Reanalysis of mtDNA mutations of human primordial germ cells (PGCs) reveals NUMT contamination and suggests that selection in PGCs may be positive

The resilience of the mitochondrial genome (mtDNA) to a high mutational pressure depends, in part, on negative purifying selection in the germline. A paradigm in the field has been that such selection, at least in part, takes place in primordial germ cells (PGCs). Specifically, Floros et al. (Nature Cell Biology 20: 144-51) reported an increase in the synonymity of mtDNA mutations (a sign of purifying selection) between early-stage and late-stage PGCs. We re-analyzed Floros' et al. data and determined that their mutational dataset was significantly contaminated with single nucleotide variants (SNVs) derived from a nuclear sequence of mtDNA origin (NUMT) located on chromosome 5. Contamination was caused by co-amplification of the NUMT sequence by cross-specific PCR primers. Importantly, when we removed NUMT-derived SNVs, the evidence of purifying selection was abolished. In addition to bulk PGCs, Floros et al. reported the analysis of single-cell late-stage PGCs, which were amplified with different sets of PCR primers that cannot amplify the NUMT sequence. Accordingly, there were no NUMT-derived SNVs among single PGC mutations. Interestingly, single PGC mutations show adecreaseof synonymity with increased intracellular mutant fraction. More specifically, nonsynonymous mutations show faster intracellular genetic drift towards higher mutant fraction than synonymous ones. This pattern is incompatible with predominantly negative selection. This suggests that germline selection of mtDNA mutations is a complex phenomenon and that the part of this process that takes place in PGCs may be predominantly positive. However counterintuitive, positive germline selection of detrimental mtDNA mutations has been reported previously andpotentially may be evolutionarily advantageous.

Interpreting Sequence-Levenshtein distance for determining error type and frequency between two embedded sequences of equal length

Levenshtein distance is a commonly used edit distance metric, typically applied in language processing, and to a lesser extent, in molecular biology analysis. Biological nucleic acid sequences are often embedded in longer sequences and are subject to insertion and deletion errors that introduce frameshift during sequencing. These frameshift errors are due to string context and should not be counted as true biological errors. Sequence-Levenshtein distance is a modification to Levenshtein distance that is permissive of frameshift error without additional penalty. However, in a biological context Levenshtein distance needs to accommodate both frameshift and weighted errors, which Sequence-Levenshtein distance cannot do. Errors are weighted when they are associated with a numerical cost that corresponds to their frequency of appearance. Here, we describe a modification that allows the use of Levenshtein distance and Sequence-Levenshtein distance to appropriately accommodate penalty-free frameshift between embedded sequences and correctly weight specific error types.

Lineage-Mismatched Mitochondrial Replacement in an Inducible Mitochondrial Depletion Model Effectively Restores the Original Proteomic Landscape of Recipient Cells

In addition to critical roles in bioenergetics, mitochondria are key contributors to the regulation of many other functions in cells, ranging from steroidogenesis to apoptosis. Numerous studies further demonstrate that cell type-specific differences exist in mitochondria, with cells of a given lineage tailoring their endogenous mitochondrial population to suit specific functional needs. These findings, coupled with studies of the therapeutic potential of mitochondrial transplantation, provide a strong impetus to better understand how mitochondria can influence cell function or fate. Here an inducible mitochondrial depletion modelis used to study how cells lacking endogenous mitochondria respond, on a global protein expression level, to transplantation with lineage-mismatched (LM) mitochondria. It is shown that LM mitochondrial transplantation does not alter the proteomic profile in nonmitochondria-depleted recipient cells; however, enforced depletion of endogenous mitochondria results in dramatic changes in the proteomic landscape, which returns to the predepletion state following internalization of LM mitochondria. These data, derived from a cell system that can be rendered free of influence by endogenous mitochondria, indicate that transplantation of mitochondria-even from a source that differs significantly from the recipient cell population, effectively restores a normal proteomic landscape to cells lacking their own mitochondria.

Characterization of Oogonial Stem Cells in Adult Mouse Ovaries with Age and Comparison to In Silico Data on Human Ovarian Aging

Many adult somatic stem cell lineages are comprised of subpopulations that differ in gene expression, mitotic activity, and differentiation status. In this study, we explored if cellular heterogeneity also exists within oogonial stem cells (OSCs), and how chronological aging impacts OSCs. In OSCs isolated from mouse ovaries by flow cytometry and established in culture, we identified subpopulations of OSCs that could be separated based on differential expression of stage-specific embryonic antigen 1 (SSEA1) and cluster of differentiation 61 (CD61). Levels of aldehyde dehydrogenase (ALDH) activity were inversely related to OSC differentiation, whereas commitment of OSCs to differentiation through transcriptional activation of stimulated by retinoic acid gene 8 was marked by a decline in ALDH activity and in SSEA1 expression. Analysis of OSCs freshly isolated from ovaries of mice between 3 and 20 months of age revealed that these subpopulations were present and persisted throughout adult life. However, expression of developmental pluripotency associated 3 (Dppa3), an epigenetic modifier that promotes OSC differentiation into oocytes, was lost as the mice transitioned from a time of reproductive compromise (10 months) to reproductive failure (15 months). Further analysis showed that OSCs from aged females could be established in culture, and that once established the cultured cells reactivated Dppa3 expression and the capacity for oogenesis. Analysis of single-nucleus RNA sequence data sets generated from ovaries of women in their 20s versus those in their late 40s to early 50s showed that the frequency of DPPA3-expressing cells decreased with advancing age, and this was paralleled by reduced expression of several key meiotic differentiation genes. These data support the existence of OSC subpopulations that differ in gene expression profiles and differentiation status. In addition, an age-related decrease in Dppa3/DPPA3 expression, which is conserved between mice and humans, may play a role in loss of the ability of OSCs to maintain oogenesis with age.

Assaying Mitochondrial Function by Multiparametric Flow Cytometry

Flow cytometry has been a vital tool in cell biology for decades based on its versatile ability to detect and quantifiably measure both physical and chemical attributes of individual cells within a larger population. More recently, advances in flow cytometry have enabled nanoparticle detection. This is particularly applicable to mitochondria, which, as intracellular organelles have distinct subpopulations that can be evaluated based on differences in functional, physical, and chemical attributes, in a manner analogous to cells. This includes distinctions based on size, mitochondrial membrane potential (ΔΨ_m), chemical properties, and protein expression on the outer mitochondrial membrane in intact, functional organelles and internally in fixed samples. This method allows for multiparametric analysis of subpopulations of mitochondria, as well as collection for downstream analysis down to the level of a single organelle. The present protocol describes a framework for analysis and sorting mitochondria by flow cytometry, termed fluorescence activated mitochondrial sorting (FAMS), based on the separation of individual mitochondria belonging to subpopulations of interest using fluorescent dyes and antibody labeling.

