Stress-induced enzyme activation primes murine embryonic stem cells to differentiate toward the first extraembryonic lineage

Novel high throughput screen reports that benzo(a)pyrene overrides mouse trophoblast stem cell multipotency, inducing SAPK activity, HAND1 and differentiated trophoblast giant cells

INTRODUCTION

Cultured mouse trophoblast stem cells (mTSC) maintain proliferation/normal stemness (NS) under FGF4, which when removed, causes normal differentiation (ND). Hypoxic, or hyperosmotic stress forces trophoblast giant cells (TGC) differentiate. Hypoxic, hyperosmotic, and genotoxic benzo(a)pyrene (BaP), which is found in tobacco smoke, force down-regulation of inhibitor of differentiation (Id)2, enabling TGC differentiation. Hypoxic and hyperosmotic stress induce TGC by SAPK-dependent HAND1 increase. Here we test whether BaP forces mTSC-to-TGC while inducing SAPK and HAND1.

METHODS

Hand1 and SAPK activity were assayed by immunoblot, mTSC-to-TGC growth and differentiation were assayed at Tfinal after 72hr exposure of BaP, NS, ND, Retinoic acid (RA), or sorbitol. Nuclear-stained cells were micrographed automatically by a live imager, and assayed by ImageJ/FIJI, Biotek Gen 5, AIVIA proprietary artificial intelligence (AI) software or open source, CellPose artificial intelligence/AI software.

RESULTS

BaP (0.05-1μM) activated SAPK and HAND1 without diminishing growth. TSC-to-TGC differentiation was assayed with increasingly accuracy for 2-4 N cycling nuclei and >4 N differentiating TGC nuclei, using ImageJ/FIJI, Gen 5, AIVIA, or CellPose AI software. The AIVIA and Cellpose AI software matches human accuracy. The lowest BaP effects on SAPK activation/HAND1 increase are >10-fold more sensitive than similar effects for mESC. RA induces 44-47% 1st lineage TGC differentiation, but the same RA dose induces only 1% 1st lineage mESC differentiation.

DISCUSSION

First, these pilot data suggest that mTSC can be used in high throughput screens (HTS) to predict toxicant exposures that force TGC differentiation. Second, mTSC differentiated more cells than mESC for similar stress exposures, Third, open source AI can replace human micrograph quantitation and enable a miscarriage-predicting HTS.

A single cell transcriptomic fingerprint of stressed premature, imbalanced differentiation of embryonic stem cells

Cultured naïve pluripotent ESC differentiate into first lineage, XEN or second lineage, formative pluripotency. Hyperosmotic stress (sorbitol), like retinoic acid, decreases naive pluripotency and increases XEN in two ESC lines, as reported by bulk and scRNAseq, analyzed by UMAP. Sorbitol overrides pluripotency in two ESC lines as reported by bulk and scRNAseq, analyzed by UMAP. UMAP analyzed the effects of 5 stimuli - three stressed (200-300mM sorbitol with leukemia inhibitory factor +LIF) and two unstressed (+LIF, normal stemness-NS and -LIF, normal differentiation-ND). Sorbitol and RA decrease naive pluripotency and increase subpopulations of 2-cell embryo-like and XEN sub-lineages; primitive, parietal, and visceral endoderm (VE). Between the naïve pluripotency and primitive endoderm clusters is a stress-induced cluster with transient intermediate cells with higher LIF receptor signaling, with increased Stat3, Klf4, and Tbx3 expression. Sorbitol, like RA, also suppresses formative pluripotency, increasing lineage imbalance. Although bulk RNAseq and gene ontology group analyses suggest that stress induces head organizer and placental markers, scRNAseq reveals few cells. But VE and placental markers/cells were in adjacent clusters, like recent reports. UMAPs show that dose-dependent stress overrides stemness to force premature lineage imbalance. Hyperosmotic stress induces lineage imbalance, and other toxicological stresses, like drugs with RA, may cause lineage imbalance, resulting in miscarriages or birth defects.

Using high throughput screens to predict miscarriages with placental stem cells and long-term stress effects with embryonic stem cells

