Epigenetic modification and imprinting of the mammalian genome during development

Polycystic Ovary Syndrome: Impact of Lipotoxicity on Metabolic and Reproductive Health

Importance

Polycystic ovary syndrome (PCOS) is the most common endocrinopathy of reproductive-aged women. Women with PCOS are at increased risk of developing several metabolic and reproductive abnormalities, including metabolic syndrome. Underlying the combined metabolic and reproductive dysfunction is lipotoxicity, defined as the ectopic deposition of lipid in nonadipose tissue where it induces oxidative stress linked with insulin resistance and inflammation.

Objective

To examine what metabolic components underlie insulin resistance in PCOS, how lipotoxicity through insulin resistance impairs metabolism and reproduction in these women, and why evidence-based, individualized management is essential for their care.

Evidence Acquisition

PubMed search was performed using relevant terms to identify journal articles related to the subject. Relevant textbook chapters were also used.

Results

Polycystic ovary syndrome by Rotterdam criteria represents a complex syndrome of heterogeneous expression with variable adverse metabolic and reproductive implications. Women with classic PCOS are often insulin resistant and at greatest risk of developing metabolic syndrome with preferential fat accumulation and weight gain. Moreover, PCOS women may also have an altered capacity to properly store fat, causing ectopic lipid accumulation in nonadipose tissue, including the ovaries, where it can perpetuate insulin resistance and inflammation and harm the oocyte.

Conclusions and Relevance

A personalized approach to managing PCOS is essential to improve the health of all PCOS women through cost-effective prevention and/or treatment, to minimize the risk of pregnancy complications in those individuals wishing to conceive, and to optimize the long-term health of PCOS women and their offspring.

The Exposome Research Paradigm: an Opportunity to Understand the Environmental Basis for Human Health and Disease

PURPOSE OF REVIEW

This paper presents an overview of the exposome research paradigm with particular application to understanding human reproduction and development and its implications for health across a lifespan.

RECENT FINDINGS

The exposome research paradigm has generated considerable discussion about its feasibility and utility for delineating the impact of environmental exposures on human health. Early initiatives are underway, including smaller proof-of-principle studies and larger concerted efforts. Despite the notable challenges underlying the exposome paradigm, analytic techniques are being developed to handle its untargeted approach and correlated and multi-level or hierarchical data structures such initiatives generate, while considering multiple comparisons. The relatively short intervals for critical and sensitive windows of human reproduction and development seem well suited for exposome research and may revolutionize our understanding of later onset diseases. Early initiatives suggest that the exposome paradigm is feasible, but its utility remains to be established with applications to population human health research.

Nutrition and maternal metabolic health in relation to oocyte and embryo quality: critical views on what we learned from the dairy cow model

Although fragmented and sometimes inconsistent, the proof of a vital link between the importance of the physiological status of the mother and her subsequent reproductive success is building up. High-yielding dairy cows are suffering from a substantial decline in fertility outcome over past decades. For many years, this decrease in reproductive output has correctly been considered multifactorial, with factors including farm management, feed ratios, breed and genetics and, last, but not least, ever-rising milk production. Because the problem is complex and requires a multidisciplinary approach, it is hard to formulate straightforward conclusions leading to improvements on the 'work floor'. However, based on remarkable similarities on the preimplantation reproductive side between cattle and humans, there is a growing tendency to consider the dairy cow's negative energy balance and accompanying fat mobilisation as an interesting model to study the impact of maternal metabolic disorders on human fertility and, more specifically, on oocyte and preimplantation embryo quality. Considering the mutual interest of human and animal scientists studying common reproductive problems, this review has several aims. First, we briefly introduce the 'dairy cow case' by describing the state of the art of research into metabolic imbalances and their possible effects on dairy cow reproduction. Second, we try to define relevant in vitro models that can clarify certain mechanisms by which aberrant metabolite levels may influence embryonic health. We report on recent advances in the assessment of embryo metabolism and meantime critically elaborate on advantages and major limitations of in vitro models used so far. Finally, we discuss hurdles to be overcome to successfully translate the scientific data to the field.