Non-neutral clonal selection and its potential role in mammalian germline stem cell dysfunction with advancing age

The concept of natural selection, or "survival of the fittest", refers to an evolutionary process in nature whereby traits emerge in individuals of a population through random gene alterations that enable those individuals to better adapt to changing environmental conditions. This genetic variance allows certain members of the population to gain an advantage over others in the same population to survive and reproduce in greater numbers under new environmental pressures, with the perpetuation of those advantageous traits in future progeny. Here we present that the behavior of adult stem cells in a tissue over time can, in many respects, be viewed in the same manner as evolution, with each stem cell clone being representative of an individual within a population. As stem cells divide or are subjected to cumulative oxidative damage over the lifespan of the organism, random genetic alterations are introduced into each clone that create variance in the population. These changes may occur in parallel to, or in response to, aging-associated changes in microenvironmental cues perceived by the stem cell population. While many of these alterations will be neutral or silent in terms of affecting cell function, a small fraction of these changes will enable certain clones to respond differently to shifts in microenvironmental conditions that arise with advancing age. In some cases, the same advantageous genetic changes that support survival and expansion of certain clones over others in the population (viz. non-neutral competition) could be detrimental to the downstream function of the differentiated stem cell descendants. In the context of the germline, such a situation would be devastating to successful propagation of the species across generations. However, even within a single generation, the “evolution” of stem cell lineages in the body over time can manifest into aging-related organ dysfunction and failure, as well as lead to chronic inflammation, hyperplasia, and cancer. Increased research efforts to evaluate stem cells within a population as individual entities will improve our understanding of how organisms age and how certain diseases develop, which in turn may open new opportunities for clinical detection and management of diverse pathologies.

Dynamics of the most common pathogenic mtDNA variant m.3243A > G demonstrate frequency-dependency in blood and positive selection in the germline

The A-to-G point mutation at position 3243 in the human mitochondrial genome (m.3243A > G) is the most common pathogenic mtDNA variant responsible for disease in humans. It is widely accepted that m.3243A > G levels decrease in blood with age, and an age correction representing ~ 2% annual decline is often applied to account for this change in mutation level. Here we report that recent data indicate that the dynamics of m.3243A > G are more complex and depend on the mutation level in blood in a bi-phasic way. Consequently, the traditional 2% correction, which is adequate 'on average', creates opposite predictive biases at high and low mutation levels. Unbiased age correction is needed to circumvent these drawbacks of the standard model. We propose to eliminate both biases by using an approach where age correction depends on mutation level in a biphasic way to account for the dynamics of m.3243A > G in blood. The utility of this approach was further tested in estimating germline selection of m.3243A > G. The biphasic approach permitted us to uncover patterns consistent with the possibility of positive selection for m.3243A > G. Germline selection of m.3243A > G shows an 'arching' profile by which selection is positive at intermediate mutant fractions and declines at high and low mutant fractions. We conclude that use of this biphasic approach will greatly improve the accuracy of modelling changes in mtDNA mutation frequencies in the germline and in somatic cells during aging.

Uncertainty quantification for epidemiological forecasts of COVID-19 through combinations of model predictions

Scientific advice to the UK government throughout the COVID-19 pandemic has been informed by ensembles of epidemiological models provided by members of the Scientific Pandemic Influenza group on Modelling. Among other applications, the model ensembles have been used to forecast daily incidence, deaths and hospitalizations. The models differ in approach (e.g. deterministic or agent-based) and in assumptions made about the disease and population. These differences capture genuine uncertainty in the understanding of disease dynamics and in the choice of simplifying assumptions underpinning the model. Although analyses of multi-model ensembles can be logistically challenging when time-frames are short, accounting for structural uncertainty can improve accuracy and reduce the risk of over-confidence in predictions. In this study, we compare the performance of various ensemble methods to combine short-term (14-day) COVID-19 forecasts within the context of the pandemic response. We address practical issues around the availability of model predictions and make some initial proposals to address the shortcomings of standard methods in this challenging situation.

Workflow Optimization for Identification of Female Germline or Oogonial Stem Cells in Human Ovarian Cortex Using Single-Cell RNA Sequence Analysis

In 2004, the identification of female germline or oogonial stem cells (OSCs) that can support post-natal oogenesis in ovaries of adult mice sparked a major paradigm shift in reproductive biology. Although these findings have been independently verified, and further extended to include identification of OSCs in adult ovaries of many species ranging from pigs and cows to non-human primates and humans, a recent study rooted in single-cell RNA sequence analysis (scRNA-seq) of adult human ovarian cortical tissue claimed that OSCs do not exist, and that other groups working with OSCs following isolation by magnetic-assisted or fluorescence-activated cell sorting have mistaken perivascular cells (PVCs) for germ cells. Here we report that rare germ lineage cells with a gene expression profile matched to OSCs but distinct from that of other cells, including oocytes and PVCs, can be identified in adult human ovarian cortical tissue by scRNA-seq after optimization of analytical workflow parameters. Deeper cell-by-cell expression profiling also uncovered evidence of germ cells undergoing meiosis-I in adult human ovaries. Lastly, we show that, if not properly controlled for, PVCs can be inadvertently isolated during flow cytometry protocols designed to sort OSCs because of inherently high cellular autofluorescence. However, human PVCs and human germ cells segregate into distinct clusters following scRNA-seq due to non-overlapping gene expression profiles, which would preclude the mistaken identification and use of PVCs as OSCs during functional characterization studies.

3GOLD: optimized Levenshtein distance for clustering third-generation sequencing data

BACKGROUND

Third-generation sequencing offers some advantages over next-generation sequencing predecessors, but with the caveat of harboring a much higher error rate. Clustering-related sequences is an essential task in modern biology. To accurately cluster sequences rich in errors, error type and frequency need to be accounted for. Levenshtein distance is a well-established mathematical algorithm for measuring the edit distance between words and can specifically weight insertions, deletions and substitutions. However, there are drawbacks to using Levenshtein distance in a biological context and hence has rarely been used for this purpose. We present novel modifications to the Levenshtein distance algorithm to optimize it for clustering error-rich biological sequencing data.

RESULTS

We successfully introduced a bidirectional frameshift allowance with end-user determined accommodation caps combined with weighted error discrimination. Furthermore, our modifications dramatically improved the computational speed of Levenstein distance. For simulated ONT MinION and PacBio Sequel datasets, the average clustering sensitivity for 3GOLD was 41.45% (S.D. 10.39) higher than Sequence-Levenstein distance, 52.14% (S.D. 9.43) higher than Levenshtein distance, 55.93% (S.D. 8.67) higher than Starcode, 42.68% (S.D. 8.09) higher than CD-HIT-EST and 61.49% (S.D. 7.81) higher than DNACLUST. For biological ONT MinION data, 3GOLD clustering sensitivity was 27.99% higher than Sequence-Levenstein distance, 52.76% higher than Levenshtein distance, 56.39% higher than Starcode, 48% higher than CD-HIT-EST and 70.4% higher than DNACLUST.

CONCLUSION

Our modifications to Levenshtein distance have improved its speed and accuracy compared to the classic Levenshtein distance, Sequence-Levenshtein distance and other commonly used clustering approaches on simulated and biological third-generation sequenced datasets. Our clustering approach is appropriate for datasets of unknown cluster centroids, such as those generated with unique molecular identifiers as well as known centroids such as barcoded datasets. A strength of our approach is high accuracy in resolving small clusters and mitigating the number of singletons.

Role of Granulosa Cells in the Aging Ovarian Landscape: A Focus on Mitochondrial and Metabolic Function

Mitochondria are at the intersection of aging and fertility, with research efforts centered largely on the role that these specialized organelles play in the relatively rapid decline in oocyte quality that occurs as females approach reproductive senescence. In addition to various roles in oocyte maturation, fertilization, and embryogenesis, mitochondria are critical to granulosa cell function. Herein, we provide a review of the literature pertaining to the role of mitochondria in granulosa cell function, with emphasis on how mitochondrial aging in granulosa cells may impact reproduction in female mammals.