A problem in developmental toxicology is the massive loss of life from fertilization through gastrulation, and the surprising lack of knowledge of causes of miscarriage. Half to two-thirds of embryos are lost, and environmental and genetic causes are nearly equal. Simply put, it can be inferred that this is a difficult period for normal embryos, but that environmental stresses may cause homeostatic responses that move from adaptive to maladaptive with increasing exposures. At the lower 50% estimate, miscarriage causes greater loss-of-life than all cancers combined or of all cardio- and cerebral-vascular accidents combined. Surprisingly, we do not know if miscarriage rates are increasing or decreasing. Overshadowed by the magnitude of miscarriages, are insufficient data on teratogenic or epigenetic imbalances in surviving embryos and their stem cells. Superimposed on the difficult normal trajectory for peri-gastrulation embryos are added malnutrition, hormonal, and environmental stresses. An overarching hypothesis is that high throughput screens (HTS) using cultured viable reporter embryonic and placental stem cells (e.g., embryonic stem cells [ESC] and trophoblast stem cells [TSC] that report status using fluorescent reporters in living cells) from the pre-gastrulation embryo will most rapidly test a range of hormonal, environmental, nutritional, drug, and diet supplement stresses that decrease stem cell proliferation and imbalance stemness/differentiation. A second hypothesis is that TSC respond with greater sensitivity in magnitude to stress that would cause miscarriage, but ESC are stress-resistant to irreversible stemness loss and are best used to predict long-term health defects. DevTox testing needs more ESC and TSC HTS to model environmental stresses leading to miscarriage or teratogenesis and more research on epidemiology of stress and miscarriage. This endeavor also requires a shift in emphasis on pre- and early gastrulation events during the difficult period of maximum loss by miscarriage.

Using Live Imaging and Fluorescence Ubiquitinated Cell Cycle Indicator Embryonic Stem Cells to Distinguish G1 Cell Cycle Delays for General Stressors like Perfluoro-Octanoic Acid and Hyperosmotic Sorbitol or G2 Cell Cycle Delay for Mutagenic Stressors like Benzo(a)pyrene

Lowest observable adverse effects level (LOAEL) is a standard point-of-departure dose in toxicology. However, first observable adverse effects level (FOAEL) was recently reported and is used, in this study, as one criterion to detect a mutagenic stimulus in a live imager. Fluorescence ubiquitinated cell cycle indicator (FUCCI) embryonic stem cells (ESC) are green in the S-G2-M phase of the cell cycle and not green in G1-phase. Standard media change here is a mild stress that delays G1-phase and media change increases green 2.5- to 5-fold. Since stress is mild, media change rapidly increases green cell number, but higher stresses of environmental toxicants and positive control hyperosmotic stress suppress increased green after media change. Perfluoro-octanoic acid (PFOA) and diethyl phthalate (DEP) previously suppressed progression of nongreen to green cell cycle progression. Here, bisphenol A (BPA), cortisol, and positive control hyperosmotic sorbitol also suppress green fluorescence, but benzo(a)pyrene (BaP) at high doses (10 μM) increases green fluorescence throughout the 74-h exposure. Since any stress can affect many cell cycle phases, messenger RNA (mRNA) markers are best interpreted in ratios as dose-dependent mutagens increase in G2/G1 and nonmutagens increase G1/G2. After 74-h exposure, RNAseq detects G1 and G2 markers and increasing BaP doses increase G2/G1 ratios but increasing hyperosmotic sorbitol and PFOA doses increase G1/G2 marker ratios. BaP causes rapid green increase in FOAEL at 2 h of stimulus, whereas retinoic acid caused significant green fluorescence increases only late in culture. Using a live imager to establish FOAEL and G2 delay with FUCCI ESC is a new method to allow commercial and basic developmental biologists to detect drugs and environmental stimuli that are mutagenic. Furthermore, it can be used to test compounds that prevent mutations. In longitudinal studies, uniquely provided by this viable reporter and live imager protocol, follow-up can be done to test whether the preventative compound itself causes harm.

Stress Decreases Host Viral Resistance and Increases Covid Susceptibility in Embryonic Stem Cells

Stress-induced changes in viral receptor and susceptibility gene expression were measured in embryonic stem cells (ESC) and differentiated progeny. Rex1 promoter-Red Fluorescence Protein reporter ESC were tested by RNAseq after 72hr exposures to control stress hyperosmotic sorbitol under stemness culture (NS) to quantify stress-forced differentiation (SFD) transcriptomic programs. Control ESC cultured with stemness factor removal produced normal differentiation (ND). Bulk RNAseq transcriptomic analysis showed significant upregulation of two genes involved in Covid-19 cell uptake, Vimentin (VIM) and Transmembrane Serine Protease 2 (TMPRSS2). SFD increased the hepatitis A virus receptor (Havcr1) and the transplacental Herpes simplex 1 (HSV1) virus receptor (Pvrl1) compared with ESC undergoing ND. Several other coronavirus receptors, Glutamyl Aminopeptidase (ENPEP) and Dipeptidyl Peptidase 4 (DPP4) were upregulated significantly in SFD>ND. Although stressed ESC are more susceptible to infection due to increased expression of viral receptors and decreased resistance, the necessary Covid-19 receptor, angiotensin converting enzyme (ACE)2, was not expressed in our experiments. TMPRSS2, ENPEP, and DPP4 mediate Coronavirus uptake, but are also markers of extra-embryonic endoderm (XEN), which arise from ESC undergoing ND or SFD. Mouse and human ESCs differentiated to XEN increase TMPRSS2 and other Covid-19 uptake-mediating gene expression, but only some lines express ACE2. Covid-19 susceptibility appears to be genotype-specific and not ubiquitous. Of the 30 gene ontology (GO) groups for viral susceptibility, 15 underwent significant stress-forced changes. Of these, 4 GO groups mediated negative viral regulation and most genes in these increase in ND and decrease with SFD, thus suggesting that stress increases ESC viral susceptibility. Taken together, the data suggest that a control hyperosmotic stress can increase Covid-19 susceptibility and decrease viral host resistance in mouse ESC. However, this limited pilot study should be followed with studies in human ESC, tests of environmental, hormonal, and pharmaceutical stressors and direct tests for infection of stressed, cultured ESC and embryos by Covid-19.