Supplement of autologous ooplasm into porcine somatic cell nuclear transfer embryos does not alter embryo development

Somatic cell nuclear transfer (SCNT) is considered as the technique in which a somatic cell is introduced into an enucleated oocyte to make a cloned animal. However, it is unavoidable to lose a small amount of the ooplasm during enucleation step during SCNT procedure. The present study was aimed to uncover whether the supplement of autologous ooplasm could ameliorate the oocyte competence so as to improve low efficiency of embryo development in porcine SCNT. Autologous ooplasm-transferred (AOT) embryos were generated by the supplementation with autologous ooplasm into SCNT embryos. They were comparatively evaluated with respect to embryo developmental potential, the number of apoptotic body formation and gene expression including embryonic lineage differentiation, apoptosis, epigenetics and mitochondrial activity in comparison with parthenogenetic, in vitro-fertilized (IVF) and SCNT embryos. Although AOT embryos showed perfect fusion of autologous donor ooplasm with recipient SCNT embryos, the supplement of autologous ooplasm could not ameliorate embryo developmental potential in regard to the rate of blastocyst formation, total cell number and the number of apoptotic body. Furthermore, overall gene expression of AOT embryos was presented with no significant alterations in comparison with that of SCNT embryos. Taken together, the results of AOT demonstrated inability to make relevant values improved from the level of SCNT embryos to their IVF counterparts.

Health of Human and Livestock Conceived by Assisted Reproduction

Assisted reproduction is used to resolve infertility problems in human and in breeding programs to generate livestock. Except for gestation length and birth weight, perinatal outcome of children conceived by In Vitro Fertilization is similar to that of spontaneously conceived children. However, large offspring syndrome observed after In Vitro Production in livestock is quite alarming. The distinct parts of assisted reproduction (oocyte maturation, fertilization and culture) have been found to contribute to abnormal fetal growth and development. Genomic imprinting is suggested to be involved in the induction of the aberrant phenotypes observed after assisted reproduction. Furthermore, current knowledge on postnatal health of offspring conceived by assisted reproduction and speculations on potential longterm effects of In Vitro Fertilization will be described.

Extensive effects of in vitro oocyte maturation on rhesus monkey cumulus cell transcriptome

The elaboration of a quality oocyte is integrally linked to the correct developmental progression of cumulus cell phenotype. In humans and nonhuman primates, oocyte quality is diminished with in vitro maturation. To determine the changes in gene expression in rhesus monkey cumulus cells (CC) that occur during the final day prior to oocyte maturation and how these changes differ between in vitro (IVM) and in vivo maturation (VVM), we completed a detailed comparison of transcriptomes using the Affymetrix gene array. We observed a large number of genes differing in expression when comparing IVM-CC and VVM-CC directly but a much larger number of differences when comparing the transitions from the prematuration to the post-IVM and post-VVM states. We observed a truncation or delay in the normal pattern of gene regulation but also remarkable compensatory changes in gene expression during IVM. Among the genes affected by IVM are those that contribute to productive cell-cell interactions between cumulus cell and oocyte and between cumulus cells. Numerous genes involved in lipid metabolism are incorrectly regulated during IVM, and the synthesis of sex hormones appears not to be suppressed during IVM. We identified a panel of 24 marker genes, the expression of which should provide the foundation for understanding how IVM can be improved for monitoring IVM conditions and for diagnosing oocyte quality.

