochondria. Differences in mtDNA genotype between recipient oocytes and potential mtDNA heteroplasmy due to persistence and replication of somatic mtDNA means that offspring generated by SCNT are not true clones. However, more importantly, the consequences of the presence of somatic mtDNA, mtDNA heteroplasmy, or possible incompatibility between nuclear and mtDNA genotypes on subsequent development and function of the embryo, fetus and offspring are unknown. Following sexual reproduction, mitochondrial function requires the biparental control of maternally inherited mtDNA, whereas following SCNT incompatibility between the recipient cell mitochondrial and transplanted nuclear genomes, or mtDNA heteroplasmy, may result in energy imbalance and initiate the onset of mtDNA-type disease, or disruption of normal developmental events. To remove the potentially adverse effects of somatic mtDNA following SCNT we have previously produced embryos using donor cells depleted to residual levels of mtDNA (mtDNA). We now report that these cells support development to term and produced live lambs in which no donor somatic mtDNA was detected, the lambs being homoplasmic for recipient oocyte DNA.

The senescence-related mitochondrial/oxidative stress pathway is repressed in human induced pluripotent stem cells

The ability of stem cells to propagate indefinitely is believed to occur via the fine modulation of pathways commonly involved in cellular senescence, including the telomerase, the p53, and the mitochondrial/oxidative stress pathways. Induced pluripotent stem cells (iPSCs) are a novel stem cell population obtained from somatic cells through forced expression of a set of genes normally expressed in embryonic stem cells (ESCs). These reprogrammed cells acquire self-renewal properties and appear almost undistinguishable from ESCs in terms of morphology, gene expression, and differentiation potential. Accordingly, iPSCs exhibit alterations of the senescence-related telomerase and p53 signaling pathways. However, although treatments with antioxidants have been recently shown to enhance cellular reprogramming, detailed information regarding the state of the mitochondrial/oxidative stress pathway in iPSCs is still lacking. Mitochondria undergo specific changes during organismal development and aging. Thus, addressing whether somatic mitochondria within iPSCs acquire ESC-like features or retain the phenotype of the parental cell is an unanswered but relevant question. Herein, we demonstrate that somatic mitochondria within human iPSCs revert to an immature ESC-like state with respect to organelle morphology and distribution, expression of nuclear factors involved in mitochondrial biogenesis, content of mitochondrial DNA, intracellular ATP level, oxidative damage, and lactate generation. Upon differentiation, mitochondria within iPSCs and ESCs exhibited analogous maturation and anaerobic-to-aerobic metabolic modifications. Overall, the data highlight that human iPSCs and ESCs, although not identical, share similar mitochondrial properties and suggest that cellular reprogramming can modulate the mitochondrial/oxidative stress pathway, thus inducing a rejuvenated state capable of escaping cellular senescence.

The relationship between pluripotency and mitochondrial DNA proliferation during early embryo development and embryonic stem cell differentiation

Pluripotent blastomeres of mammalian pre-implantation embryos and embryonic stem cells (ESCs) are characterized by limited oxidative capacity and great reliance on anaerobic respiration. Early pre-implantation embryos and undifferentiated ESCs possess small and immature mitochondria located around the nucleus, have low oxygen consumption and express high levels of glycolytic enzymes. However, as embryonic cells and ESCs lose pluripotency and commit to a specific cell fate, the expression of mtDNA transcription and replication factors is upregulated and the number of mitochondria and mtDNA copies/cell increases. Moreover, upon cellular differentiation, mitochondria acquire an elongated morphology with swollen cristae and dense matrices, migrate into wider cytoplasmic areas and increase the levels of oxygen consumption and ATP production as a result of the activation of the more efficient, aerobic metabolism. Since pluripotency seems to be associated with anaerobic metabolism and a poorly developed mitochondrial network and differentiation leads to activation of mitochondrial biogenesis according to the metabolic requirements of the specific cell type, it is hypothesized that reprogramming of somatic cells towards a pluripotent state, by somatic cell nuclear transfer (SCNT), transcription-induced pluripotency or creation of pluripotent cell hybrids, requires acquisition of mitochondrial properties characteristic of pluripotent blastomeres and ESCs.