Using Live Imaging and FUCCI Embryonic Stem Cells to Rank DevTox Risks: Adverse Growth Effects of PFOA Compared With DEP Are 26 Times Faster, 1,000 Times More Sensitive, and 13 Times Greater in Magnitude

Fluorescent ubiquitination-based cell cycle indicator (FUCCI) embryonic stem cells (ESCs), which fluoresce green during the S-G2-M phases, generate an S-shaped curve for the accumulation of cells during normal stemness (NS) culture with leukemia-inhibitory factor (LIF). Since it was hypothesized that a culture of ESCs was heterogeneous in the cell cycle, it was expected that increased S-G2-M-phases of the cell cycle would make an S-shaped curve parallel to the accumulation curve. Unexpectedly, it was observed that the fraction of FUCCI ESCs in green decreases over time to a nadir at ∼24 h after previous feeding and then rapidly enters S-G2-M-phases after medium change. G1 delay by infrequent medium change is a mild stress, as it does not affect growth significantly when frequency is increased to 12 h. Perfluoro-octanoic acid (PFOA) and diethyl phthalate (DEP) were used as examples of members of the per- and polyfluoroalkyl substances (PFAS) and phthalate families of chemicals, respectively. Two adverse outcomes were used to compare dose- and time-dependent effects of PFOA and DEP. The first was cell accumulation assay by time-lapse confluence measurements, largely at Tfinal/T74 h. The second was by quantifying dominant toxicant stress shown by the suppression of mild stress that creates a green fed/unfed peak. In terms of speed, PFOA is 26 times faster than DEP for producing a time-dependent LOAEL dose at 100 uM (that is, 2 h for PFOA and 52 h for DEP). PFOA has 1000-fold more sensitive LOAEL doses than DEP for suppressing ESC accumulation (confluence) at day 3 and day 2. There were two means to compare the magnitude of the growth suppression of PFOA and DEP. For the suppression of the accumulation of cells measured by confluence at Tfinal/T74h, there was a 13-fold suppression at the highest dose of PFOA > the highest dose of DEP. For the suppression of entry into the cell cycle after the G1 phase by stress on day 1 and 2, there is 10-fold more suppression by PFOA than DEP. The data presented here suggest that FUCCI ESCs can assay the suppression of accumulated growth or predict the suppression of future growth by the suppression of fed/unfed green fluorescence peaks and that PFOA's adverse effects are faster and larger and can occur at more sensitive lower doses than DEP.

Hypertonic pressure affects the pluripotency and self-renewal of mouse embryonic stem cells

As an important mechanical cue in the extracellular microenvironment, osmotic stress directly affects the proliferation, migration, and differentiation of cells. In this paper, we focused on the influence of hypertonic pressure on the colony morphology, stemness, and self-renew of mouse embryonic stem cells (mESCs). Our results showed that culture media with hypertonic pressure are more conducive to the maintenance of 3D colony morphology and pluripotency of mESCs after withdrawing the glycogen synthase kinase 3β (GSK3β) inhibitor CHIR99021 and the mitogen-activated protein kinase (MEK) inhibitor PD0325901 (hereinafter referred to as 2i) for 48 h. Furthermore, we revealed the microscopic mechanisms of the this finding: hypertonic pressure resulted in the depolymerization of F-actin cytoskeleton and limits Yes-associated protein (hereinafter referred to as YAP) transmission into the nucleus which play a vital role in the regulation of cell proliferation, and resulting in cell-cycle arrest at last.

Mechanical Roles of F-Actin in the Differentiation of Stem Cells: A Review

In the development and differentiation of stem cells, mechanical forces associated with filamentous actin (F-actin) play a crucial role. The present review aims to reveal the relationship among the chemical components, microscopic structures, mechanical properties, and biological functions of F-actin. Particular attention is given to the functions of the cytoplasmic and nuclear microfilament cytoskeleton and their regulation mechanisms in the differentiation of stem cells. The distributions of different types of actin monomers in mammal cells and the functions of actin-binding proteins are summarized. We discuss how the fate of stem cells is regulated by intra/extracellular mechanical and chemical cues associated with microfilament-related proteins, intercellular adhesion molecules, etc. In addition, we also address the differentiation-induced variation in the stiffness of stem cells and the correlation between the fate and geometric shape change of stem cells. This review not only deepens our understanding of the biophysical mechanisms underlying the fates of stem cells under different culture conditions but also provides inspirations for the tissue engineering of stem cells.