Biomechanical Strain Promotes the Differentiation of Murine Oogonial Stem Cells

Cells within tissues are routinely subjected to physiological stress and strain, arising from direct interactions with neighboring cells as well as with extracellular matrix components. Accordingly, there is tremendous interest in deciphering how cells sense, and respond to, changes in biomechanical forces. In this study, we explored the effects of mechanostimulation on the differentiation of mouse female germline or oogonial stem cells (OSCs) as a model for adult stem cell function. We report that increasing levels, or repeated application of a subthreshold fixed level, of radial strain to OSCs in culture significantly increased rates of in vitro oocyte formation as a measure of stem cell differentiation. These responses involved changes in F-actin-mediated cytoskeletal tension as well as in activation of intracellular signaling by Rho-associated protein kinase (ROCK) and Yes-associated protein (YAP) phosphorylation. In addition, application of mechanical strain to OSCs enhanced association of YAP with muscle-specific cytidine-adenosine-thymidine (MCAT) response elements in the promoter stimulated by retinoic acid gene 8 (Stra8), the transcriptional activation of which is required for germline meiotic commitment. These data indicate that biomechanical strain directly promotes the differentiation of adult female germline stem cells through a signaling pathway involving F-actin, ROCK, YAP, and Stra8.

LUCS: a high-resolution nucleic acid sequencing tool for accurate long-read analysis of individual DNA molecules

Nucleic acid sequence analyses are fundamental to all aspects of biological research, spanning aging, mitochondrial DNA (mtDNA) and cancer, as well as microbial and viral evolution. Over the past several years, significant improvements in DNA sequencing, including consensus sequence analysis, have proven invaluable for high-throughput studies. However, all current DNA sequencing platforms have limited utility for studies of complex mixtures or of individual long molecules, the latter of which is crucial to understanding evolution and consequences of single nucleotide variants and their combinations. Here we report a new technology termed LUCS (Long-molecule UMI-driven Consensus Sequencing), in which reads from third-generation sequencing are aggregated by unique molecular identifiers (UMIs) specific for each individual DNA molecule. This enables in-silico reconstruction of highly accurate consensus reads of each DNA molecule independent of other molecules in the sample. Additionally, use of two UMIs enables detection of artificial recombinants (chimeras). As proof of concept, we show that application of LUCS to assessment of mitochondrial genomes in complex mixtures from single cells was associated with an error rate of 1X10^-4 errors/nucleotide. Thus, LUCS represents a major step forward in DNA sequencing that offers high-throughput capacity and high-accuracy reads in studies of long DNA templates and nucleotide variants in heterogenous samples.

The obligate need for accuracy in reporting preclinical studies relevant to clinical trials: autologous germline mitochondrial supplementation for assisted human reproduction as a case study

A now large body of work has solidified the central role that mitochondria play in oocyte development, fertilization, and embryogenesis. From these studies, a new technology termed autologous germline mitochondrial energy transfer was developed for improving pregnancy success rates in assisted reproduction. Unlike prior clinical studies that relied on the use of donor, or nonautologous, mitochondria for microinjection into eggs of women with a history of repeated in vitro fertilization failure to enhance pregnancy success, autologous germline mitochondrial energy transfer uses autologous mitochondria collected from oogonial stem cells of the same woman undergoing the fertility treatment. Initial trials of autologous germline mitochondrial energy transfer during - in vitro fertilization at three different sites with a total of 104 patients indicated a benefit of the procedure for improving pregnancy success rates, with the birth of children conceived through the inclusion of autologous germline mitochondrial energy transfer during in vitro fertilization. However, a fourth clinical study, consisting of 57 patients, failed to show a benefit of autologous germline mitochondrial energy transfer-in vitro fertilization versus in vitro fertilization alone for improving cumulative live birth rates. Complicating this area of work further, a recent mouse study, which claimed to test the long-term safety of autologous mitochondrial supplementation during in vitro fertilization, raised concerns over the use of the procedure for reproduction. However, autologous mitochondria were not actually used for preclinical testing in this mouse study. The unwarranted fears that this new study's erroneous conclusions could cause in women who have become pregnant through the use of autologous germline mitochondrial energy transfer during-in vitro fertilization highlight the critical need for accurate reporting of preclinical work that has immediate bearing on human clinical studies.

Estrogen regulation of germline stem cell differentiation as a mechanism contributing to female reproductive aging

Progressive loss of ovarian estrogen (E2) production is a hallmark feature of, if not a driving force behind, reproductive aging and the menopause. Recent genetic studies in mice have shown that female germline or oogonial stem cells (OSCs) contribute to maintenance of adult ovarian function and fertility under physiological conditions through support of de-novo oogenesis. Here we show that mouse OSCs express E2 receptor-α (ERα). In the presence of E2, ERα interacts with the stimulated by retinoic acid gene 8 (Stra8) promoter to drive Stra8 expression followed by oogenesis. Treatment of mice with E2 in vivo increases Stra8 expression and oogenesis, and these effects are nullified by ERα (Esr1), but not ERβ (Esr2), gene disruption. Although mice lacking ERα are born with a normal quota of oocytes, ERα-deficient females develop premature ovarian insufficiency in adulthood due to impaired oogenesis. Lastly, mice treated with reversible ER antagonists show a loss of Stra8 expression and oocyte numbers; however, both endpoints rebound to control levels after ceasing drug treatment. These findings establish a key physiological role for E2-ERα signaling in promoting OSC differentiation as a potential mechanism to maintain adequate numbers of ovarian follicles during reproductive life.

Inherent mitochondrial activity influences specification of the germ line in pluripotent stem cells

Herein we investigated whether inherent differences in mitochondrial activity in mouse pluripotent cells could be used to identify populations with an intrinsic ability to differentiate into primordial germ cells (PGCs). Notably, we determined that stem cells sorted based on differences in mitochondrial membrane activity exhibited altered germline differentiation capacity, with low-mitochondrial membrane potential associated with an increase in PGC-like cells. This specification was not further enhanced by hypoxia. We additionally noted differences between these populations in metabolism, transcriptome, and cell-cycle. These data contribute to a growing body of work demonstrating that pluripotent cells exhibit a large range of mitochondrial activity, which impacts cellular function and differentiation potential. Furthermore, pluripotent cells possess a subpopulation of cells with an improved ability to differentiate into the germ lineage that can be identified based on differences in mitochondrial membrane potential.

Copula-based robust optimal block designs

Blocking is often used to reduce known variability in designed experiments by collecting together homogeneous experimental units. A common modeling assumption for such experiments is that responses from units within a block are dependent. Accounting for such dependencies in both the design of the experiment and the modeling of the resulting data when the response is not normally distributed can be challenging, particularly in terms of the computation required to find an optimal design. The application of copulas and marginal modeling provides a computationally efficient approach for estimating population-average treatment effects. Motivated by an experiment from materials testing, we develop and demonstrate designs with blocks of size two using copula models. Such designs are also important in applications ranging from microarray experiments to experiments on human eyes or limbs with naturally occurring blocks of size two. We present a methodology for design selection, make comparisons to existing approaches in the literature, and assess the robustness of the designs to modeling assumptions.