Nuclear reprogramming: what has been done and potential avenues for improvements

A major challenge for reproductive biologists is the development of novel strategies to improve cloning efficiency. Even in species for which cloning is relatively successful, like cattle, the efficiency is still unacceptably low. In this review article we critically analyse all approaches that have been suggested by different laboratories in the field so far. As will be discussed below, so far none of these gives rise to a dramatic increase in cloning efficiency. Possibly, a multi-step approach including a pre-treatment of donor cells to modify their chromatin, along with improved culture system for cloned embryos would be the most promising.

Characterization of a donor mitochondrial DNA transmission bottleneck in nuclear transfer derived cow lineages

In embryos derived by nuclear-transfer (NT), fusion of donor cells with recipient oocytes resulted in varying patterns of mitochondrial DNA (mtDNA) transmission in NT animals. Distribution of donor cell mtDNA (D-mtDNA) found in offspring of NT-derived founders may also vary from donor cell and host embryo heteroplasmy to host embryo homoplasmy. Here we examined the transmission of mtDNA from NT cows to G(1) offspring. Eleven NT founder cows were produced by fusion of enucleated oocytes (Holstein/Japanese Black) with Jersey/ Holstein oviduct epithelial cells, or Holstein/Japanese Black cumulus cells. Transmission of mtDNA was analyzed by PCR mediated single-strand conformation polymorphism of the D-loop region. In six of seven animals sampled postmortem, heteroplasmy were detected in various tissues, while D-mtDNA could not be detected in blood or hair samples from four live animals. The average proportion of D-mtDNA detected in one NT cow was 7.6%, and those in other cows were <5%. Heteroplasmic NT cows (n = 6) generated a total 12 G(1) offspring. Four of 12 G(1) offspring exhibited high percentages of D-mtDNA populations (range 17-51%). The remaining eight G(1) offspring had slightly or undetectable D-mtDNA (<5%). Generally, a genetic bottleneck in the female germ-line should favor a homoplasmic state. However, proportions of some G(1) offspring maintained heteroplasmy with a much higher percentage of D-mtDNA than their NT dams, which may also reflect a segregation distortion caused by the proposed mitochondrial bottleneck. These results demonstrate that D-mtDNA in NT cows is transmitted to G(1) offspring with varying efficiencies.

The kinetics of donor cell mtDNA in embryonic and somatic donor cell-derived bovine embryos

The mechanisms controlling the outcome of donor cell-derived mitochondrial DNA (mtDNA) in cloned animals remain largely unknown. This research was designed to investigate the kinetics of somatic and embryonic mtDNA in reconstructed bovine embryos during preimplantation development, as well as in cloned animals. The experiment involved two different procedures of embryo reconstruction and their evaluation at five distinct phases of embryo development to measure the proportion of donor cell mtDNA (Bos indicus), as well as the segregation of this mtDNA during cleavage. The ratio of donor cell (B. indicus) to host oocyte (B. taurus) mtDNA (heteroplasmy) from blastomere(NT-B) and fibroblast(NT-F) reconstructed embryos was estimated using an allele-specific PCR with fluorochrome-stained specific primers in each sampled blastomere, in whole blastocysts, and in the tissues of a fibroblast-derived newborn clone. NT-B zygotes and blastocysts show similar levels of heteroplasmy (11.0% and 14.0%, respectively), despite a significant decrease at the 9-16 cell stage (5.8%; p<0.05). Heteroplasmy levels in NT-F reconstructed zygotes, however, increased from an initial low level (4.7%), to 12.9% (p<0.05) at the 9-16 cell stage. The NT-F blastocysts contained low levels of heteroplasmy (2.2%) and no somatic-derived mtDNA was detected in the gametes or the tissues of the newborn calf cloned. These results suggest that, in contrast to the mtDNA of blastomeres, that of somatic cells either undergoes replication or escapes degradation during cleavage, although it is degraded later after the blastocyst stage or lost during somatic development, as revealed by the lack of donor cell mtDNA at birth.