Stress Forces First Lineage Differentiation of Mouse Embryonic Stem Cells; Validation of a High-Throughput Screen for Toxicant Stress

Mouse Embryonic Stem Cells (mESCs) are unique in their self-renewal and pluripotency. Hypothetically, mESCs model gestational stress effects or stresses of in vitro fertilization/assisted reproductive technologies or drug/environmental exposures that endanger embryos. Testing mESCs stress responses should diminish and expedite in vivo embryo screening. Transgenic mESCs for green fluorescent protein (GFP) reporters of differentiation use the promoter for platelet-derived growth factor receptor (Pdgfr)a driving GFP expression to monitor hyperosmotic stress-forced mESC proliferation decrease (stunting), and differentiation increase that further stunts mESC population growth. In differentiating mESCs Pdgfra marks the first-lineage extraembryonic primitive endoderm (ExEndo). Hyperosmotic stress forces mESC differentiation gain (Pdgfra-GFP) in monolayer or three-dimensional embryoid bodies. Despite culture with potency-maintaining leukemia inhibitory factor (LIF), stress forces ExEndo as assayed using microplate readers and validated by coexpression of Pdgfra-GFP, Disabled 2 (Dab2), and laminin by immunofluorescence and GFP protein and Dab2 by immunoblot. In agreement with previous reports, Rex1 and Oct4 loss was inversely proportional to increased Pdgfra-GFP mESC after treatment with high hyperosmotic sorbitol despite LIF. The increase in subpopulations of Pdgfra-GFP⁺ cells>background at ∼23% was similar to the previously reported ∼25% increase in Rex1-red fluorescent protein (RFP)-negative subpopulation at matched high sorbitol doses. By microplate reader, there is a ∼7-11-fold increase in GFP at a high nonmorbid and a morbid dose despite LIF, compared with LIF alone. By flow cytometry (FACS), the subpopulation of Pdgfra-GFP⁺ cells>background increases ∼8-16-fold at these doses. Taken together, the microplate, FACS, immunoblot, and immunofluorescence data suggest that retinoic acid or hyperosmotic stress forces dose-dependent differentiation whether LIF is present or not and this is negatively correlated with and possibly compensates for stress-forced diminished ESC population expansion and potency loss.

Why AMPK agonists not known to be stressors may surprisingly contribute to miscarriage or hinder IVF/ART

Here we examine recent evidence suggesting that many drugs and diet supplements (DS), experimental AMP-activated protein kinase (AMPK) agonists as well as energy-depleting stress, lead to decreases in anabolism, growth or proliferation, and potency of cultured oocytes, embryos, and stem cells in an AMPK-dependent manner. Surprising data for DS and drugs that have some activity as AMPK agonists in in vitro experiments show possible toxicity. This needs to be balanced against a preponderance of evidence in vivo that these drugs and DS are beneficial for reproduction. We here discuss and analyze data that leads to two possible conclusions: First, although DS and drugs that have some of their therapeutic mechanisms mediated by AMPK activity associated with low ATP levels, some of the associated health problems in vivo and in vitro fertilization/assisted reproductive technologies (IVF/ART) may be better-treated by increasing ATP production using CoQ10 (Ben-Meir et al., Aging Cell 14:887-895, 2015). This enables high developmental trajectories simultaneous with solving stress by energy-requiring responses. In IVF/ART, it is ultimately best to maintain handling and culture of gametes and embryos in the quietest state with low metabolic activity (Leese et al., Mol Hum Reprod 14:667-672, 2008; Leese, Bioessays 24 (9):845-849, 2002) using back-to-nature or simplex algorithms to identify optima (Biggers, Reprod Biomed Online 4 Suppl 1:30-38, 2002). Stress markers, such as checkpoint proteins like TRP53 (aka p53) (Ganeshan et al., Exp Cell Res 358:227-233, 2017); Ganeshan et al., Biol Reprod 83:958-964, 2010) and a small set of kinases from the protein kinome that mediate enzymatic stress responses, can also be used to define optima. But, some gametes or embryos may have been stressed in vivo prior to IVF/ART or IVF/ART optimized for one outcome may be suboptimal for another. Increasing nutrition or adding CoQ10 to increase ATP production (Yang et al., Stem Cell Rev 13:454-464, 2017), managing stress enzyme levels with inhibitors (Xie et al., Mol Hum Reprod 12:217-224, 2006), or adding growth factors such as GM-CSF (Robertson et al., J Reprod Immunol 125:80-88, 2018); Chin et al., Hum Reprod 24:2997-3009, 2009) may increase survival and health of cultured embryos during different stress exposure contexts (Puscheck et al., Adv Exp Med Biol 843:77-128, 2015). We define "stress" as negative stimuli which decrease normal magnitude and speed of development, and these can be stress hormones, reactive oxygen species, inflammatory cytokines, or physical stimuli such as hypoxia. AMPK is normally activated by high AMP, commensurate with low ATP, but it was recently shown that if glucose is present inside the cell, AMPK activation by low ATP/high AMP is suppressed (Zhang et al., Nature 548:112-116, 2017). As we discuss in more detail below, this may also lead to greater AMPK agonist toxicity observed in two-cell embryos that do not import glucose. Stress in embryos and stem cells increases AMPK in large stimulation indexes but also direness indexes; the fastest AMPK activation occurs when stem cells are shifted from optimal oxygen to lower or high levels (Yang et al., J Reprod Dev 63:87-94, 2017). CoQ10 use may be better than risking AMPK-dependent metabolic and developmental toxicity when ATP is depleted and AMPK activated. Second, the use of AMPK agonists, DS, and drugs may best be rationalized when insulin resistance or obesity leads to aberrant hyperglycemia and hypertriglyceridemia, and obesity that negatively affect fertility. Under these conditions, beneficial effects of AMPK on increasing triglyceride and fatty acid and glucose uptake are important, as long as AMPK agonist exposures are not too high or do not occur during developmental windows of sensitivity. During these windows of sensitivity suppression of anabolism, proliferation, and stemness/potency due to AMPK activity, or overexposure may stunt or kill embryos or cause deleterious epigenetic changes.

