Mitochondrial biogenesis in early mouse embryos: expression of the mRNAs for subunits IV, Vb, and VIIc of cytochrome c oxidase and subunit 9 (P1) of H(+)-ATP synthase

Distinct Mitochondrial Remodeling During Mesoderm Differentiation in a Human-Based Stem Cell Model

Given the considerable interest in using stem cells for modeling and treating disease, it is essential to understand what regulates self-renewal and differentiation. Remodeling of mitochondria and metabolism, with the shift from glycolysis to oxidative phosphorylation (OXPHOS), plays a fundamental role in maintaining pluripotency and stem cell fate. It has been suggested that the metabolic “switch” from glycolysis to OXPHOS is germ layer-specific as glycolysis remains active during early ectoderm commitment but is downregulated during the transition to mesoderm and endoderm lineages. How mitochondria adapt during these metabolic changes and whether mitochondria remodeling is tissue specific remain unclear. Here, we address the question of mitochondrial adaptation by examining the differentiation of human pluripotent stem cells to cardiac progenitors and further to differentiated mesodermal derivatives, including functional cardiomyocytes. In contrast to recent findings in neuronal differentiation, we found that mitochondrial content decreases continuously during mesoderm differentiation, despite increased mitochondrial activity and higher levels of ATP-linked respiration. Thus, our work highlights similarities in mitochondrial remodeling during the transition from pluripotent to multipotent state in ectodermal and mesodermal lineages, while at the same time demonstrating cell-lineage-specific adaptations upon further differentiation. Our results improve the understanding of how mitochondrial remodeling and the metabolism interact during mesoderm differentiation and show that it is erroneous to assume that increased OXPHOS activity during differentiation requires a simultaneous expansion of mitochondrial content.

Parental diet-induced obesity leads to retarded early mouse embryo development and altered carbohydrate utilisation by the blastocyst

Maternal obesity results in reproductive complications, whereas the impact of paternal obesity is unclear. In the present study, the effects of parental obesity on preimplantation embryo cell cycle length and carbohydrate utilisation were investigated. Maternal and paternal obesity were assessed independently by deriving zygotes from normal or obese C57BL/6 female mice mated with normal Swiss male mice (maternal obesity), or from normal Swiss female mice mated with normal or obese C57BL/6 male mice (paternal obesity). Zygotes were cultured in vitro and development was then assessed by time-lapse microscopy and metabolism determined using ultramicrofluorescence. Maternal obesity was associated with a significant delay in precompaction cell cycle kinetics from the 1-cell stage. A significant increase in glucose consumption by embryos from obese compared with normal females occurred after compaction, although glycolysis remained unchanged. Similarly, paternal obesity led to significant delays in cell cycle progression during preimplantation embryo development. However, this developmental delay was observed from the second cleavage stage onwards, following embryonic genome activation. Blastocysts from obese males showed disproportionate changes in carbohydrate metabolism, with significantly increased glycolysis. Overall, metabolic changes were not inhibitory to blastocyst formation; however, blastocyst cell numbers were significantly lower when either parent was obese. These data suggest that both maternal and paternal obesity significantly impacts preimplantation embryo physiology.

Effect of serum on the mitochondrial active area on developmental days 1 to 4 in in vitro-produced bovine embryos

Certain morphological changes at the subcellular level caused by the current techniques for in vitro embryo production seem to affect mitochondria. Many of these, including dysfunctional changes, have been associated with the presence of serum in the culture medium. Thus, the aim of the present work was to assess the mitochondrial dynamics occurring in embryos during the first 4 days of development, in order to analyze the most appropriate time for adding the serum. We used transmission electron microscopy (TEM) micrographs to calculate the embryo area occupied by the different morphological types of mitochondria, and analyzed them with Image Pro Plus analyzer. The results showed hooded mitochondria as the most representative type in 1- to 4-day-old embryos. Swollen, on-fusion, orthodox and vacuolated types were also present. When analyzed in embryos cultured without serum, the dynamics of the different mitochondrial types appeared to be similar, a fact that may provide evidence that the developmental changes control the mitochondrial dynamics, and that swollen mitochondria may not be completely inactive. In contrast, in culture medium supplemented with serum from estrous cows, we observed an increased area of hooded mitochondria by developmental day 4, a fact that may indicate an increased production of energy compared with previous days. According to these results, the bovine serum added to the culture medium seems not to be responsible for the functional changes in mitochondria.