Oocyte quality and maternal control of development

The oocyte is a unique and highly specialized cell responsible for creating, activating, and controlling the embryonic genome, as well as supporting basic processes such as cellular homeostasis, metabolism, and cell cycle progression in the early embryo. During oogenesis, the oocyte accumulates a myriad of factors to execute these processes. Oogenesis is critically dependent upon correct oocyte-follicle cell interactions. Disruptions in oogenesis through environmental factors and changes in maternal health and physiology can compromise oocyte quality, leading to arrested development, reduced fertility, and epigenetic defects that affect long-term health of the offspring. Our expanding understanding of the molecular determinants of oocyte quality and how these determinants can be disrupted has revealed exciting new insights into the role of oocyte functions in development and evolution.

Mitochondrial DNA heteroplasmy in ovine fetuses and sheep cloned by somatic cell nuclear transfer

Background

The mitochondrial DNA (mtDNA) of the cloned sheep "Dolly" and nine other ovine clones produced by somatic cell nuclear transfer (SCNT) was reported to consist only of recipient oocyte mtDNA without any detectable mtDNA contribution from the nucleus donor cell. In cattle, mouse and pig several or most of the clones showed transmission of nuclear donor mtDNA resulting in mitochondrial heteroplasmy. To clarify the discrepant transmission pattern of donor mtDNA in sheep clones we analysed the mtDNA composition of seven fetuses and five lambs cloned from fetal fibroblasts.

Results

The three fetal fibroblast donor cells used for SCNT harboured low mtDNA copy numbers per cell (A: 753 ± 54, B: 292 ± 33 and C: 561 ± 88). The ratio of donor to recipient oocyte mtDNAs was determined using a quantitative amplification refractory mutation system (ARMS) PCR (i.e. ARMS-qPCR). For quantification of SNP variants with frequencies below 0.1% we developed a restriction endonuclease-mediated selective quantitative PCR (REMS-qPCR). We report the first cases (n = 4 fetuses, n = 3 lambs) of recipient oocyte/nuclear donor mtDNA heteroplasmy in SCNT-derived ovine clones demonstrating that there is no species-effect hindering ovine nucleus-donor mtDNA from being transmitted to the somatic clonal offspring. Most of the heteroplasmic clones exhibited low-level heteroplasmy (0.1% to 0.9%, n = 6) indicating neutral transmission of parental mtDNAs. High-level heteroplasmy (6.8% to 46.5%) was observed in one case. This clone possessed a divergent recipient oocyte-derived mtDNA genotype with three rare amino acid changes compared to the donor including one substitution at an evolutionary conserved site.

Conclusion

Our study using state-of-the-art techniques for mtDNA quantification, like ARMS-qPCR and the novel REMS-qPCR, documents for the first time the transmission of donor mtDNA into somatic sheep clones. MtDNA heteroplasmy was detected in seven of 12 clones tested, whereby all but one case revealed less than 1% mtDNA contribution from the nuclear donor cell suggesting neutral segregation.

Mitochondrial DNA replication during differentiation of murine embryonic stem cells

Oxidative phosphorylation (OXPHOS), the intracellular process that generates the majority of the ATP of a cell through the electron-transfer chain, is highly dependent on proteins encoded by the mitochondrial genome (mtDNA). MtDNA replication is regulated by the nuclear-encoded mitochondrial transcription factor A (TFAM) and the mitochondrial-specific DNA polymerase gamma, which consists of a catalytic (POLG) and an accessory (POLG2) subunit. Differentiation of pluripotent embryonic stem cells (ESCs) into specific cell types requires expansion of discrete populations of mitochondria and mtDNA replication to meet the specific metabolic requirements of the cell. We determined by real-time PCR that expression of pluripotent markers is reduced before the upregulation of Polg, Polg2 and Tfam in spontaneously differentiating R1 murine (m)ESCs, along with transient increases in mtDNA copy number. In D3 mESCs, the initial transient increase did not take place. However, precursors of neuronal and cardiomyocyte differentiation were positive for both POLG and TFAM. Similar-stage ESCs also showed active mtDNA replication, identified by 5-bromo-2'-deoxy-uridine labelling, as mtDNA copy number increased. Retinoic-acid-induced differentiation resulted in more consistent patterns of replication and upregulation of Polg, Polg2 and Tfam, whereas siRNA knockdown demonstrated that steady-state expression of POLG is essential for maintaining pluripotency.