Hypoxic Stress Forces Adaptive and Maladaptive Placental Stress Responses in Early Pregnancy

This review focuses on hypoxic stress and its effects on the placental lineage and the earliest differentiation events in mouse and human placental trophoblast stem cells (TSCs). Although the placenta is a decidual organ at the end of pregnancy, its earliest rapid growth and function at the start of pregnancy precedes and supports growth and function of the embryo. Earliest function requires that TSCs differentiate, however, "hypoxia" supports rapid growth, but not differentiation of TSCs. Most of the literature on earliest placental "hypoxia" studies used 2% oxygen which is normoxic for TSCs. Hypoxic stress happens when oxygen level drops below 2%. It decreases anabolism, proliferation, potency/stemness and increases differentiation, despite culture conditions that would sustain proliferation and potency. Thus, to study the pathogenesis due to TSC dysfunction, it is important to study hypoxic stress below 2%. Many studies have been performed using 0.5 to 1% oxygen in cultured mouse TSCs. From all these studies, a small number has examined human trophoblast lines and primary first trimester placental hypoxic stress responses in culture. Some other stress stimuli, aside from hypoxic stress, are used to elucidate common and unique aspects of hypoxic stress. The key outcomes produced by hypoxic stress are mitochondrial, anabolic, and proliferation arrest, and this is coupled with stemness loss and differentiation. Hypoxic stress can lead to depletion of stem cells and miscarriage, or can lead to later dysfunctions in placentation and fetal development. Birth Defects Research 109:1330-1344, 2017. © 2017 Wiley Periodicals, Inc.

Blastocyst-Derived Stem Cell Populations under Stress: Impact of Nutrition and Metabolism on Stem Cell Potency Loss and Miscarriage

Data from in vitro and in vivo models suggest that malnutrition and stress trigger adaptive responses, leading to small for gestational age (SGA) blastocysts with fewer cell numbers. These stress responses are initially adaptive, but become maladaptive with increasing stress exposures. The common stress responses of the blastocyst-derived stem cells, pluripotent embryonic and multipotent placental trophoblast stem cells (ESCs and TSCs), are decreased growth and potency, and increased, imbalanced and irreversible differentiation. SGA embryos may fail to produce sufficient antiluteolytic placental hormone to maintain corpus luteum progesterone secretion that provides nutrition at the implantation site. Myriad stress inputs for the stem cells in the embryo can occur in vitro during in vitro fertilization/assisted reproductive technology (IVF/ART) or in vivo. Paradoxically, stresses that diminish stem cell growth lead to a higher level of differentiation simultaneously which further decreases ESC or TSC numbers in an attempt to functionally compensate for fewer cells. In addition, prolonged or strong stress can cause irreversible differentiation. Resultant stem cell depletion is proposed as a cause of miscarriage via a "quiet" death of an ostensibly adaptive response of stem cells instead of a reactive, violent loss of stem cells or their differentiated progenies.

Two-cell embryos are more sensitive than blastocysts to AMPK-dependent suppression of anabolism and stemness by commonly used fertility drugs, a diet supplement, and stress

PURPOSE

This study tests whether metformin or diet supplement BR-DIM-induced AMP-activated protein kinase (AMPK) mediated effects on development are more pronounced in blastocysts or 2-cell mouse embryos.