Molecular control of mitochondrial function in developing rhesus monkey oocytes and preimplantation-stage embryos

The mitochondrion undergoes significant functional and structural changes, as well as an increase in number, during preimplantation embryonic development. The mitochondrion generates ATP and regulates a range of cellular processes, such as signal transduction and apoptosis. Therefore, mitochondria contribute to overall oocyte quality and embryo developmental competence. The present study identified, for the first time, the detailed temporal expression of mRNAs related to mitochondrial biogenesis in rhesus monkey oocytes and embryos. Persistent expression of maternally encoded mRNAs was observed, in combination with transcriptional activation and mRNA accumulation at the eight-cell stage, around the time of embryonic genome activation. The expression of these transcripts was significantly altered in oocytes and embryos with reduced developmental potential. In these embryos, most maternally encoded transcripts were precociously depleted. Embryo culture and specific culture media affected the expression of some of these transcripts, including a deficiency in the expression of key transcriptional regulators. Several genes involved in regulating mitochondrial transcription and replication are similarly affected by in vitro conditions and their downregulation may be instrumental in maintaining the mRNA profiles of mitochondrially encoded genes observed in the present study. These data support the hypothesis that the molecular control of mitochondrial biogenesis, and therefore mitochondrial function, is impaired in in vitro-cultured embryos. These results highlight the need for additional studies in human and non-human primate model species to determine how mitochondrial biogenesis can be altered by oocyte and embryo manipulation protocols and whether this affects physiological function in progeny.

Role of glucose in cloned mouse embryo development

Cloned mouse embryos display a marked preference for glucose-containing culture medium, with enhanced development to the blastocyst stage in glucose-containing medium attributable mainly to an early beneficial effect during the first cell cycle. This early beneficial effect of glucose is not displayed by parthenogenetic, fertilized, or tetraploid nuclear transfer control embryos, indicating that it is specific to diploid clones. Precocious localization of the glucose transporter SLC2A1 to the cell surface, as well as increased expression of glucose transporters and increased uptake of glucose at the one- and two-cell stages, is also seen in cloned embryos. To examine the role of glucose in early cloned embryo development, we examined glucose metabolism and associated metabolites, as well as mitochondrial ultrastructure, distribution, and number. Clones prepared with cumulus cell nuclei displayed significantly enhanced glucose metabolism at the two-cell stage relative to parthenogenetic controls. Despite the increase in metabolism, ATP content was reduced in clones relative to parthenotes and fertilized controls. Clones at both stages displayed elevated concentrations of glycogen compared with parthenogenetic controls. There was no difference in the number of mitochondria, but clone mitochondria displayed ultrastructural alterations. Interestingly, glucose availability positively affected mitochondrial structure and localization. We conclude that cloned embryos may be severely compromised in terms of ATP-dependent processes during the first two cell cycles and that glucose may exert its early beneficial effects via positive effects on the mitochondria.

The modifier of Min 2 (Mom2) locus: embryonic lethality of a mutation in the Atp5a1 gene suggests a novel mechanism of polyp suppression

Inactivation of the APC gene is considered the initiating event in human colorectal cancer. Modifier genes that influence the penetrance of mutations in tumor-suppressor genes hold great potential for preventing the development of cancer. The mechanism by which modifier genes alter adenoma incidence can be readily studied in mice that inherit mutations in the Apc gene. We identified a new modifier locus of ApcMin-induced intestinal tumorigenesis called Modifier of Min 2 (Mom2). The polyp-resistant Mom2R phenotype resulted from a spontaneous mutation and linkage analysis localized Mom2 to distal chromosome 18. To obtain recombinant chromosomes for use in refining the Mom2 interval, we generated congenic DBA.B6 ApcMin/+, Mom2R/+ mice. An intercross revealed that Mom2R encodes a recessive embryonic lethal mutation. We devised an exclusion strategy for mapping the Mom2 locus using embryonic lethality as a method of selection. Expression and sequence analyses of candidate genes identified a duplication of four nucleotides within exon 3 of the alpha subunit of the ATP synthase (Atp5a1) gene. Tumor analyses revealed a novel mechanism of polyp suppression by Mom2R in Min mice. Furthermore, we show that more adenomas progress to carcinomas in Min mice that carry the Mom2R mutation. The absence of loss of heterozygosity (LOH) at the Apc locus, combined with the tendency of adenomas to progress to carcinomas, indicates that the sequence of events leading to tumors in ApcMin/+ Mom2R/+ mice is consistent with the features of human tumor initiation and progression.