A nanoscale, multi-parametric flow cytometry-based platform to study mitochondrial heterogeneity and mitochondrial DNA dynamics

Mitochondria are well-characterized regarding their function in both energy production and regulation of cell death; however, the heterogeneity that exists within mitochondrial populations is poorly understood. Typically analyzed as pooled samples comprised of millions of individual mitochondria, there is little information regarding potentially different functionality across subpopulations of mitochondria. Herein we present a new methodology to analyze mitochondria as individual components of a complex and heterogeneous network, using a nanoscale and multi-parametric flow cytometry-based platform. We validate the platform using multiple downstream assays, including electron microscopy, ATP generation, quantitative mass-spectrometry proteomic profiling, and mtDNA analysis at the level of single organelles. These strategies allow robust analysis and isolation of mitochondrial subpopulations to more broadly elucidate the underlying complexities of mitochondria as these organelles function collectively within a cell.

Female Fertility Preservation through Stem Cell-based Ovarian Tissue Reconstitution In Vitro and Ovarian Regeneration In Vivo

Historically, approaches designed to offer women diagnosed with cancer the prospects of having a genetically matched child after completion of their cytotoxic treatments focused on the existing oocyte population as the sole resource available for clinical management of infertility. In this regard, elective oocyte and embryo cryopreservation, as well as autologous ovarian cortical tissue grafting posttreatment, have gained widespread support as options for young girls and reproductive-age women who are faced with cancer to consider. In addition, the use of ovarian protective therapies, including gonadotropin-releasing hormone agonists and sphingosine-1-phosphate analogs, has been put forth as an alternative way to preserve fertility by shielding existing oocytes in the ovaries in vivo from the side-effect damage caused by radiotherapy and many chemotherapeutic regimens. This viewpoint changed with the publication of now numerous reports that adult ovaries of many mammalian species, including humans, contain a rare population of oocyte-producing germ cells-referred to as female germline or oogonial stem cells (OSCs). This new line of study has fueled research into the prospects of generating new oocytes, rather than working with existing oocytes, as a novel approach to sustain or restore fertility in female cancer survivors. Here, we overview the history of work from laboratories around the world focused on improving our understanding of the biology of OSCs and how these cells may be used to reconstitute "artificial" ovarian tissue in vitro or to regenerate damaged ovarian tissue in vivo as future fertility-preservation options.

Extracellular matrix signaling activates differentiation of adult ovary-derived oogonial stem cells in a species-specific manner

OBJECTIVE

To test if ovarian microenvironmental cues affect oogonial stem cell (OSC) function in a species-specific manner.

DESIGN

Animal and human study.

SETTING

Research laboratory.

PATIENT(S)/ANIMAL(S)

Human ovarian cells obtained from cryopreserved ovarian cortical tissue of reproductive-age women, and ovarian cells and tissues from female C57BL/6 mice.

INTERVENTION(S)

Mouse ovarian tissue, mouse OSCs (mOSCs) and human OSCs (hOSCs) were analyzed for extracellular matrix (ECM) protein expression, and OSCs isolated from adult mouse and human ovaries were cultured in the absence or presence of ECM proteins without or with an integrin signaling inhibitor.

MAIN OUTCOME MEASURE(S)

Gene expression and in vitro derived (IVD) oocyte formation.

RESULT(S)

Culture of mOSCs on a collagen-based ECM significantly elevated the rate of differentiation of the cells into IVD oocytes. Mouse OSCs expressed many integrins, including Arg-Gly-Asp (RGD)-binding subunits, and ECM-mediated increases in mOSC differentiation were blocked by addition of integrin-antagonizing RGD peptides. In comparison, hOSCs expressed a different pattern of integrin subunits compared with mOSCs, and hOSCs were unresponsive to a collagen-based ECM; however, hOSCs exhibited increased differentiation into IVD oocytes when cultured on laminin.

CONCLUSION(S)

These data, along with in silico analysis of ECM protein profiles in human ovaries, indicate that ovarian ECM-based niche components function in a species-specific manner to control OSC differentiation.

Implications and Current Limitations of Oogenesis from Female Germline or Oogonial Stem Cells in Adult Mammalian Ovaries

A now large body of evidence supports the existence of mitotically active germ cells in postnatal ovaries of diverse mammalian species, including humans. This opens the possibility that adult stem cells naturally committed to a germline fate could be leveraged for the production of female gametes outside of the body. The functional properties of these cells, referred to as female germline or oogonial stem cells (OSCs), in ovaries of women have recently been tested in various ways, including a very recent investigation of the differentiation capacity of human OSCs at a single cell level. The exciting insights gained from these experiments, coupled with other data derived from intraovarian transplantation and genetic tracing analyses in animal models that have established the capacity of OSCs to generate healthy eggs, embryos and offspring, should drive constructive discussions in this relatively new field to further exploring the value of these cells to the study, and potential management, of human female fertility. Here, we provide a brief history of the discovery and characterization of OSCs in mammals, as well as of the in-vivo significance of postnatal oogenesis to adult ovarian function. We then highlight several key observations made recently on the biology of OSCs, and integrate this information into a broader discussion of the potential value and limitations of these adult stem cells to achieving a greater understanding of human female gametogenesis in vivo and in vitro.

Dynamics of WNT signaling components in the human ovary from development to adulthood

WNT signaling has been shown to play a pivotal role in mammalian gonad development and sex differentiation; however, its role in the developing human ovary has not been investigated. We analyzed a quantitative mass spectrometry dataset to determine the expression of WNT signaling components between 47 and 137 days of development and in adult ovarian cortex tissue. WNT signaling was identified within the top ten canonical pathways of proteins detected at every developmental stage examined. We further examined the specific localization of WNT signaling components glycogen synthase kinase 3 (GSK3B), frizzled 2 (FZD2), and β-catenin (CTNNB1) within ovarian tissue. GSK3B was nearly ubiquitously expressed during fetal development, while FZD2 was specific to germ cell nests during early development. β-catenin exhibited translocation from primarily membrane bound during early ovarian development to cytoplasmic and nuclear staining specifically in early primordial follicles in the fetal ovary. This cytoplasmic and nuclear β-catenin persisted in primordial follicles in adult ovarian tissue, but returned to membrane-bound localization in secondary follicles. We conclude that WNT signaling components are expressed in the human ovary from early to mid-gestation and remain in the adult ovary, and observed evidence for canonical WNT signaling only in the oocytes of primordial follicles. Together, these data are indicative of a role for canonical WNT signaling via β-catenin nuclear translocation during human follicle formation and follicle maintenance.

Calorie restriction does not influence oocyte quality in oocytes from POLG mitochondrial mutator mice

It has recently been demonstrated that moderate adult onset caloric restriction (e.g. calorie restriction; CR) has a positive impact on female fertility in aged mice, due in large to preventing the age-associated decline in the quality of oocytes available for fertilization. The impact of CR on oocyte quality has been attributed, at least in part, to mitochondrial functions. In mitochondrial DNA (mtDNA) mutator mice (Polg^D257A/D257A), which harbor a mutation in the proofreading mtDNA polymerase-gamma (POLG), mitochondrial mutations rapidly accumulate, resulting in a premature aging phenotype and female infertility. As CR has been shown to extend both lifespan and ‘healthspan’ as well as improve oocyte quality in aged mice, we investigated whether adult onset CR could improve oocyte quality in the POLG mouse. Female Polg^D257A/D257A mice exhibited infertility based on an inability to produce litters through natural mating. Analysis of oocytes from 8–9-month-old Polg^D257A/D257A mice on CR following hormone stimulation revealed no improvement in the number of oocytes ovulated. Furthermore, CR did not result in a greater percentage of metaphase II oocytes, with the majority of the oocytes prematurely arrested at the germinal vesicle stage. Finally, CR did not improve the abnormal mitochondrial distribution or pronounced defects in meiotic spindle assembly and chromosomal distribution observed in the ad libitum fed Polg^D257A/D257A. Taken together, these data suggest that although CR benefits oocyte quality and fertility outcomes in naturally aged female mice, it does not sufficiently improve oocyte quality in Polg^D257A/D257A.