Developmental stage-specific imprinting of IPL in domestic pigs (Sus scrofa)

Imprinted in placenta and liver (IPL) gene has been identified as an imprinted gene in the mouse and human. Its sequence and imprinting status, however, have not been determined in the domestic pigs. In the present study, a 259 base pair-specific sequence for IPL gene of the domestic pig was obtained and a novel SNP, a T/C transition, was identified in IPL exon 1. The C allele of this polymorphism was found to be the predominant allele in Landrace,Yorkshire, and Duroc. The frequency of CC genotype and C allele are different in Duroc as compared with Yorkshire (P = .038 and P = .005, resp.). Variable imprinting status of this gene was observed in different developmental stages. For example, it is imprinted in 1-dayold newborns (expressed from the maternal allele), but imprinting was lost in 180-day-old adult (expressed from both parental alleles). Real-time PCR analysis showed the porcine IPL gene is expressed in all tested eight organ/tissues. The expression level was significantly higher in spleen, duodenum, lung, and bladder of 180-day-old Lantang adult compared to that in 1-day-old newborns Lantang pigs (P < .05). In conclusion, the imprinting of the porcine IPL gene is developmental stage and tissue specific.

Molecular and enzymatic profiles of mammalian DNA methyltransferases: structures and targets for drugs

DNA methylation is an epigenetic event involved in a variety array of processes that may be the foundation of genetic phenomena and diseases. DNA methyltransferase is a key enzyme for cytosine methylation in DNA, and can be divided into two functional families (Dnmt1 and Dnmt3) in mammals. All mammalian DNA methyltransferases are encoded by their own single gene, and consisted of catalytic and regulatory regions (except Dnmt2). Via interactions between functional domains in the regulatory or catalytic regions and other adaptors or cofactors, DNA methyltransferases can be localized at selective areas (specific DNA/nucleotide sequence) and linked to specific chromosome status (euchromatin/heterochromatin, various histone modification status). With assistance from UHRF1 and Dnmt3L or other factors in Dnmt1 and Dnmt3a/Dnmt3b, mammalian DNA methyltransferases can be recruited, and then specifically bind to hemimethylated and unmethylated double-stranded DNA sequence to maintain and de novo setup patterns for DNA methylation. Complicated enzymatic steps catalyzed by DNA methyltransferases include methyl group transferred from cofactor Ado-Met to C5 position of the flipped-out cytosine in targeted DNA duplex. In the light of the fact that different DNA methyltransferases are divergent in both structures and functions, and use unique reprogrammed or distorted routines in development of diseases, design of new drugs targeting specific mammalian DNA methyltransferases or their adaptors in the control of key steps in either maintenance or de novo DNA methylation processes will contribute to individually treating diseases related to DNA methyltransferases.

Effects of ooplasm transfer on paternal genome function in mice

BACKGROUND

The ooplasm plays a central role in forming the paternal pronucleus, and subsequently in regulating the expression of paternally inherited chromosomes. Previous studies in mice have revealed genetic differences in paternal genome processing by ooplasm of different genotypes. Ooplasm donation coupled to intracytoplasmic sperm injection (ICSI) has been used in human assisted reproductive technology (ART). This procedure exposes the developing paternal pronucleus to 'foreign' ooplasm, which may direct aberrant epigenetic processing. The potential effects of the foreign ooplasm on epigenetic information in the paternal pronucleus are unknown; however, some human progeny from ooplasm donation procedures display abnormalities.

METHODS

In this study, we employed inter-genotype ooplasm transfer followed by ICSI using two mouse strains, C57BL/6 and DBA/2, to explore the influence of foreign ooplasm on paternal pronucleus function. In order to assay for effects on the paternal genome without masking effects of the maternal genome, we examined ooplasm effects in diploid androgenones, which are produced by pronuclear transfer to contain exclusively two paternal sets of chromosomes, in combination with ICSI.

RESULTS

There was no significant effect of intra-strain ooplasm transfer among androgenones made with either C57BL/6 or DBA/2 oocytes. There was a significant negative effect on androgenone blastocyst development with inter-genotype transfer (10% volume) of DBA/2 ooplasm to C57BL/6 oocytes (P < 0.05). The reciprocal inter-genotype ooplasm transfer had no significant effect.