Characterization of mitochondrial replication and transcription control during rat early development in vivo and in vitro

In vitroculture (IVC), used in assisted reproductive technologies, is a major environmental stress on the embryo. To evaluate the effect of IVC on mitochondrial transcription and the control of mtDNA replication, we measured the mtDNA copy number and relative amount of mRNA for mitochondrial-related genes in individual rat oocytes, zygotes and embryos using real-time PCR. The average mtDNA copy number was 147 600 (±3000) in metaphase II oocytes. The mtDNA copy number was stable throughoutin vivoearly development and IVC induced an increase in mtDNA copy number from the 8-cell stage onwards.GapdmRNA levels vary during early development and IVC did not change the patterns of these housekeeping gene transcripts.PolrmtmRNA levels did not vary during early development up to the morula stage but increased at the blastocyst stage. IVC induced the up-regulation ofPolrmtmRNA, one of the key genes regulating mtDNA transcription and replication, at the blastocyst stage. An increase inmt-Nd4mRNA preceded the blastocyst-related event observed in nuclear-encodedGapdandPolrmt, suggesting that the expression of mitochondrial encoded genes is controlled differently from nuclear encoded genes. We conclude that the IVC system can perturb mitochondrial transcription and the control of mtDNA replication in rat embryos. This perturbation of mtDNA regulation may be responsible for the abnormal physiology, metabolism and viability ofin vitro-derived embryos.

Quantitative analysis of mitochondrial RNA in goat–sheep cloned embryos

Mitochondria are the key generators of cellular ATP, and contain extranuclear genome-mitochondrial DNA (mtDNA). In the process of nuclear transfer (NT), heteroplasmic sources of mtDNA from a donor cell and a recipient oocyte are mixed in the cytoplasm of the reconstituted embryo. Previous studies showed inconsistent patterns of mtDNA inheritance in offspring and early fetuses generated through interspecies NT. The quantitative analysis of mitochondrial RNA (mtRNA) in interspecies cloned embryos is useful for better understanding the fate of two types of mitochondria. The components of nicotinamide adenine dinucleotide (NADH) dehydrogenase were coded by both nuclear DNA (nDNA) and mtDNA. The Subunit 1 (ND-1) is one of seven NADH dehydrogenase subunits coded by mtDNA. In present study, using real-time and reverse-transcription PCR, the copy number of species-specific ND-1 mRNA was examined in goat-sheep cloned embryos of various developmental stages, and was applied to evaluate the expression pattern of species-specific mtDNA. The results of showed that (1) the expression of mtDNA derived from goat fetal fibroblast (GFF) decreased from 1-cell stage (immediately after fused) to 2-cell stage, and could not be detected from 4-cell stage onward to blastocyst stage; (2) the expression of mtDNA derived from sheep oocyte was roughly constant from 1-cell stage to the 8-cell stage, increased gradually from 16-cell stage, and sharply at morula and blastocyst stage. Moreover, we strongly argued a mechanism, that is GFF-derived mitochondria were degraded for the depression of bioenergetic functions, and then selectively eliminated during the embryogenesis of goat-sheep cloned embryos.

Abnormal mitochondrial structure in human unfertilized oocytes and arrested embryos

To clarify the relationship between mitochondria and embryo development, we collected human unfertilized oocytes, early embryos, and arrested embryos. Unfertilized oocytes and poor-quality embryos were collected, and the ultrastructure of mitochondria was determined by transmission electron micrography. Four criteria for determining the mitochondrial state were mitochondrial morphology, cristae shape, location, and number of mitochondria. In mature oocytes, mitochondria were rounded with arched cristae and a dense matrix and were distributed evenly in the ooplasm. In pronuclear zygotes, the size and shape of mitochondria were similar to those in mature oocytes; however, mitochondria appeared to migrate and concentrate around pronuclei. In this study, 67% of examined unfertilized oocytes had fewer mitochondria in the cytoplasm. A decreased number of mitochondria located near the nucleus was also demonstrated in 60% of arrested embryos. Fewer differentiated cristae were determined in all three arrested blastocyst stages of embryos. The relative expressions of oxidative phosphorylation genes in oocytes and embryos were also determined. These data imply that inadequate redistribution of mitochondria, unsuccessful mitochondrial differentiation, or decreased mitochondrial transcription may result in poor oocyte fertilization and compromised embryo development.

Molecular control of mitochondrial function in preimplantation mouse embryos

Mitochondria play a key role in a number of physiological events during all stages of life, including the very first stages following fertilization. It is, therefore, important to understand the mechanisms controlling mitochondrial activity during early embryogenesis to determine their role in development outcome. The objective of this study was to investigate the molecular control of mitochondrial transcription and mitochondrial DNA (mtDNA) replication in mouse preimplantation embryos. We estimated the mtDNA copy number and characterized the expression patterns of two mitochondrial genes and several nuclear genes that encode mitochondrial transcription and replication factors throughout preimplantation development. Mitochondrial gene transcripts were present in larger quantities in morula and blastocyst stage embryos relative to other stages. A significant increase in the amount of mRNA for nuclear genes encoding mtDNA transcription factors was observed in eight-cell stage embryos. Although a similar increase in the mRNA levels of nuclear genes encoding mtDNA replication factors was observed in morula and blastocyst stage embryos, the number of mtDNA molecules remained stable during preimplantation stages, suggesting that nuclear-encoded mitochondrial transcription factors are involved in the regulation of mtDNA transcription during early development. Although transcripts of replication factors are abundant at the morula and blastocyst stage, mtDNA replication did not occur until the blastocyst stage, suggesting that the inhibition of mtDNA replication is controlled at the post-transcriptional level during early embryogenesis.