Impact of exercise on oocyte quality in the POLG mitochondrial DNA mutator mouse

The mtDNA 'mutator' mouse, also called the 'POLG' mouse, is a well-characterized model frequently used for studies of progeroid aging. Harboring a mutation in the proofreading domain of the mitochondrial polymerase, polymerase-γ (Polg), POLG mice acquire mtDNA mutations at an accelerated rate. This results in premature mitochondrial dysfunction and a systemic aging phenotype. Previous work has demonstrated that the progeroid phenotype in POLG is attenuated following endurance exercise, the only reported intervention to extend health span and lifespan of these mice. Herein, oocyte quality was evaluated in sedentary and exercised POLG mice. In mice homozygous for the Polg mutation, litter size is dramatically reduced as compared to heterozygous Polg mice. Following ovarian hyper-stimulation, oocytes were retrieved until 9 months of age in exercised and sedentary groups, with no oocytes ovulated thereafter. Although ovulated oocyte numbers were not impacted by exercise, we did find a modest improvement in both the ovarian follicle reserve and in oocyte quality based on meiotic spindle assembly, chromosomal segregation and mitochondrial distribution at 7 months of age in exercised POLG mice as compared to sedentary counterparts. Of note, analysis of mtDNA mutational load revealed no differences between exercised and sedentary groups. Collectively, these data indicate that exercise differentially influences somatic tissues of the POLG mouse as compared to oocytes, highlighting important mechanistic differences between mitochondrial regulatory mechanisms in the soma and the germline.

Quantitative Proteomic Profiling of the Human Ovary from Early to Mid-Gestation Reveals Protein Expression Dynamics of Oogenesis and Folliculogenesis

The in vivo gene networks involved in coordinating human fetal ovarian development remain obscure. In this study, quantitative mass spectrometry was conducted on ovarian tissue collected at key stages during the first two trimesters of human gestational development, confirming the expression profiling data using immunofluorescence, as well as in vitro modeling with human oogonial stem cells (OSCs) and human embryonic stem cells (ESCs). A total of 3,837 proteins were identified in samples spanning developmental days 47-137. Bioinformatics clustering and Ingenuity Pathway Analysis identified DNA mismatch repair and base excision repair as major pathways upregulated during this time. In addition, MAEL and TEX11, two key meiosis-related proteins, were identified as highly expressed during the developmental window associated with fetal oogenesis. These findings were confirmed and extended using in vitro differentiation of OSCs into in vitro derived oocytes and of ESCs into primordial germ cell-like cells and oocyte-like cells, as models. In conclusion, the global protein expression profiling data generated by this study have provided novel insights into human fetal ovarian development in vivo and will serve as a valuable new resource for future studies of the signaling pathways used to orchestrate human oogenesis and folliculogenesis.

Influence of Maternal Aging on Mitochondrial Heterogeneity, Inheritance, and Function in Oocytes and Preimplantation Embryos

Contrasting the equal contribution of nuclear genetic material from maternal and paternal sources to offspring, passage of mitochondria, and thus mitochondrial DNA (mtDNA), is uniparental through the egg. Since mitochondria in eggs are ancestral to all somatic mitochondria of the next generation and to all cells of future generations, oocytes must prepare for the high energetic demands of maturation, fertilization and embryogenesis while simultaneously ensuring that their mitochondrial genomes are inherited in an undamaged state. Although significant effort has been made to understand how the mtDNA bottleneck and purifying selection act coordinately to prevent silent and unchecked spreading of invisible mtDNA mutations through the female germ line across successive generations, it is unknown if and how somatic cells of the immediate next generation are spared from inheritance of detrimental mtDNA molecules. Here, we review unique aspects of mitochondrial activity and segregation in eggs and early embryos, and how these events play into embryonic developmental competency in the face of advancing maternal age.

A method for freeze-fracture and scanning electron microscopy of isolated mitochondria

Electron microscopy as a methodology for the study of mitochondria based on morphological features is a standard technique that has experienced little evolution over the course of several decades. This technology has identified heterogeneity of mitochondria populations across both whole tissues, as well between individual cells, using primarily ultrathin sections for transmission electron microscopy (TEM). However, this technique constrains the evaluation of a sample to a single two-dimensional plane. To overcome this limitation, scanning electron microscopy (SEM) has been successfully utilized to observe three-dimensional mitochondria structures within the complex microenvironment containing total cellular components. In response to these dual technical caveats of existing electron microscopy protocols, we developed a methodology to evaluate the three-dimensional ultrastructure of isolated mitochondria, utilizing a freeze-fracture step and rigorous preservation of sample morphology. This protocol allows for a more high-throughput analysis of mitochondria populations from a specimen of interest, as the sample has been previously purified, as well as a finer resolution of complex intra-mitochondrial structures, using the depth of field created by SEM.

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Protocol designed for SEM of isolated mitochondria samples.

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SEM visualizes mitochondria ultrastructure in 3-D.

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Freeze-fracture creates cross-sectional plane for view of interior organelle structures.

Ovarian regeneration: The potential for stem cell contribution in the postnatal ovary to sustained endocrine function

The endocrine function of the ovary is dependent upon the ovarian follicle, which on a cellular basis consists of an oocyte surrounded by adjacent somatic cells responsible for generating sex steroid hormones and maintenance of hormonal stasis with the hypothalamic-pituitary axis. As females age, both fertility and the endocrine function of the ovary decline due to waning follicle numbers as well as aging-related cellular dysfunction. Although there is currently no cure for ovarian failure and endocrine disruption, recent advances in ovarian biology centered on ovarian stem cell and progenitor cell populations have brought the prospects of cell- or tissue-based therapeutic strategies closer to fruition. Herein, we review the relative contributions of ovarian stem cells to ovarian function during the reproductive lifespan, and postulate steps toward the development of ovarian stem cell-based approaches to advance fertility treatments, and also importantly to provide a physiological long-term means of endocrine support.

Isolation of Mammalian Oogonial Stem Cells by Antibody-Based Fluorescence-Activated Cell Sorting

The ability to isolate and subsequently culture mitotically active female germ cells from adult ovaries, referred to as either oogonial stem cells (OSCs) or adult female germline stem cells (aFGSCs), has provided a robust system to study female germ cell development under multiple experimental conditions, and in many species. Flow cytometry or fluorescence-activated cell sorting (FACS) is an integral part of many isolation and characterization protocols. Here, we provide methodological details for antibody-based flow cytometric isolation of OSCs using antibodies specific for external epitopes of the proteins Ddx4 or Ifitm3, alone or in combination with the use of fluorescent reporter mice. Beginning with sample preparation, we provide point-by-point instructions to guide researchers on how to isolate OSCs using flow cytometry.