CONCLUSIONS

Thus, inter-genotype ooplasm transfer in conjunction with ICSI can alter the function of the paternal genome. However, the effect of foreign ooplasm is restricted to a negative effect, with no evidence of a positive effect. This study provides important new information about the possible consequences of ooplasm donation in human ART.

Altered gene expression in cloned piglets

Studies on cloned pigs are scant compared with those in mice and cattle. Expression profiles of cloned pig embryos on full-term cloned pigs are even more limited owing to the limited availability of DNA microarray technology in the pig. We have conducted expression profile comparisons between pigs from somatic cell nuclear transfer and pigs from conventional breeding at birth and 1 month of age. Differentially expressed genes that are subjected to DNA methylation were also examined for their DNA methylation status. These data will be presented in the 2009 Annual Meeting of the International Embryo Transfer Society in San Diego. In the present review, we focus on summarising existing findings on epigenetic and other changes in cloned embryo, cloned pigs and their offspring by conventional breeding.

Hybrid vigor and transgenerational epigenetic effects on early mouse embryo phenotype

Mouse embryos display a strain-dependent propensity for blastomere cytofragmentation at the two-cell stage. The maternal pronucleus exerts a predominant, transcription-dependent effect on this phenotype, with lesser effects of the ooplasm and the paternal pronucleus. A parental origin effect has been observed as an inequality in the cytofragmentation rate of embryos produced through genetic crosses of reciprocal F(1) hybrid females. To understand the basis for this, we conducted an extensive series of pronuclear transfer studies employing different combinations of inbred and F(1) hybrid maternal and paternal genotypes. We find that the parental origin effect is the result of a transgenerational epigenetic modification, whereby the inherited maternal grandpaternal contribution interacts with the fertilizing paternal genome and the ooplasm. This result indicates that some epigenetic information related to grandparental origins of chromosomes (i.e., imprinting of chromosomes in the mother) is retained through oogenesis and transmitted to progeny, where it affects gene expression from the maternal pronucleus and subsequent embryo phenotype. These results reveal for the first time that mammalian embryonic development can be affected by the epigenotype of at least three individuals. Additionally, we observe a significant suppression of fragmentation by F(1) hybrid ooplasm when it is separated from the F(1) hybrid maternal pronucleus. This latter effect is a striking example of heterosis in the early mammalian embryo, and it provides a new opportunity for examining the molecular mechanisms of heterosis. These results are relevant to our understanding of the mechanisms of epigenetic effects on development and the possible fertility effects of genetic and epigenetic interactions in reproductive medicine.

Implications of polycystic ovary syndrome on oocyte development

Human follicle development requires the recruitment of primordial follicles into a cohort of growing follicles from which one follicle is selected to ovulate a mature oocyte. During this developmental process, complex endocrine and intraovarian paracrine signals create a changing intrafollicular hormonal milieu. With this microenvironment, appropriate cumulus cell-oocyte signaling governs oocyte developmental competence, defined as the ability of the oocyte to complete meiosis and undergo fertilization, embryogenesis, and term development. Many of these mechanisms are perturbed in polycystic ovary syndrome (PCOS), a heterogeneous syndrome characterized by ovarian hyperandrogenism, hyperinsulinemia from insulin resistance, and reduced fecundity. In addition to these endocrinopathies, PCOS also is characterized by paracrine dysregulation of follicle development by intraovarian proteins of the transforming growth factor-beta family. Consequently, PCOS patients undergoing ovarian stimulation for in vitro fertilization are at increased risks of impaired oocyte developmental competence, implantation failure, and pregnancy loss. Recent data demonstrate links between endocrine/paracrine factors and oocyte gene expression in PCOS and suggest that new clinical strategies to optimize developmental competence of PCOS oocytes should target correction of the entire follicle growth and oocyte development process.