Role of the mitochondrial genome in preimplantation development and assisted reproductive technologies

Our fascination for mitochondria relates to their origin as symbiotic, semi-independent organisms on which we, as eukaryotic beings, rely nearly exclusively to produce energy for every cell function. Therefore, it is not surprising that these organelles play an essential role in many events during early development and in artificial reproductive technologies (ARTs) applied to humans and domestic animals. However, much needs to be learned about the interactions between the nucleus and the mitochondrial genome (mtDNA), particularly with respect to the control of transcription, replication and segregation during preimplantation. Nuclear-encoded factors that control transcription and replication are expressed during preimplantation development in mice and are followed by mtDNA transcription, but these result in no change in mtDNA copy number. However, in cattle, mtDNA copy number increases during blastocyst expansion and hatching. Nuclear genes influence the mtDNA segregation patterns in heteroplasmic animals. Because many ARTs markedly modify the mtDNA content in embryos, it is essential that their application is preceded by careful experimental scrutiny, using suitable animal models.

Response characteristics of the mitochondrial DNA genome in developmental health and disease

This review focuses on mitochondrial biology in mammalian development; specifically, the dynamics of information transfer from nucleus to mitochondrion in the regulation of mitochondrial DNA genomic expression, and the reverse signaling of mitochondrion to nucleus as an adaptive response to the environment. Data from recent studies suggest that the capacity of embryonic cells to react to oxygenation involves a tradeoff between factors that influence prenatal growth/development and postnatal growth/function. For example, mitochondrial DNA replication and metabolic set points in nematodes may be determined by mitochondrial activity early in life. The mitochondrial drug PK11195, a ligand of the peripheral benzodiazepine receptor, has antiteratogenic and antidisease action in several developmental contexts in mice. Protein malnutrition during early life in rats can program mitochondrial DNA levels in adult tissues and, in humans, epidemiological data suggest an association between impaired fetal growth and insulin resistance. Taken together, these findings raise the provocative hypothesis that environmental programming of mitochondrial status during early life may be linked with diseases that manifest during adulthood. Genetic defects that affect mitochondrial function may involve the mitochondrial DNA genome directly (maternal inheritance) or indirectly (Mendelian inheritance) through nuclear-coded mitochondrial proteins. In a growing number of cases, the depletion of, or deletion in, mitochondrial DNA is seen to be secondary to mutation of key nuclear-coded mitochondrial proteins that affect mitochondrial DNA replication, expression, or stability. These defects of intergenomic regulation may disrupt the normal cross-talk or structural compartmentation of signals that ultimately regulate mitochondrial DNA integrity and copy number, leading to depletion of mitochondrial DNA.

[Mitochondria and reproduction]

Mitochondria play a primary role in cellular energetic metabolism. They possess their own DNA, which is exclusively maternally transmitted. The relatively recent idea that mitochondria may be directly involved in human reproduction is arousing increasing interest in the scientific and medical community. It has been shown that the functional status of mitochondria contributes to the quality of oocytes and spermatozoa, and plays a part in the process of fertilisation and embryo development. Moreover, new techniques, such as ooplasm transfer, compromise the uniquely maternal inheritance of mitochondrial DNA, raising important ethical questions. This review discusses recent information about mitochondria in the field of human fertility and reproduction.

Transcript profiling during preimplantation mouse development

Studies using low-resolution methods to assess gene expression during preimplantation mouse development indicate that changes in gene expression either precede or occur concomitantly with the major morphological transitions, that is, conversion of the oocyte to totipotent 2-cell blastomeres, compaction, and blastocyst formation. Using microarrays, we characterized global changes in gene expression and used Expression Analysis Systematic Explorer (EASE) to identify biological and molecular processes that accompany and likely underlie these transitions. The analysis confirmed previously described processes or events, but more important, EASE revealed new insights. Response to DNA damage and DNA repair genes are overrepresented in the oocyte compared to 1-cell through blastocyst stages and may reflect the oocyte's response to selective pressures to insure genomic integrity; fertilization results in changes in the transcript profile in the 1-cell embryo that are far greater than previously recognized; and genome activation during 2-cell stage may not be as global and promiscuous as previously proposed, but rather far more selective, with genes involved in transcription and RNA processing being preferentially expressed. These results validate this hypothesis-generating approach by identifying genes involved in critical biological processes that can be the subject of a more traditional hypothesis-driven approach.