Contrasting effects of in vitro fertilization and nuclear transfer on the expression of mtDNA replication factors

Mitochondrial DNA (mtDNA) is normally only inherited through the oocyte. However, nuclear transfer (NT), the fusion of a donor cell with an enucleated oocyte, can transmit both donor cell and recipient oocyte mtDNA. mtDNA replication is under the control of nuclear-encoded replication factors, such as polymerase gamma (POLG) and mitochondrial transcription factor A (TFAM). These are first expressed during late preimplantation embryo development. To account for the persistence of donor cell mtDNA, even when introduced at residual levels (mtDNA(R)), we hypothesized that POLG and TFAM would be upregulated in intra- and interspecific (ovine-ovine) and intergeneric (caprine-ovine) NT embryos when compared to in vitro fertilized (IVF) embryos. For the intra- and interspecific crosses, PolGA (catalytic subunit), PolGB (accessory subunit), and TFAM mRNA were expressed at the 2-cell stage in both nondepleted (mtDNA(+)) and mtDNA(R) embryos with protein being expressed up to the 16-cell stage for POLGA and TFAM. However, at the 16-cell stage, there was significantly more PolGA expression in the mtDNA(R) embryos compared to their mtDNA(+) counterparts. Expression for all three genes first matched IVF embryos at the blastocyst stage. In the intergeneric model, POLG was upregulated during preimplantation development. Although these embryos did not persist further than the 16+-cell stage, significantly more mtDNA(R) embryos reached this stage. However, the vast majority of these embryos were homoplasmic for recipient oocyte mtDNA. The upreglation in mtDNA replication factors was most likely due to the donor cells still expressing these factors prior to NT.

Transmission of Mitochondrial DNA Disorders: Possibilities for The Elimination of Mutated Mitochondria

The recent article by D.T. Brown et al. (2006) discusses the transmission of mitochondrial DNA disorders. In the second part, the authors describe certain techniques that can be potentially used to eliminate the transmission of mutated mtDNA from mother to offspring. It is our opinion that this brief account needs broader explanation. Moreover, some approaches mentioned are not always correctly interpreted.

The expression of polymerase gamma and mitochondrial transcription factor A and the regulation of mitochondrial DNA content in mature human sperm

BACKGROUND

Human mitochondrial DNA (mtDNA) encodes 13 polypeptides of the electron transfer chain. Its replication is dependent on the nuclear-encoded polymerase gamma (POLG) and mitochondrial transcription factor A (TFAM). For POLG, only the polyglutamine tract, characterized by a series of CAG repeats, has been investigated in human sperm. However, TFAM is associated with the reduction in mtDNA content of testicular sperm. We have determined whether POLG and TFAM have functional roles in post-ejaculatory sperm mtDNA.

METHODS

Sperm samples were categorized as: normals, samples with one or two abnormal sperm parameters and oligoasthenoteratozoospermics (OATs). These were analysed by fluorescent PCR to determine the number of CAG repeats, real-time PCR for mtDNA copy number and immunocytochemistry and western blotting for patterns of expression for POLG, TFAM and the mtDNA-encoded COXI.

RESULTS

Only the OAT group presented with a significantly higher incidence of heterozygosity for CAG repeats, higher mtDNA content and a lower percentage of sperm expressing POLG and TFAM. Paradoxically, good-quality sperm had fewer mtDNA copies but significantly more sperm expressed POLG, TFAM and COXI.