Polycystic ovary syndrome and oocyte developmental competence

Folliculogenesis is a complex process, in which multiple endocrine and intraovarian paracrine interactions create a changing intrafollicular microenvironment for appropriate oocyte development. Within this microenvironment, bidirectional cumulus cell-oocyte signaling governs the gradual acquisition of developmental competence by the oocyte, defined as the ability of the oocyte to complete meiosis and undergo fertilization, embryogenesis, and term development. These regulatory mechanisms of follicle growth, controlled in part by the oocyte itself, are susceptible to derangement in polycystic ovary syndrome (PCOS), a heterogeneous syndrome characterized by ovarian hyperandrogenism, insulin resistance, and paracrine dysregulation of follicle development. Consequently, only a subset of PCOS patients experience reduced pregnancy outcome after ovarian stimulation for in vitro fertilization. Recent data implicate functional associations between endocrine/paracrine abnormalities, metabolic dysfunction, and altered oocyte gene expression with impaired oocyte developmental competence in women with PCOS. Therefore, an understanding of how developmentally relevant endocrine/paracrine factors interact to promote optimal oocyte developmental is crucial to identify those PCOS patients who might benefit from long-term correction of follicle growth to improve fertility, optimize follicular responsiveness to gonadotropin therapy, and enhance pregnancy outcome by in vitro fertilization.

Rescue of the mouse DDK syndrome by parent-of-origin-dependent modifiers

When females of the DDK inbred mouse strain are mated to males of other strains, 90-100% of the resulting embryos die during early embryonic development. This DDK syndrome lethality results from incompatibility between an ooplasmic DDK factor and a non-DDK paternal gene, which map to closely linked loci on chromosome 11. It has been proposed that the expression of the gene that encodes the ooplasmic factor is subject to allelic exclusion in oocytes. Previous studies have demonstrated the existence of recessive modifiers that increase lethality in the C57BL/6 and BALB/c strains. These modifiers are thought to skew the choice of allele undergoing allelic exclusion in the oocytes of heterozygous females. In the present study, we demonstrate the presence of modifiers in three Mus musculus domesticus wild-derived strains, PERA, PERC, and RBA. These modifiers completely rescued DDK syndrome lethality. We mapped the major locus that is responsible for rescue in PERA and PERC crosses to proximal chromosome 13 and named this locus Rmod1 (Rescue Modifier of the DDK Syndrome 1). Our experiments demonstrate that PERA or PERC alleles at Rmod1 rescue lethality independently of allelic exclusion. In addition, rescue of the lethal phenotype depends on the parental origin of the Rmod1 alleles; transmission through the dam leads to rescue, while transmission through the sire has no effect.

Dynamic developmental regulation of the large non-coding RNA associated with the mouse 7C imprinted chromosomal region

The mouse ortholog of the Prader-Willi/Angelman syndrome imprinted domain contains several paternal-specific transcripts and the maternally expressed gene encoding ubiquitin protein ligase E3A (Ube3a). A Large paternal Non-Coding RNA, encompassing Snurf-Snrpn exons and the Ube3a Antisense Transcript (Ube3a-ATS), has been recently characterized and named here LNCAT. Potential roles of LNCAT in imprinting, gene regulation, and disease are likely but have not been investigated. In order to establish the function(s) of LNCAT, we first determined its in vivo spatio-temporal expression pattern at the cellular level during development and in different adult brain tissues. We show here that LNCAT is developmentally regulated, with transcript variants being specifically expressed through neuronal differentiation in postmitotic neurons. We demonstrate that the LNCAT and Snurf-Snrpn transcripts are independent although they share common exons. We show an absence of expression of LNCAT through gametogenesis and in early embryo excluding a role of LNCAT in the imprint establishment. We also report a range of observations that challenges the widely accepted model of imprinted gene silencing of Ube3a. Although these last data do not completely exclude that the LNCAT variants including "Ube3a-ATS"exons could repress the paternal allele of Ube3a, they do allow us to propose an alternative and consistent model.