Mitochondrial dysfunction in mouse oocytes results in preimplantation embryo arrest in vitro

Oocyte mitochondrial dysfunction has been proposed as a cause of high levels of developmental retardation and arrest that occur in human preimplantation embryos generated using assisted reproductive technology in the treatment of some causes of female infertility. To investigate this, a model of mitochondrial dysfunction was developed in mouse oocytes using a method of photosensitization of the mitochondrion-specific dye, rhodamine-123. After in vitro fertilization, dye-loaded and photosensitized oocytes showed developmental arrest in proportion to irradiation time. Morphological and metabolic assessments of zygotes indicated an increase in mitochondrial permeability that subsequently resulted in apoptotic degeneration. Development was partially restored by inhibition of mitochondrial permeability transition pore formation by oocyte pretreatment with cyclosporin A. Oocyte mitochondria are therefore physiological regulators of early embryo development and potential sites of pathological insult that may perturb oocyte and subsequent preimplantation embryo viability. These findings have important implications for the treatment of clinically infertile women using assisted reproductive technologies.

The role of mitochondria in the establishment of oocyte functional competence

Mitochondria are maternally inherited, semi-autonomous organelles with their own genomes (mtDNA), largely responsible for the generation of energy in the form of cellular ATP. However, mitochondrial replication and transcription of mtDNA do not commence until well into embryonic differentiation. This means that the oocyte needs to contain sufficient stocks of functioning mitochondria to fuel the first few days of embryonic development. In this review, I examine how qualitative and quantitative aspects of mitochondria help us define the notion of functional competence.

Decreased expression of mitochondrial genes in human unfertilized oocytes and arrested embryos

OBJECTIVE

To evaluate the relationship between mitochondrial gene expression of oocytes/embryos and their fertilizability in unfertilized oocytes, arrested embryos, and tripronucleate zygotes, because both nuclear and cytoplasmic factors contribute to oocyte activation, fertilization, and subsequent development.

DESIGN

Prospective laboratory research.

SETTING

In vitro fertilization (IVF) laboratory in a university hospital.

PATIENT(S)

Seventy-five unfertilized oocytes, 45 arrested embryos, and 24 tripronucleate (3PN) embryos from 45 female patients undergoing IVF.

INTERVENTION(S)

Analysis of mitochondrial gene expression by semiquantitative reverse transcription polymerase chain reaction (RT-PCR).

MAIN OUTCOME MEASURE(S)

Comparison of the expression levels of mitochondrial genes including ND2, CO I, CO II, ATPase 6, CO III, ND3, ND6, and Cyt b in three groups.

RESULT(S)

Significantly decreased transcription levels were expressed in unfertilized oocytes and arrested embryos. The average expression levels of the eight determined genes compared with the control (GAPDH) was 4.4 +/- 0.7, 6.4 +/- 1.1, and 13.2 +/- 1.1 in unfertilized oocytes, arrested embryos, and 3PN embryos, respectively. Significantly decreased expressions of the ATPase 6, CO III, and ND3 genes were detected from samples with 4977-bp common deletion in the mitochondrial DNA (mtDNA) compared with the non-deletion group.

CONCLUSION(S)

The present study is the first report to present globally decreased mitochondrial gene expression levels in human compromised oocytes and embryos. These data support the notion that the down-regulation of mitochondrial RNA by defective oxidative phosphorylation genes possibly affects oocyte quality including fertilization and further embryo development.

The potential risks of abnormal transmission of mtDNA through assisted reproductive technologies

The recent introduction of more invasive assisted reproductive techniques offers the possibility to provide a wider treatment profile to patients. However, some of these technologies are of considerable concern as they are fraught with the possible transmission of genetic abnormalities to the offspring they create. To date, much analysis of these technologies has been conducted at the chromosomal DNA level. While some analysis has been conducted on the extranuclear, mitochondrial genome (mtDNA), this has been mainly descriptive. In the vast majority of cases, it appears that mtDNA is maternally inherited. The impact that leakage of sperm mtDNA transmission might have for the offspring is discussed in the light of the recent identification of sperm mtDNA presence in a patient with mtDNA disease. The implications of introducing donor mtDNA into a recipient oocyte through both cytoplasmic and nuclear transfer are also discussed. Again, the implications for offspring survival are discussed and suggestions made as to why the techniques might provide valuable insights into mtDNA transmission, replication and transcription. In order to be confident that patients and their offspring are being offered safe treatment, it is argued that potentially some of these treatments may be of considerable benefit in the future but significant scientific research is required before these treatments can be effectively employed in the clinic.