METHODS

Culture mouse zygotes to two-cell embryos and test effects after 0.5-1 h AMPK agonists' (e.g., Met, BR-DIM) exposure on AMPK-dependent ACCser79P phosphorylation and/or Oct4 by immunofluorescence. Culture morulae to blastocysts and test for increased ACCser79P, decreased Oct4 and for AMPK dependence by coculture with AMPK inhibitor compound C (CC). Test whether Met or BR-DIM decrease growth rates of morulae cultured to blastocyst by counting cells.

RESULT(S)

Aspirin, metformin, and hyperosmotic sorbitol increased pACC ser79P ~ 20-fold, and BR-DIM caused a ~ 30-fold increase over two-cell embryos cultured for 1 h in KSOMaa but only 3- to 6-fold increase in blastocysts. We previously showed that these stimuli decreased Oct4 40-85% in two-cell embryos that was ~ 60-90% reversible by coculture with AMPK inhibitor CC. However, Oct4 decreased only 30-50% in blastocysts, although reversibility of loss by CC was similar at both embryo stages. Met and BR-DIM previously caused a near-complete cell proliferation arrest in two-cell embryos and here Met caused lower CC-reversible growth decrease and AMPK-independent BR-DIM-induced blastocyst growth decrease.

CONCLUSION

Inducing drug or diet supplements decreased anabolism, growth, and stemness have a greater impact on AMPK-dependent processes in two-cell embryos compared to blastocysts.

CoQ10 increases mitochondrial mass and polarization, ATP and Oct4 potency levels, and bovine oocyte MII during IVM while decreasing AMPK activity and oocyte death

PURPOSE

We tested whether mitochondrial electron transport chain electron carrier coenzyme Q10 (CoQ10) increases ATP during bovine IVM and increases %M2 oocytes, mitochondrial polarization/mass, and Oct4, and decreases pAMPK and oocyte death.

METHODS

Bovine oocytes were aspirated from ovaries and cultured in IVM media for 24 h with 0, 20, 40, or 60 μM CoQ10. Oocytes were assayed for ATP by luciferase-based luminescence. Oocyte micrographs were quantitated for Oct4, pAMPK (i.e., activity), polarization by JC1 staining, and mitochondrial mass by MitoTracker Green staining.

RESULTS

CoQ10 at 40 μM was optimal. Oocytes at 40 μM enabled 1.9-fold more ATP than 0 μM CoQ10. There was 4.3-fold less oocyte death, 1.7-fold more mitochondrial charge polarization, and 3.1-fold more mitochondrial mass at 40 μM than at 0 μM CoQ10. Increased ATP was associated with 2.2-fold lower AMPK thr172P activation and 2.1-fold higher nuclear Oct4 stemness/potency protein at 40 μM than at 0 μM CoQ10. CoQ10 is hydrophobic, and at all doses, 50% was lost from media into oil by ~ 12 h. Replenishing CoQ10 at 12 h did not significantly diminish dead oocytes.

CONCLUSIONS

The data suggest that CoQ10 improves mitochondrial function in IVM where unwanted stress, higher AMPK activity, and Oct4 potency loss are induced.

Using stem cell oxygen physiology to optimize blastocyst culture while minimizing hypoxic stress

This review is a response to the Fellows Forum on testing 2% oxygen for best culture of human blastocysts (J Ass Reprod Gen 34:303-8, 1; J Ass Reprod Gen 34:309-14, 2) prior to embryo transfer. It is a general analysis in support of the position that an understanding of stem cell physiology and responses to oxygen are necessary for optimization of blastocyst culture in IVF and to enhance reproductive success in fertile women.

Hypoxic Stress Forces Irreversible Differentiation of a Majority of Mouse Trophoblast Stem Cells Despite FGF4

Hypoxic, hyperosmotic, and genotoxic stress slow mouse trophoblast stem cell (mTSC) proliferation, decrease potency/stemness, and increase differentiation. Previous reports suggest a period of reversibility in stress-induced mTSC differentiation. Here we show that hypoxic stress at 0.5% O₂ decreased potency factor protein by ∼60%–90% and reduced growth to nil. Hypoxia caused a 35-fold increase in apoptosis at Day 3 and a 2-fold increase at Day 6 above baseline. The baseline apoptosis rate was only 0.3%. Total protein was never less than baseline during hypoxic treatment, suggesting 0.5% O₂ is a robust, nonmorbid stressor. Hypoxic stress induced ∼50% of trophoblast giant cell (TGC) differentiation with a simultaneous 5- to 6-fold increase in the TGC product antiluteolytic prolactin family 3, subfamily d, member 1 (PRL3D1), despite the presence of fibroblast growth factor 4 (FGF4). Hypoxia-induced TGC differentiation was also supported by potency and differentiation mRNA marker analysis. FGF4 removal at 20% O₂ committed cell fate towards irreversible differentiation at 2 days, with similar TGC percentages after an additional 3 days of culture under potency conditions when FGF4 was readded or under differentiation conditions without FGF4. However, hypoxic stress required 4 days to irreversibly differentiate cells. Runted stem cell growth, forced differentiation of fewer cells, and irreversible differentiation limit total available stem cell population. Were mTSCs to respond to stress in a similar mode in vivo, miscarriage might occur as a result, which should be tested in the future.