Environmental exposures, toxicologic mechanisms, and adverse pregnancy outcomes

Environmental risk factors (defined as those agents and stresses that are generally the responsibility of environmental agencies) are often tangible indicators of economic and social disparity in the United States. Many site-specific analyses have reported that communities of color and poverty are exposed more often and more intensively to such environmental hazards as lead, air pollution, agrochemicals, incinerator emissions, and releases from hazardous waste sites. Thus, exposures to these toxicants may explain part of the socioeconomic disparity that is observed in terms of risks of adverse pregnancy outcomes. The purpose of this study was to describe the associations between certain environmental exposures and reproductive outcomes through a discussion of both epidemiologic and animal model studies. In addition, we list potential sources of exposure data and describe physiologic changes in pregnancy that may increase the likelihood of both external exposures and increased internal dose. Several models for further study of environmental risk factors are suggested to increase our understanding of gene-environment interactions toward the goal of indentifying preventable risk factors to improve reproductive outcomes of particular concern to disadvantaged populations.

Recapitulation of the Ovum Mutant (Om) Phenotype and Loss of Om Locus Polarity in Cloned Mouse Embryos1

The ovum mutant (Om) locus in mice affects early interactions between sperm and egg that in turn affect viability of embryos beyond the morula stage. Crosses of DDK females to males of many other inbred strains are 95% lethal around the morula stage, whereas reciprocal crosses are fully viable. Available data indicate that the early lethality is the result of an interaction between a factor in the ooplasm and the paternal genome. In this study, we examined whether this lethal interaction would likewise occur in cloned embryos produced by somatic cell nuclear transfer. We find that the Om effect is recapitulated but that the parental origin effect at the Om locus is no longer evident in cloned embryos.

Nuclear transfer in rodents

Cloning is the asexual reproduction of an individual, such that the offspring have an essentially identical nuclear genome. Nuclear transfer and cloning have been achieved in a number of species, namely sheep, cows, goats, rabbits, cats and mice, but have been largely unsuccessful, so far, in dogs, primates and rats. Clearly, contributory factors which affect the outcome of successful cloning experiments are not universally applicable to all species. One theme common to all cloning experiments, however, is the overall inefficiency of the process, typically 0-4%. A number of factors contribute to nuclear transfer inefficiency, and we will review mouse cloning experiments, which address these problems, highlighting the importance of donor nucleus choice (somatic or ES cell, fetal or adult, quiescent or actively dividing). Finally, we will summarize the emerging principles which appear to govern nuclear reprogramming and production of clones, and will consider the application of nuclear transfer to the rat.

Cytoplasmic transfer in the mouse in conjunction with intracytoplasmic sperm injection

Cytoplasmic transfer (CT) from a donor to a recipient oocyte has been used clinically to facilitate human pregnancies. Data reported here describe the first characterization of CT coincident with intracytoplasmic sperm injection in the mouse system. Sibling oocytes were used to transfer 2, 4, or 6 pl of ooplasm to a recipient egg along with a sperm head using piezo-actuated injection. Survival and fertilization after CT were comparable to controls at 2 pl and 4 pl, but survival was significantly reduced with 6 pl volumes. Development to the blastocyst stage was also inversely related to CT volume, with some decline beginning with the 4 pl CT group. However, some blastocysts did develop in all of the groups. The results are in contrast with human eggs, which tolerate larger CT volumes. Results indicate that the mouse system can be used to characterize the transfer of exogenous materials concomitant with sperm injection, provided that the CT volume is not excessive.

Reprogramming of epigenetic inheritance by somatic cell nuclear transfer

Somatic cloning by nuclear transfer returns a differentiated cell to a totipotent stage, a process termed nuclear reprogramming. During this de-differentiation process, genes inactivated during tissue differentiation are re-activated in a temporal and spatial special manner. It is believed that tissue differentiation occurs through epigenetic mechanisms, genetic inheritance that does not involve changes in DNA sequences. Developmental abnormalities and a high mortality rate in cloned offspring have frequently been observed and probably result from incomplete nuclear reprogramming. In this review, the reprogramming of two epigenetic mechanisms, imprinting and X chromosome inactivation, as well as recent attempts to modify pre-existing epigenetic marks in donor cells to improve nuclear transfer efficacy, are discussed.