A role for retinoids in human oocyte fertilization: regulation of connexin 43 by retinoic acid in cumulus granulosa cells

Retinoids are essential for ovarian steroid production and oocyte maturation in mammals. Oocyte competency is known to positively correlate with efficient gap junction intercellular communication (GJIC) among granulosa cells in the cumulus-oocyte complex. Connexin 43 (C x 43) is the main subunit of gap junction channels in human cumulus granulosa cells (CGC) and is regulated by all-trans retinoic acid (ATRA) in other hormone responsive cell types. The objectives of this study were to quantify retinoid levels in human CGC obtained during IVF oocyte retrievals, to investigate the potential relationship between CGC ATRA levels and successful oocyte fertilization, and to determine the effects of ATRA on C x 43 protein expression in CGC. Results showed that CGC cultures actively metabolize retinol to produce ATRA. Grouped according to fertilization rate tertiles, mean ATRA levels were 2-fold higher in pooled CGC from women in the highest versus the lowest tertile (P < 0.05). ATRA induced a rapid dephosphorylation of C x 43 in CGC and granulosa cell line (KGN) cultures resulting in a >2-fold increase in the expression of the functional non-phosphorylated (P0) species (P < 0.02). Similar enhancement of P0 by ATRA was shown in CGC and KGN cultures co-treated with LH or hCG which, by themselves, enhanced the protein levels of C x 43 without altering its phosphorylation profile. Correspondingly, the combination of ATRA+hCG treatment of KGN caused a significant increase in GJIC compared with single agent treatments (P < 0.025) and a doubling of GJIC from that seen in untreated cells (P < 0.01). These findings indicate that CGC are a primary site of retinoid uptake and ATRA biosynthesis. Regulation of C x 43 by ATRA may serve an important role in folliculogenesis, development of oocyte competency, and successful fertilization by increasing GJIC in CGC.

Bone morphogenetic protein 4 promotes mammalian oogonial stem cell differentiation via Smad1/5/8 signaling

OBJECTIVE

To test whether bone morphogenetic protein 4 (BMP4) directly regulates differentiation of adult mouse ovary-derived oogonial stem cells (OSCs) in vitro.

DESIGN

Animal study.

SETTING

Research laboratory.

ANIMAL(S)

Adult C57BL/6 female mice.

INTERVENTION(S)

After purification from adult ovaries by fluorescence-activated cell sorting, OSCs were cultured without or with BMP4 in the absence or presence of the BMP4 antagonist, Noggin.

MAIN OUTCOME MEASURE(S)

Rates of in vitro-derived (IVD) oocyte formation and changes in gene expression were assessed.

RESULT(S)

Cultured OSCs expressed BMP receptor (BMPR) 1A (BMPR1A), BMPR1B, and BMPR2, suggesting that BMP signaling can directly affect OSC function. In agreement with this, BMP4 significantly increased the number of IVD oocytes formed by cultured OSCs in a dose-dependent manner, and this response was inhibited in a dose-dependent fashion by cotreatment with Noggin. Exposure of OSCs to BMP4 was associated with rapid phosphorylation of BMPR-regulated Smad1/5/8 proteins, and this response was followed by increased expression of the meiosis initiation factors, stimulated by retinoic acid gene 8 (Stra8), muscle-segment homeobox 1 (Msx1), and Msx2. In keeping with the IVD oocyte formation data, the ability of BMP4 to activate Smad1/5/8 signaling and meiotic gene expression in OSCs was abolished by cotreatment with Noggin.

CONCLUSION(S)

Engagement of BMP4-mediated signaling in adult mouse ovary-derived OSCs cultured in vitro drives differentiation of these cells into IVD oocytes through Smad1/5/8 activation and transcriptional up-regulation of key meiosis-initiating genes.

Comparative gene expression profiling of adult mouse ovary-derived oogonial stem cells supports a distinct cellular identity

OBJECTIVE

Perform gene expression profiling of adult mouse ovary-derived oogonial stem cells (OSCs).

DESIGN

Experimental animal study.

SETTING

Research laboratory.

ANIMAL(S)

Adult C57BL/6 female mice.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Gene expression profiles were compared between freshly isolated and cultured OSCs, as well as between OSCs and embryonic stem cells (ESCs), fetal primordial germ cells (PGCs), and spermatogonial stem cells (SSCs); OSC yield from ovaries versus meiotic gene activation during the estrous cycle was determined.

RESULT(S)

Freshly isolated OSCs, PGCs, and SSCs exhibited distinct gene expression profiles. Cultured OSCs maintained their germline gene expression pattern but gained expression of pluripotency markers found in PGCs and ESCs. Cultured OSCs also expressed the meiotic marker, stimulated by retinoic acid gene 8 (Stra8). In vivo, OSC yield was higher from luteal versus follicular phase ovaries, and this was inversely related to Stra8 expression.

CONCLUSION(S)

Freshly isolated OSCs exhibit a germline gene expression profile that overlaps with, but is distinct from, that of PGCs and SSCs. After in vitro expansion, OSCs activate expression of pluripotency genes found in freshly isolated PGCs. In vivo, OSC numbers in the ovaries fluctuate during the estrous cycle, with the highest numbers noted during the luteal phase. This is followed by activation of Stra8 expression during the follicular phase, which may signify a wave of neo-oogenesis to partially offset follicular loss through atresia and ovulation in the prior cycle.

Embryonic stem cell-derived granulosa cells participate in ovarian follicle formation in vitro and in vivo

Differentiating embryonic stem cells (ESCs) can form ovarian follicle-like structures in vitro, consisting of an oocyte-like cell surrounded by somatic cells capable of steroidogenesis. Using a dual-fluorescence reporter system in which mouse ESCs express green fluorescent protein (GFP) under the control of a germ cell-specific Pou5f1 gene promoter and red fluorescent protein (Discosoma sp red [DsRed]) driven by the granulosa cell-specific Forkhead box L2 (Foxl2) gene promoter, we first confirmed in vitro formation of follicle-like structures containing GFP-positive cells surrounded by DsRed-positive cells. Isolated DsRed-positive cells specified from ECSs exhibited a gene expression profile consistent with granulosa cells, as revealed by the detection of messenger RNAs (mRNAs) for Foxl2, follistatin (Fst), anti-Müllerian hormone (Amh), and follicle-stimulating hormone receptor (Fshr) as well as by production of both progesterone and estradiol. In addition, treatment of isolated DsRed-expressing cells with follicle-stimulating hormone (FSH) significantly increased estradiol production over basal levels, confirming the presence of functional FSH receptors in these cells. Last, ESC-derived DsRed-positive cells injected into neonatal mouse ovaries became incorporated within the granulosa cell layer of immature follicles. These studies demonstrate that Foxl2-expressing ovarian somatic cells derived in vitro from differentiating ESCs express granulosa cell markers, actively associate with germ cells in vitro, synthesize steroids, respond to FSH, and participate in folliculogenesis in vivo.

Embryonic stem cell-derived granulosa cells participate in ovarian follicle formation in vitro and in vivo

Differentiating embryonic stem cells (ESCs) can form ovarian follicle-like structures in vitro, consisting of an oocyte-like cell surrounded by somatic cells capable of steroidogenesis. Using a dual-fluorescence reporter system in which mouse ESCs express green fluorescent protein (GFP) under the control of a germ cell-specific Pou5f1 gene promoter and red fluorescent protein (Discosoma sp red [DsRed]) driven by the granulosa cell-specific Forkhead box L2 (Foxl2) gene promoter, we first confirmed in vitro formation of follicle-like structures containing GFP-positive cells surrounded by DsRed-positive cells. Isolated DsRed-positive cells specified from ECSs exhibited a gene expression profile consistent with granulosa cells, as revealed by the detection of messenger RNAs (mRNAs) for Foxl2, follistatin (Fst), anti-Müllerian hormone (Amh), and follicle-stimulating hormone receptor (Fshr) as well as by production of both progesterone and estradiol. In addition, treatment of isolated DsRed-expressing cells with follicle-stimulating hormone (FSH) significantly increased estradiol production over basal levels, confirming the presence of functional FSH receptors in these cells. Last, ESC-derived DsRed-positive cells injected into neonatal mouse ovaries became incorporated within the granulosa cell layer of immature follicles. These studies demonstrate that Foxl2-expressing ovarian somatic cells derived in vitro from differentiating ESCs express granulosa cell markers, actively associate with germ cells in vitro, synthesize steroids, respond to FSH, and participate in folliculogenesis in vivo.