Commonly used fertility drugs, a diet supplement, and stress force AMPK-dependent block of stemness and development in cultured mammalian embryos

PURPOSE

The purpose of the present study is to test whether metformin, aspirin, or diet supplement (DS) BioResponse-3,3'-Diindolylmethane (BR-DIM) can induce AMP-activated protein kinase (AMPK)-dependent potency loss in cultured embryos and whether metformin (Met) + Aspirin (Asa) or BR-DIM causes an AMPK-dependent decrease in embryonic development.

METHODS

The methods used were as follows: culture post-thaw mouse zygotes to the two-cell embryo stage and test effects after 1-h AMPK agonists' (e.g., Met, Asa, BR-DIM, control hyperosmotic stress) exposure on AMPK-dependent loss of Oct4 and/or Rex1 nuclear potency factors, confirm AMPK dependence by reversing potency loss in two-cell-stage embryos with AMPK inhibitor compound C (CC), test whether Met + Asa (i.e., co-added) or DS BR-DIM decreases development of two-cell to blastocyst stage in an AMPK-dependent (CC-sensitive) manner, and evaluate the level of Rex1 and Oct4 nuclear fluorescence in two-cell-stage embryos and rate of two-cell-stage embryo development to blastocysts.

RESULT(S)

Met, Asa, BR-DIM, or hyperosmotic sorbitol stress induces rapid ~50-85 % Rex1 and/or Oct4 protein loss in two-cell embryos. This loss is ~60-90 % reversible by co-culture with AMPK inhibitor CC. Embryo development from two-cell to blastocyst stage is decreased in culture with either Met + Asa or BR-DIM, and this is either >90 or ~60 % reversible with CC, respectively.

CONCLUSION

These experimental designs here showed that Met-, Asa-, BR-DIM-, or sorbitol stress-induced rapid potency loss in two-cell embryos is AMPK dependent as suggested by inhibition of Rex1 and/or Oct4 protein loss with an AMPK inhibitor. The DS BR-DIM or fertility drugs (e.g., Met + Asa) that are used to enhance maternal metabolism to support fertility can also chronically slow embryo growth and block development in an AMPK-dependent manner.

Development and Validation of a Rex1-RFP Potency Activity Reporter Assay That Quantifies Stress-Forced Potency Loss in Mouse Embryonic Stem Cells

Assays for embryonic stem cells (ESCs) of the blastocyst are needed to quantify stress-induced decreases of potent subpopulations. High-throughput screens (HTSs) of stressed ESCs quantify embryonic stress, diminishing laboratory animal needs. Normal or stress-induced ESC differentiation is marked by Rex1 potency factor loss. Potency reporter ESC assays were developed, using low-stress techniques to create transgenic ESCs. Rex1 and Oct4 promoters drove RFP and green fluorescent protein (GFP) expression, respectively. Lentivirus infection and fluorescence-activated cell sorting selection of ESCs obviated the need for stressful electroporation and antibiotic selection, respectively. We showed using immunoblots, microscopic analysis, flow cytometry, and fluorescence microplate reader that the response to stress of potency-reporter ESCs is similar to parental ESCs assayed by biochemical means. Stress caused a dose-dependent decrease in bright Rex1-RFP(+) ESCs and increase in Rex1 dim ESCs. At highest stress, ∼ 20% of bright Rex1-RFP cells are lost coinciding with a 2.8-fold increase in Rex1-RFP dim cells that approach 20%. This conversion of bright to dim cells tested by flow cytometry is commensurate with about 60% loss in fluorescence measured by microplate reader. Dose-dependent stress-induced Rex1-RFP and endogenous Rex1 protein decreases are similar. The data show that Rex1 reporter ESCs accurately report stress in a microplate reader-based HTS. The increasing dim Rex1 subpopulation size is balanced by the decreasing total ESC number during culture at multiple sorbitol doses. This is consistent with previous observations that stress forces potency decrease and differentiation increase to compensate for stress-induced diminished stem cell population growth.