Natural Selection and the Evolution of Genome Imprinting

Sexual reproduction results from the fusion of gametes in which the chromatin configuration of maternal and paternal chromosomes is distinct at fertilization. Although many of the differences are erased during successive cellular divisions and chromatin modifications, some are retained in both somatic and germline cells. These epigenetic modifications can confer different characteristics on maternal and paternal chromosomes and such differences can be selected during any process that has the ability to distinguish between homologues. The end result of these selective forces are parental origin effects, writ large. The range of effects observed, including transcriptional imprinting and effects on chromosome segregation and heterochromatization, reflects the diversity of selective forces in operation. However, a closer look at these effects suggests that parental origin-dependent differences in chromatin structure might be subject to some common forces and that these forces may explain many of the "nontranscriptional" parental origin effects observed in mammals.

Ooplasmic transfer: animal models assist human studies

Ooplasmic transplantation is based on the premise that ooplasmic components are compromised in some individuals. In theory, the transfer of small amounts of healthy ooplasm can correct such deficits, allowing for improved development and implantation. The technique is based on a well-established background of experimental embryology demonstrating that cytoplasmic manipulation in oocytes and early embryos can be entirely compatible with normal development. Cytoplasm has been manipulated via karyoplast and cytoplast transfer and by cytoplasmic injection. Term development has been obtained following such manipulations in a variety of mammalian species. While some manipulative scenarios have exhibited compromised development, others have exhibited improved development. Developmental problems involving specific epigenetic and mitochondrial incompatibilities have been observed in a very limited subset of animal studies. These studies are based on genetic and physical models that have little relation to the actual substance of ooplasmic transplantation in the human. In fact, the majority of animal studies suggest that ooplasmic transplantation is well-founded and unlikely to result in negative developmental consequences. Furthermore, there are considerable physical, physiological and developmental differences between human and rodent eggs and embryos. These differences suggest that potentially negative issues raised by rodent results may not be relevant in the human.

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.

Factors controlling embryo viability

Many diverse factors affect embryo viability. The morphological measures routinely used to grade human embryos are of limited use as many of the factors influencing the longterm viability of embryos are genetic or molecular and are undetectable or inconclusive from visualization of living embryos by microscopy. This article presents examples of factors that are known to affect embryo viability, including gamete formation, embryonic genome activation, and im-printing. Aspects of both gamete and embryo development are addressed, and the possibility that various anomalies remain hidden for extended periods before impacting upon a later aspect of development is hypothesized. In future, more detailed and informative assessments of embryo viability before embryo transfer may require invasive approaches to study the composition of embryos at various stages of preimplantation development; however, indirect, non-invasive measures would be preferable.

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.

Unequal expression of allelic kainate receptor GluR7 mRNAs in human brains

We describe here the first example of an exonic polymorphism that affects the primary structure of a human ionotropic glutamate receptor. The human kainate receptor GluR7 gene contains a thymine (T)/guanine (G) nucleotide variation that determines a serine or alanine at position 310 in the extracellular region of GluR7 receptor subunits. Our finding contrasts with a previous report that suggested that GluR7 transcripts were RNA-edited at this site. Whole-cell patch-clamp recordings did not detect differences in receptor activation and desensitization between the human GluR7 receptor isoforms expressed in HEK-293 cells. Analysis of 41 tissue samples obtained from 30 human brains revealed expression level differences between GluR7 alleles expressed in the same brain. The expression level of the allelic GluR7 mRNAs differed in 27 samples from 1.2- to 12.7-fold. Unequal expression level of allelic mRNAs is characteristic for genes that are affected by genomic imprinting or that contain mutations. Genomic imprinting in most cases is conserved between human and mice. However, we did not detect unequal expression of allelic GluR7 mRNAs in mice. Our results are important for future studies that explore a potential role or roles for GluR7 receptors in the brain and for neurological disorders.