An evolutionary perspective on adult female germline stem cell function from flies to humans

The concept that oogenesis continues into reproductive life has been well established in nonmammalian species. Recent studies of mice and women indicate that oocyte formation is also not, as traditionally believed, restricted to the fetal or perinatal periods. Analogous to de novo oocyte formation in flies and fish, newly formed oocytes in adult mammalian ovaries arise from germline stem cells (GSCs) or, more specifically, oogonial stem cells (OSCs). Studies of mice have confirmed that isolated OSCs, once delivered back into adult ovaries, are capable of generating fully functional eggs that fertilize to produce healthy embryos and offspring. Parallel studies of OSCs recently purified from ovaries of reproductive-age women indicate that these cells closely resemble their mouse ovary-derived counterparts, although the fertilization competency of oocytes generated by human OSCs awaits clarification. Despite the ability of OSCs to produce new oocytes during adulthood, oogenesis will still ultimately cease with age, contributing to ovarian failure. The causal mechanisms behind these events in mammals are unknown, but studies of flies have revealed that GSC niche dysfunction plays a critical role in age-related oogenic failure. Such insights derived from evaluation of nonmammalian species, in which postnatal oogenesis has been studied in depth, may aid in development of new strategies to alleviate ovarian failure and infertility in mammals.

Hot topic: Comparison of sex-sorted and conventional semen within a fixed-time artificial insemination protocol designed for dairy heifers

The objective was to compare pregnancy per AI (P/AI) with conventional (CON) or sex-sorted (SS) semen from a single sire within a fixed-time AI (FTAI) program designed for dairy heifers. Holstein heifers (n=240) were assigned to treatment (CON or SS) according to body weight and reproductive tract score. All heifers underwent FTAI by using the "Show-Me-Synch" protocol [controlled internal drug release (CIDR) insert from d 0 to 14 followed by PGF(2α) (25mg i.m.) 16d after insert removal (d 30) with GnRH (100 µg i.m.) and FTAI at 66 h after PGF(2α)]. A single professional technician performed the FTAI. Heifers were fitted with heat detection patches at PGF(2α) to characterize estrous response. Estrous response did not differ between CON (63/120; 53%) and SS (70/120; 58%) treatments. The CON heifers, however, achieved greater FTAI P/AI (82/120; 68%) compared with SS (45/120; 38%) heifers. The P/AI did not differ for CON heifers that exhibited or failed to exhibit estrus before FTAI [44/63 (70%) vs. 38/57(67%), respectively]. For SS heifers, however, those that exhibited estrus had greater P/AI compared with those that failed to exhibit estrus [32/70 (46%) vs. 13/50 (26%)]. Pregnancy per AI resulting from FTAI was greater for heifers that were inseminated with CON semen compared with those that received SS semen. The expression of estrus before FTAI did not affect P/AI when CON semen was used, whereas the P/AI with SS semen was greater for heifers detected in estrus. Further studies are required to develop strategies for using sex-sorted semen when inseminating heifers at predetermined fixed times on the basis of expression of estrus before FTAI.

The next (re)generation of ovarian biology and fertility in women: is current science tomorrow's practice?

Stem cell-based strategies for ovarian regeneration and oocyte production have been proposed as future clinical therapies for treating infertility in women. However, utilization of embryonic stem cells or induced pluripotent stem cells to produce oocytes has had limited success in vitro. A recent report of the isolation and characterization of endogenous oocyte-producing or oogonial stem cells (OSCs) from ovaries of reproductive age women describes the first stable and pure human female germ cell culture model in which a subset of cells appear to initiate and complete meiosis. In addition, purified human OSCs introduced into adult human ovarian cortical tissue generate oocytes that arrest at the diplotene stage of meiosis and successfully recruit granulosa cells to form new primordial follicles. This overview examines the current landscape of in vitro and in vivo gametogenesis from stem cells, with emphasis on generation of human oocytes. Future research objectives for this area of work, as well as potential clinical applications involving the use of human OSCs, are discussed.

A transgenic zebrafish model of targeted oocyte ablation and de novo oogenesis

We describe here a novel transgenic zebrafish, Tg(zpc:G4VP16/UAS:nfsB-mCherry) that effectively demonstrates the targeted oocyte ablation in the adult zebrafish ovary. This transgenic line expresses bacterial nitroreductase enzyme (nfsB) under the control of the oocyte-specific zona pellucida C (zpc) gene promoter. Adult transgenic females exposed to the prodrug metronidazole demonstrated near-complete ablation of growing oocytes, resulting in ovarian degeneration and complete cessation of reproductive function. Within 4 weeks of prodrug removal, treated fish demonstrated complete anatomical regeneration of the ovary and, within 7 weeks, ovarian function (fertility) was fully restored. Together, these results demonstrate functional renewal of the oocyte pool in the adult zebrafish ovary. Accordingly, this transgenic zebrafish model system provides a novel means to investigate ovarian growth dynamics in a genetically tractable vertebrate, and may be useful for evaluating signaling interactions that regulate gonadal development processes such as de novo oogenesis.

TLR4 activates NF-κB in human ovarian granulosa tumor cells

Previous studies have demonstrated expression of Toll-like receptors (TLRs) in the surface epithelium of normal ovaries (OSE) and in epithelial ovarian tumors. Most notably, OSE-derived cancers express TLR4, which activates the nuclear factor-kappa B (NF-κB) signaling cascade as a mediator of inflammatory response. Currently, there is considerable interest in elucidating the role of TLR-mediated signaling in cancers. Nevertheless, the expression of TLRs in granulosa cell tumors (GCTs) of the ovary, and the extent to which GCT expression of TLRs may influence cell-signaling pathways and/or modulate the efficacy of chemotherapeutics, has yet to be determined. In the present study, human GCT lines (COV434 and KGN) were utilized to evaluate expression of functional TLR4. TLR4 is expressed in GCT cell lines and ligation of TLR4 with bacterial lipopolysaccharide (LPS) led to IκB degradation and activation of NF-κB. NF-κB activation was confirmed by nuclear localization of NF-κB p65 following treatment with LPS and the naturally occurring ligand, HSP60. Notably, immunoneutralization of TLR4 blocked nuclear localization, and inhibition of NF-κB signaling attenuated LPS-induced TNFα plus increased doubling time in both cell lines. Contradictory to reports using human OSE cell lines, inhibition of NF-κB signaling failed to sensitize GCT lines to TRAIL or cisplatin. In summary, findings herein are the first to demonstrate a functional TLR-signaling pathway specifically in GCTs, and indicate that in contrast to OSE-derived cancers, inhibition of NF-κB does not sensitize GCTs to TRAIL or cisplatin.