Antenatal manifestations of mitochondrial respiratory chain deficiency

OBJECTIVE

To review the antenatal manifestations of disorders of oxidative phosphorylation.

STUDY DESIGN

A total of 300 cases of proven respiratory chain enzyme deficiency were retrospectively reviewed for fetal development, based on course and duration of pregnancy, antenatal ultrasonography and birth weight, length, and head circumference. Particular attention was given to fetal movements, oligo/hydramnios, fetal cardiac rhythm, fetal heart ultrasound, and ultrasonography/echo Doppler signs of brain, facial, trunk, limb, and organ anomalies.

RESULTS

Retrospective analyses detected low birth weight (<3rd percentile for gestational age) in 22.7% of cases (68/300, P<.000001). Intrauterine growth retardation was either isolated (48/300, 16%) or associated with otherwise unexplained anomalies (20/300, 6.7%, P<.0001). Antenatal anomalies were usually multiple and involved several organs sharing no common function or embryologic origin. They included polyhydramnios (6/20), oligoamnios (2/20), arthrogryposis (1/20), decreased fetal movements (1/20), ventricular septal defects (2/20), hypertrophic cardiomyopathy (4/20), cardiac rhythm anomalies (4/20), hydronephrosis (3/20), vertebral abnormalities, anal atresia, cardiac abnormalities, tracheoesophageal fistula/atresia, renal agenesis and dysplasia, and limb defects (VACTERL) association (2/20), and a complex gastrointestinal malformation (1/20).

CONCLUSIONS

Although a number of metabolic diseases undergo a symptom-free period, respiratory chain deficiency may have an early antenatal expression, presumably related to the time course of the disease gene expression in the embryofetal period. The mechanism triggering malformations is unknown and may include decreased ATP formation and/or an alteration of apoptotic events controlled by the mitochondria.

The role of maternal mitochondria during oogenesis, fertilization and embryogenesis

This review examines the place of mitochondria in the life cycle through oogenesis, ovulation and early embryogenesis. Mitochondria are semi-autonomous organelles responsible for the bulk of oxidative energy production in the body. They play central roles in ageing, in apoptosis and in many non-Mendelian-inherited bioenergetic and neurological diseases. Originating as free alpha-proteobacteria that entered into a symbiotic relationship with the ancestral eukaryotic organisms, they now have a highly restricted genome of ~16 kb, encoding for 37 genes of the oxidative phosphorylation pathway. Mitochondria are inherited through the mother and special mechanisms have evolved to eliminate the contribution of the spermatozoon in early embryonic development. Most mitochondrial genes have become translocated to the nucleus, and nuclear and mitochondrial genes have co-evolved. This, coupled with a high mutation rate in the remaining mitochondrial DNA, has resulted in a high degree of concordance between them. Disharmony between nuclear and mitochondrial genes is thus likely to complicate cloning technology and the experimental reconstruction of chimeric embryos by cytoplasmic or nuclear transfer.

Rat mitochondrial ATP synthase ATP5G3: cloning and upregulation in pancreas after chronic ethanol feeding

Individuals with chronic excessive alcohol ingestion are put at the risk of acute and chronic pancreatitis. Underlying molecular mechanisms are unknown. Differential gene expression in the pancreas was profiled using mRNA differential display by comparison between control and ethanol-consuming rats. Male Wistar rats were fed with diets containing 6.7% (vol/vol) ethanol for 4 wk. A cDNA tag that was overexpressed in the pancreas of rats fed ethanol was isolated. A 723-bp cDNA was cloned from a rat pancreatic cDNA library, which encodes a novel rat mitochondrial ATP synthase subunit 9, isoform 3 (ATP5G3), which is homologous to a human ATP5G3 gene. Real-time PCR demonstrated that all three nuclear gene isoforms (ATP5G1, ATP5G2, and ATP5G3) were consistently upregulated in the pancreas of alcohol-consuming rats, parallel with mitochondrial injury. The cellular response to mitochondrial damage and metabolic stress may reflect an adaptive process for mitochondrial repair in pancreatic acinar cells during chronic ethanol ingestion.

Cytoplasmic inheritance and its implications for animal biotechnology

At fertilization, the mammalian sperm transmits the haploid paternal genome. However, it also carries a variety of other factors into the oocyte that have the potential to affect embryo development. These include mRNAs left over from spermatogenesis, mitochondria with their own DNA, cytoskeletal and contractile elements, remnants of the sperm plasma membrane and, in many species, the sperm centriole. While most of these elements are eliminated, some play essential roles in early embryogenesis. In this review, I summarize the latest information on these phenomena and indicate some of the implications for animal biotechnology and, in particular, cloning.