Molecular biology of the stress response in the early embryo and its stem cells

Stress is normal during early embryogenesis and transient, elevated stress is commonplace. Stress in the milieu of the peri-implantation embryo is a summation of maternal hormones, and other elements of the maternal milieu, that signal preparedness for development and implantation. Examples discussed here are leptin, adrenaline, cortisol, and progesterone. These hormones signal maternal nutritional status and provide energy, but also signal stress that diverts maternal and embryonic energy from an optimal embryonic developmental trajectory. These hormones communicate endocrine maternal effects and local embryonic effects although signaling mechanisms are not well understood. Other in vivo stresses affect the embryo such as local infection and inflammation, hypoxia, environmental toxins such as benzopyrene, dioxin, or metals, heat shock, and hyperosmotic stress due to dehydration or diabetes. In vitro, stresses include shear during handling, improper culture media and oxygen levels, cryopreservation, and manipulations of the embryo to introduce sperm or mitochondria. We define stress as any stimulus that slows stem cell accumulation or diminishes the ability of cells to produce normal and sufficient parenchymal products upon differentiation. Thus stress deflects downwards the normal trajectories of development, growth and differentiation. Typically stress is inversely proportional to embryonic developmental and proliferative rates, but can be proportional to induction of differentiation of stem cells in the peri-implantation embryo. When modeling stress it is most interesting to produce a 'runting model' where stress exposures slow accumulation but do not create excessive apoptosis or morbidity. Windows of stress sensitivity may occur when major new embryonic developmental programs require large amounts of energy and are exacerbated if nutritional flow decreases and removes energy from the normal developmental programs and stress responses. These windows correspond to zygotic genome activation, the large mRNA program initiated at compaction, ion pumping required for cavitation, the differentiation of the first lineages, integration with the uterine environment at implantation, rapid proliferation of stem cells, and production of certain lineages which require the highest energy and are most sensitive to mitochondrial inhibition. Stress response mechanisms insure that stem cells for the early embryo and placenta survive at lower stress exposures, and that the organism survives through compensatory and prioritized stem cell differentiation, at higher stress exposures. These servomechanisms include a small set of stress enzymes from the 500 protein kinases in the kinome; the part of the genome coding for protein kinases that hierarchically regulate the activity of other proteins and enzymes. Important protein kinases that mediate the stress response of embryos and their stem cells are SAPK, p38MAPK, AMPK, PI3K, Akt, MEK1/2, MEKK4, PKA, IRE1 and PERK. These stress enzymes have cytosolic function in cell survival at low stress exposures and nuclear function in modifying transcription factor activity at higher stress exposures. Some of the transcription factors (TFs) that are most important in the stress response are JunC, JunB, MAPKAPs, ATF4, XBP1, Oct1, Oct4, HIFs, Nrf2/KEAP, NFKB, MT1, Nfat5, HSF1/2 and potency-maintaining factors Id2, Cdx2, Eomes, Sox2, Nanog, Rex1, and Oct4. Clearly the stress enzymes have a large number of cytosolic and nuclear substrates and the TFs regulate large numbers of genes. The interaction of stress enzymes and TFs in the early embryo and its stem cells are a continuing central focus of research. In vitro regulation of TFs by stress enzymes leads to reprogramming of the stem cell when stress diminishes stem cell accumulation. Since more differentiated product is produced by fewer cells, the process compensates for fewer cells. Coupled with stress-induced compensatory differentiation of stem cells is a tendency to prioritize differentiation by increasing the first essential lineage and decreasing later lineages. These mechanisms include stress enzymes that regulate TFs and provide stress-specific, shared homeostatic cellular and organismal responses of prioritized differentiation.

Hypoxic stress induces, but cannot sustain trophoblast stem cell differentiation to labyrinthine placenta due to mitochondrial insufficiency

Dysfunctional stem cell differentiation into placental lineages is associated with gestational diseases. Of the differentiated lineages available to trophoblast stem cells (TSC), elevated O2 and mitochondrial function are necessary to placental lineages at the maternal-placental surface and important in the etiology of preeclampsia. TSC lineage imbalance leads to embryonic failure during uterine implantation. Stress at implantation exacerbates stem cell depletion by decreasing proliferation and increasing differentiation. In an implantation site O2 is normally ~2%. In culture, exposure to 2% O2 and fibroblast growth factor 4 (FGF4) enabled the highest mouse TSC multipotency and proliferation. In contrast, hypoxic stress (0.5% O2) initiated the most TSC differentiation after 24h despite exposure to FGF4. However, hypoxic stress supported differentiation poorly after 4-7 days, despite FGF4 removal. At all tested O2 levels, FGF4 maintained Warburg metabolism; mitochondrial inactivity and aerobic glycolysis. However, hypoxic stress suppressed mitochondrial membrane potential and maintained low mitochondrial cytochrome c oxidase (oxidative phosphorylation/OxPhos), and high pyruvate kinase M2 (glycolysis) despite FGF4 removal. Inhibiting OxPhos inhibited optimum differentiation at 20% O2. Moreover, adding differentiation-inducing hyperosmolar stress failed to induce differentiation during hypoxia. Thus, differentiation depended on OxPhos at 20% O2; hypoxic and hyperosmolar stresses did not induce differentiation at 0.5% O2. Hypoxia-limited differentiation and mitochondrial inhibition and activation suggest that differentiation into two lineages of the labyrinthine placenta requires O2>0.5-2% and mitochondrial function. Stress-activated protein kinase increases an early lineage and suppresses later lineages in proportion to the deviation from optimal O2 for multipotency, thus it is the first enzyme reported to prioritize differentiation.