Dynamics of avian ovarian follicle development: cellular mechanisms of granulosa cell differentiation

In vertebrate species that ovulate one or a limited number of ovarian follicles per reproductive cycle, the cellular processes by which follicle selection (cyclic recruitment) is mediated and final differentiation is initiated remain largely unknown. In the hen ovary, the selection of a single follicle into the preovulatory hierarchy on an approximate daily basis occurs from a small cohort of prehierarchal follicles measuring approximately 6- to 8-mm in diameter. Given that the granulosa layer undergoes a dramatic alteration in phenotype subsequent to follicle selection, of particular interest are the cell signaling and associated transcriptional mechanisms that regulate this transition. Recent studies suggest that granulosa cells from prehierarchal follicles are normally maintained in an undifferentiated state by inhibitory MAP kinase (MAPK) signaling mediated by epidermal growth factor receptor ligands (EGFRLs). One of the earliest markers for differentiating granulosa cells is elevated expression of FSH receptor (fshr) mRNA and enhanced FSH-induced cyclic adenosine monophosphate (cAMP) production. EGFRL/MAPK signaling is proposed to inhibit fshr transcription via its ability to induce Inhibitor of differentiation/DNA binding (Id) protein isoforms, Id1, Id3 and Id4. Subsequent to follicle selection, cAMP-induced Id2 expression is considered both sufficient and necessary for fshr transcription. Two working models are proposed which predict that enhanced FSHR expression and the progression of granulosa cell differentiation occurs as a result of a decline in MAPK signaling from within granulosa cells (internal model for differentiation) and/or elevated cAMP signaling promoted by an endocrine, neuroendocrine or neuronal factor (external model).

Toll-like receptor signaling in hen ovarian granulosa cells is dependent on stage of follicle maturation

The recent identification of toll-like receptor (TLR) signaling within ovarian granulosa cells has broad implications for ovarian physiology. Functions of TLRs within granulosa cells of the laying hen are of particular interest due to the method of transovarian transmission of Salmonella enteritidis, which results in egg contamination. This study utilized hen granulosa cells to evaluate the expression and function of Gallus TLR-signaling at distinct stages of follicular maturity. Data presented herein demonstrate the presence of TLR2, TLR4, and TLR15 mRNAs in undifferentiated granulosa cells from prehierarchal follicles and differentiated granulosa cells from preovulatory follicles, together with mRNAs encoding adaptor proteins and signaling components required for TLR signaling gene. Treatment with lipopolysaccharide (LPS) or LH, in vitro, led to the differential regulation of TLRs based on the stage of follicle maturation, with the largest (F1) follicle granulosa cells having the most rapid response. Furthermore, treatment with LPS resulted in attenuation of agonist-induced progesterone synthesis in undifferentiated, but not differentiated, granulosa cells. Additionally, undifferentiated granulosa cells were significantly more sensitive to LPS-induced apoptosis than differentiated granulosa cells from the F1 follicle. Together, these data provide evidence for a complete and functional TLR signaling pathway in hen granulosa cells, with effects on steroidogenesis and cell viability dependent upon stage of maturation. These differences may reflect the susceptibility of granulosa cells at early stages of maturation to undergo apoptosis in response to select pathogenic stimuli, thus attenuating transovarian transmission, whereas granulosa cells from preovulatory follicles are comparably resistant to LPS-mediated apoptosis.

Role for inhibitor of differentiation/deoxyribonucleic acid-binding (Id) proteins in granulosa cell differentiation

Recent studies in the hen ovary have linked the initiation of granulosa cell differentiation at follicle selection to the alleviation of inhibitory MAPK signaling. The present studies assessed a role for individual inhibitor of differentiation (Id) protein isoforms as modulators of key transcriptional events occurring within granulosa cells at or immediately subsequent to differentiation. Findings from freshly collected granulosa cells collected at different stages of follicle development demonstrated a negative association between expression levels for Id2 mRNA compared with levels of Id1, Id3, and Id4. Elevated levels of Id2 are related to a differentiating/differentiated phenotype, whereas elevated Id1, Id3, and Id4 are associated with an undifferentiated phenotype. This negative relationship extends to cell signal transduction, because factors that promote inhibitory MAPK signaling (TGF-alpha and betacellulin) block expression of Id2 mRNA but increase levels of Id1, Id3, and Id4. Furthermore, overexpression of Gallus Id2 in cultured granulosa was found to significantly decrease levels of Id1, Id3, and Id4 mRNA but facilitate FSHR mRNA expression and, importantly, initiate LHR mRNA expression plus LH-induced progesterone production. Finally, knockdown studies using small interfering RNA specific for Id2 revealed reduced expression of FSHR and LHR mRNA and attenuated FSH- and LH-induced levels of StAR and p450 cholesterol side-chain cleavage enzyme mRNA plus progesterone production. Collectively, these data demonstrate that Id2 expression is both sufficient and necessary for increasing LHR expression and, as a result, promoting gonadotropin-induced differentiation in hen granulosa cells subsequent to follicle selection.

Inhibition of proteasome activity sensitizes human granulosa tumor cells to TRAIL-induced cell death

Human granulosa tumor cell (GCT) lines (KGN and COV434) were utilized to establish the combinatorial effects of TRAIL treatment and a proteasome inhibitor on cell viability, in vitro. TRAIL induced a slight, but consistent, decrease in viability for both cell lines, and pharmacologic inhibition of proteasome activity, using Z-LLF-CHO (Z-LLF), synergistically enhanced TRAIL-induced loss of viability. This enhanced sensitization was associated with the up-regulation of a TRAIL receptor, DR5, and pro-apoptotic Bax. Targeted reduction of p53 expression revealed that the ability of Z-LLF to enhance DR5 and Bax expression occurs independent of p53 activity. These studies underscore the potential to develop targeted treatments for GCTs using established cell lines.

Protein kinase C activity mediates LH-induced ErbB/Erk signaling in differentiated hen granulosa cells

While there is accumulating evidence that mitogen-activated protein kinase/Erk and protein kinase C (PKC) signaling inhibits premature differentiation of granulosa cells in hen prehierarchal follicles, it has only recently been established that these signaling pathways play an important facilitory role in promoting steroidogenesis in differentiated granulosa cells from preovulatory follicles. The present studies were conducted with differentiated granulosa cells to establish the ability of LH to initiate PKC activity, and the subsequent requirement for PKC activity in promoting the ErbB/Erk signaling cascade that ultimately facilitates LH-induced progesterone production. Incubation of differentiated granulosa cells with LH increases PKC activity within 15 min, and latently promotes Erk phosphorylation (P-Erk) by 180 min. Inhibition of PKC activity with GF109203X attenuates LH- and 8-bromo-cAMP (8-br-cAMP)-induced P-Erk, but not P-Erk promoted by an epidermal growth factor (EGF) family ligand (e.g., transforming growth factor alpha). Importantly, inhibition of PKC activity also blocks the LH-induced increase in the autocrine expression of mRNA encoding the EGF family ligands, such as EGF, amphiregulin, and betacellulin. Furthermore, inhibition of EGF ligand shedding at the level of the cell membrane using the matrix metalloprotease activity inhibitor, GM6001, prevents both LH- and 8-br-cAMP-induced P-Erk and progesterone production. These findings provide evidence for a facilitory role of PKC and ErbB/Erk signaling in LH-induced progesterone production, place the requirement for PKC activation upstream of ErbB/Erk activity, and demonstrate for the first time in a non-mammalian vertebrate the requirement for PKC activity in LH-induced expression of EGF family ligands in granulosa cells.