Analysis of gene expression in mouse 2-cell embryos using fluorescein differential display: comparison of culture environments

The effect of the oviductal environment on gene expression in 2-cell mouse embryos was examined with mRNA differential display. Embryos used for experiments were cultured in modified Whitten medium with or without oviductal tissue until late 2-cell stage. The results of sequencing indicated that the genes for ATP synthase (ATPase 6), S:-adenosylmethionine decarboxylase (S:-AMDC) and nuclear autoantigenic sperm protein (NASP) were differentially expressed in embryos cultured in the oviductal environment (nonblocking culture condition). The ATPase 6 gene is encoded by mitochondrial DNA and is essential for the production of ATP. This indicates that the expression of ATP synthesis-related genes at the 2-cell stage may be required to maintain normal development in vitro. S:-Adenosylmethionine decarboxylase decarboxylates adenosylmethionine, which is a substrate of DNA methylation. The expression of S:-AMDC may be responsible for the low level of methylation of preimplantation development. As NASP is a histone-binding protein that is thought to be testis and sperm specific, its function in embryos remains unclear. On the other hand, the Tcl1 gene and a novel gene, the c-1 gene, were strongly expressed in embryos cultured without oviductal tissue (blocking culture condition). The expression patterns of these genes are quite similar. However, the detailed functions of these genes in embryos remain to be determined.

Oxidative phosphorylation-dependent and -independent oxygen consumption by individual preimplantation mouse embryos

The self-referencing electrode technique was employed to noninvasively measure gradients of dissolved oxygen in the medium immediately surrounding developing mouse embryos and, thereby, characterized changes in oxygen consumption and utilization during development. A gradient of depleted oxygen surrounded each embryo and could be detected >50 microm from the embryo. Blastocysts depleted the surrounding medium of 0.6+/-0.1 microM of oxygen, whereas early cleavage stage embryos depleted the medium of only 0.3+/-0.1 microM of oxygen, suggesting a twofold increase in oxygen consumption at the blastocyst stage. Mitochondrial oxidative phosphorylation (OXPHOS) accounted for 60-70% of the oxygen consumed by blastocysts, while it accounted for only 30% of the total oxygen consumed by cleavage-stage embryos. The amount of oxygen consumed by non-OXPHOS mechanisms remained relatively constant throughout preimplantation development. By contrast, the amount of oxygen consumed by OXPHOS in blastocysts is greater than that consumed by OXPHOS in cleavage-stage embryos. The amount of oxygen consumed by one-cell embryos was modulated by the absence of pyruvate from the culture medium. Treatment of one-cell embryos and blastocysts with diamide, an agent known to induce cell death in embryos, resulted in a decline in oxygen consumption, such that the medium surrounding dying embryos was not as depleted of oxygen as that surrounding untreated control embryos. Together these results validate the self-referencing electrode technique for analyzing oxygen consumption and utilization by preimplantation embryos and demonstrate that changes in oxygen consumption accompany important physiological events, such as development, response to medium metabolites, or cell death.

Mitochondrial ATPase 6 gene expression in unfertilized oocytes and cleavage-stage embryos

OBJECTIVE

To compare the level of mitochondrial ATPase 6 gene expression in unfertilized oocytes and cleavage-stage embryos.

DESIGN

Reverse transcription polymerase chain reaction was performed in unfertilized oocytes and cleavage-stage embryos derived from tripronucleate embryos to determine ATPase 6 gene expression.

SETTING

Department of Obstetrics and Gynecology, Human Genetics Laboratory, Infertility Medical Center of CHA General Hospital, College of Medicine, Pochon CHA University, Seoul, Korea.

PATIENT(S)

Oocytes were obtained from infertile couples undergoing in vitro fertilization.

INTERVENTION(S)

Unfertilized oocytes collected at 48 hours after retrieval and cleavage-stage embryos derived from tripronucleate embryos were prepared for evaluation of mitochondrial gene expression.

MAIN OUTCOME MEASURE(S)

Comparison of ATPase 6 gene expression by using single-cell reverse transcription polymerase chain reaction.

RESULT(S)

Expression of unfertilized oocytes decreased compared with early cleavage-stage embryos.

CONCLUSION(S)

Our findings of decreased ATPase 6 expression in unfertilized oocytes suggest that there may be a decrease in the mitochondrial functional capacity of oxidative phosphorylation.