CONCLUSIONS

Our data support the original findings that an association between sperm quality and POLG CAG repeats does exist. However, the biological significance of these variants in male infertility remains unclear, as these do not seem to affect mtDNA maintenance. The reduction in mtDNA content in normal samples likely reflects normal spermiogenesis, whereas increases in POLG and TFAM expression possibly compensate for the low mtDNA content, maintaining mitochondrial homeostasis.

Nuclear Transfer: Preservation of a Nuclear Genome at the Expense of Its Associated mtDNA Genome(s)

Nuclear transfer technology has uses across theoretical and applied applications, but advances are restricted by continued poor success rates and health problems associated with live offspring. Development of reconstructed embryos is dependent upon numerous interlinking factors relating both to the donor cell and the recipient oocyte. For example, abnormalities in gene expression following somatic cell nuclear transfer (SCNT) have been linked with an inability of the oocyte cytoplasm to sufficiently epigenetically reprogram the nucleus. Furthermore, influences on the propagation of mitochondria and mitochondrial DNA (mtDNA) could be of great importance in determining the early developmental potential of NT embryos and contributing to their genetic identity. mtDNA encodes some of the subunits of the electron transfer chain, responsible for cellular ATP production. The remaining subunits and those factors required for mtDNA replication, transcription and translation are encoded by the nucleus, necessitating precise intergenomic communication. Additionally, regulation of mtDNA copy number, via the processes of mtDNA transcription and replication, is essential for normal preimplantation embryo development and differentiation. Unimaternal transmission following natural fertilization usually results in the presence of a single identical population of mtDNA, homoplasmy. Heteroplasmy can result if mixed populations of mtDNA genomes co-exist. Many abnormalities observed in NT embryos, fetuses, and offspring may be caused by deficiencies in OXPHOS, perhaps resulting in part from heteroplasmic mtDNA populations. Additionally, incompatibilities between the somatic nucleus and the cytoplast may be exacerbated by increased genetic divergence between the two genomes. It is important to ensure that the nucleus is capable of sufficiently regulating mtDNA, requiring a level of compatibility between the two genomes, which may be a function of evolutionary distance. We suggest that abnormal expression of factors such as TFAM and POLG in NT embryos will prematurely drive mtDNA replication, hence impacting on early development.

Regulated mitochondrial DNA replication during oocyte maturation is essential for successful porcine embryonic development

Cellular ATP is mainly generated through mitochondrial oxidative phosphorylation, which is dependent on mitochondrial DNA (mtDNA). We have previously demonstrated the importance of oocyte mtDNA for porcine and human fertilization. However, the role of nuclear-encoded mitochondrial replication factors during oocyte and embryo development is not yet understood. We have analyzed two key factors, mitochondrial transcription factor A (TFAM) and polymerase gamma (POLG), to determine their role in oocyte and early embryo development. Competent and incompetent oocytes, as determined by brilliant cresyl blue (BCB) dye, were assessed intermittently during the maturation process for TFAM and POLG mRNA using real-time RT-PCR, for TFAM and POLG protein using immunocytochemistry, and for mtDNA copy number using real-time PCR. Analysis was also carried out following treatment of maturing oocytes with the mtDNA replication inhibitor, 2',3'-dideoxycytidine (ddC). Following in vitro fertilization, preimplantation embryos were also analyzed. Despite increased levels of TFAM and POLG mRNA and protein at the four-cell stage, no increase in mtDNA copy number was observed in early preimplantation development. To compensate for this, mtDNA appeared to be replicated during oocyte maturation. However, significant differences in nuclear-encoded regulatory protein expression were observed between BCB(+) and BCB(-) oocytes and between untreated oocytes and those treated with ddC. These changes resulted in delayed mtDNA replication, which correlated to reduced fertilization and embryonic development. We therefore conclude that adherence to the regulation of the timing of mtDNA replication during oocyte maturation is essential for successful embryonic development.