Conditions that affect acquisition of developmental competence by mouse oocytes in vitro: FSH, insulin, glucose and ascorbic acid

The simplest unit required for the support of oocyte growth and development is the oocyte-granulosa cell complex. Therefore, a culture system was established that utilizes these complexes to assess mechanisms promoting nuclear, cytoplasmic and genomic maturation in mammalian oocytes. Deletion of serum from the culture, results in increased apoptosis in oocyte-associated granulosa cells (OAGCs), however, addition of ascorbic acid (0.5 mM) significantly reduced the level of apoptosis in the OAGCs, although no improvement of oocyte developmental competence was detected. The effects of reducing glucose during oocyte growth were studied since, under some culture conditions, glucose has deleterious effects on early preimplantation development. Reducing the glucose concentration to 1 mM resulted in the production of oocytes with greatly reduced developmental competence. Deleterious effects of FSH plus insulin during oocyte growth in vitro on preimplantation development are reviewed and discussed in terms of the communication of oocytes with inappropriately developing granulosa cells. Evidence that oocytes promote the appropriate differentiation of OAGCs in intact follicles in vivo is also discussed. It is hypothesized that oocytes control the differentiation of these cells, in order to promote intercellular signaling essential for the acquisition of competence to undergo normal embryogenesis.

Improving the safety of embryo technologies: possible role of genomic imprinting

Although developments in mammalian in vitro embryo technologies have allowed many new clinical and agricultural achievements, their application has been hindered by limitations in the developmental potential of resulting embryos. Low efficiencies of development to the pre-implantation blastocyst stage have been consistently observed in most species, including humans, rabbits, pigs and ruminants. Furthermore, in cattle and sheep a wide range of congenital abnormalities currently termed "Large Offspring syndrome" (LOS) are commonly observed as a result of several embryo culture and manipulation procedures. This paper reviews the hypothesis that at least some of the problems associated with embryo technologies may result from disruptions in imprinted genes. Several imprinted genes (i.e. genes which express only the maternal or paternal allele) are known to have significant effects on fetal size and survival in other species and are possible candidates for involvement in livestock LOS. Major changes in putative imprinting mechanisms such as DNA methylation of imprinted genes occur in the mouse embryo during pre-implantation development. Alterations in DNA methylation are stabley transmitted through repeated cell cycles such that changes in the embryo may still act at the fetal stages. Thus any disruption in establishment and/or maintenance of imprinting during the vulnerable periods of embryo culture or manipulation is a plausible candidate mechanism for inducing fetal loss and Large Offspring Syndrome. Identification of these disruptions may provide crucial means to improve the success of current procedures.

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.

A macaque model for studying mechanisms controlling oocyte development and maturation in human and non-human primates

A model to study mechanisms controlling nuclear and cytoplasmic maturation of primate oocytes is being developed in our laboratory. The high incidence of pregnancy failure in women following in-vitro fertilization (IVF) may be partly attributed to inadequate cytoplasmic maturation of oocytes. Advancement of knowledge of mechanisms controlling primate oocyte maturation would have important implications for treatment of human infertility, and would potentially increase numbers of viable non-human primate embryos for biomedical research. Use of a non-human primate model to study oocyte and embryo biology avoids legal, ethical and experimental limitations encountered in a clinical situation. Using this model, the meiotic and developmental capacity of oocytes from three sources have been compared: (i) in-vivo matured oocytes from monkeys stimulated with follicle-stimulating hormone (FSH) and human chorionic gonadotrophin, (ii) in-vitro matured oocytes from monkeys primed with FSH, and (iii) in-vitro matured oocytes from non-stimulated monkeys. This work demonstrates that oocyte developmental competence is likely acquired both during follicle development, before meiotic resumption, and during meiotic progression, concurrent with nuclear maturation. Potential causes of developmental failure of in-vitro matured oocytes, implications for human infertility, and future strategies to study the regulation of primate oocyte maturation are discussed.