Mitochondrial genotype segregation and effects during mammalian development: applications to biotechnology

Mitochondria are endosymbiotic organelles responsible for energy production in practically every eukaryotic cell. Their uniparental fashion of inheritance, maternally inherited in mammals, and the homogeneity of mitochondrial DNA (mtDNA) within individuals and matrilineages, are biological phenomena that remain unexplained. This paper reviews some of the recent findings on mitochondrial influences on the manner in which embryos develop and how their genotypes are inherited in mammals, with particular emphasis on the genetic "bottleneck" effect. Animal models carrying a mix of mtDNAs (heteroplasmic) have been produced by karyoplast and cytoplast transplantation to analyze the segregation patterns at different stages during embryogenesis, in fetuses and offspring. Comparisons performed between murine and bovine reveal interesting changes in segregation and replication of transplanted mtDNAs. We have recently obtained Bos indicus and Bos taurus fetuses and calves from embryos reconstructed using enucleated polymorphic oocytes of Bos taurus origin. These and other findings on mitochondrial biology will have important implications in determining the cytoplasmic genotype of clones and in the preservation of endangered breeds and species.

Altered expression of mitochondrial 16S ribosomal RNA in p53-deficient mouse embryos revealed by differential display

Inactivation of the tumor suppressor p53 is associated with neural tube defects and altered teratogenicity in early embryos. To gain insight into the function of p53 during early embryogenesis, RNA profiles of wild-type p53(+/+) and p53(-/-) null mutant mouse embryos were compared at the head-fold stage (day 8 post coitum) using HPLC-based mRNA differential display. The results of this screen revealed a deficiency of mitochondrial 16S ribosomal RNA in p53(-/-) embryos. RT-PCR showed abnormalities in 16S rRNA levels relative to some representative nuclear (COIV, beta-actin) and mitochondrial (COIII) transcripts in p53(-/-) embryos, and that 16S rRNA expression increased with development of p53(+/+) embryos during neurulation. Embryos that lack p53 also displayed weakened cytochrome c oxidase staining and reduced ATP content. During neurulation, the mouse embryo switches from an anaerobic (glycolytic) to an aerobic (oxidative) metabolism. The preliminary results of the present study suggest that p53 may be involved, directly or indirectly, in this transition.

Mitochondrial genotype segregation during preimplantation development in mouse heteroplasmic embryos

Mitochondrial DNA content remains constant between the mature egg and the blastocyst stage in mammals, making this the only period in development when genotypes segregate to daughter cells without the confounding effect of genotype replication. To analyze the segregation patterns of mitochondrial DNA during preimplantation development, we introduced polymorphic mitochondria either peripherally (cytoplast transplantation) or in the perinuclear vicinity (karyplast transplantation) into zygotes. Genotype ratios were significantly more variable among blastomeres from cytoplast (coefficient of variation = 83.8%) than karyoplast (coefficient of variation = 34.7%) reconstructed zygotes. These results suggest that heteroplasmy caused by polymorphic mitochondria positioned in the periphery of oocytes at the time of fertilization shows a more stringent segregation pattern than when the organelle is in the vicinity of the nucleus. Moreover, donor-to-host mitochondrial genotype ratios in karyoplast-derived groups increased significantly during development, particularly in the C57BL/6 group, where the ratio practically doubled between the four-cell (17.3%) and the blastocyst stage (29.6%). Although the mechanisms controlling this preferential replication of nuclear-type mitochondrial DNA are unknown, it is suggested that access to nuclear-derived transcription and replication factors could lead to the preferential replication of perinuclear mitochondrial genotypes during morula and blastocyst formation.

Multiple members of the connexin gene family participate in preimplantation development of the mouse

The connexin gene family, of which there are at least 12 members in rodents, encodes the protein subunits intercellular membrane channels (gap junction channels). Because of the diverse structural and biophysical properties exhibited by the different connexins, it has been proposed that each may play a unique role in development or homeostasis. We have begun to test this hypothesis in the preimplantation mouse embryo in which de novo gap junction assembly is a developmentally regulated event. As a first step, we have used reverse transcription-polymerase chain reaction (RT-PCR) to determine the connexin mRNA phenotype of mouse blastocysts, and have identified transcripts of connexins 30.3, 31, 31.1, 40, 43, and 45. Quantitative measurements indicated that all six of these connexin genes are transcribed after fertilization. They can be divided into two groups with respect to the timing of mRNA accumulation: Cx31, Cx43, and Cx45 mRNAs accumulate continuously from the two- or four-cell stage, whereas Cx30.3, Cx31.1, and Cx40 mRNAs accumulate beginning in the eight-cell stage. All six mRNAs were found to co-sediment with polyribosomes from their time of first appearance, indicating that all six are translated. The expression of Cx31.1 and Cx40 was examined by confocal immunofluorescence microscopy; whereas both could be detected in compacting embryos, only Cx31.1 could be seen in punctate membrane foci indicative of gap junctions. Taken together with other results (published or submitted), our findings indicate that at least four connexins (Cx31, 31.1, 43 and 45) contribute to gap junctions in preimplantation development. The expression of multiple connexin genes during this early period of embryogenesis (when there are only two distinct cell types) raises questions about the functional significance of connexin diversity in this context.