The intracellular pH-regulatory HCO3-/Cl- exchanger in the mouse oocyte is inactivated during first meiotic metaphase and reactivated after egg activation via the MAP kinase pathway

Acidosis defense mechanisms in the preimplantation stages of embryos in BALB/c strain mice

Regulation of intracellular pH (pHi) is an important homeostatic function of cells. There are three major pHi regulatory mechanisms: the HCO3-/Cl- exchanger (AE), which alleviates alkalosis, and the Na+/H+ exchanger (NHE) and Na+,HCO3-/Cl- exchanger (NDBCE), both of which counteract acidosis. NHE activity, which is high at the germinal vesicle stage of oocyte, is inhibited during meiotic maturation, while this inhibition is abolished when the oocyte reaches the pronuclear (PN) stage of the zygote. On the other hand, we have previously found that NDBCE performs complementary regulation against acidosis during meiotic maturation. Additionally, we found that AE activity, which is a defense mechanism against alkalosis, gradually decreases during preimplantation period of embryonic development. Considering that NHE activity is inhibited during meiotic maturation and AE activity gradually decreases during embryonic development stages, we investigated whether NHE and NDBCE activities, both of which act against acidosis, functionally change from the PN zygote to the blastocyst stage of the embryo and identified these pH-regulating proteins at the molecular level in mice of the Balb/c strain. PN zygotes, two-cell (2-c), four-cell (4-c), morula and blastocyst stage embryos were obtained from 5-8-week-old, sexually mature female Balb/c mice by using the classical superovulation procedure. pHi was recorded by using the microspectrofluorometric technique on zygotes and embryos simultaneously loaded with the pH-sensitive fluorophore, 2',7'-Bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). The activities of NHE and NDBCE were determined from the recovery curve of induced-acidosis in bicarbonate-free and bicarbonate-containing media, respectively. Specific inhibitors such as cariporide (1 μM), S3226 (1 and 10 μM), EIPA (1, 5, and 25 μM), and amiloride (1 mM) were used to functionally identify NHE isoforms, and the nonspecific inhibitor 4,4'-diisocyanatostilbene-2,2' disulphonic acid, disodium salt (DIDS) was used to confirm NDBCE activity. The isoforms of the pHi-regulatory proteins were also identified by molecular biology using real-time PCR. We found that NHE activity was high at all embryonic stages, and differences between stages were not significant. Functional and molecular findings indicated that isoforms of NHE 1 and 5 are present in the blastocyst, whereas isoforms of NHE 1, 3, and 4 are functional at earlier embryonic stages. Although the contribution of NDBCE activity to recovery from induced-acidosis was detected at all embryonic stages, it was significant only in the PN zygote and the 2-c embryo. This finding was confirmed by molecular analysis, which detected the expression of SLC4A8 encoding NDBCE at all embryonic stages. In conclusion, NHE is an active and important defense mechanism against acidosis and is encoded by at least two protein isoforms in all stages of the Balb/c strain of mice. NDBCE has a supportive function in all embryonic stages, especially in the PN zygote and the 2-c embryo. Preimplantation stage embryos have effective mechanisms to defend against acidosis in response to their metabolic end products (increased acid load) and the acidic environment in utero.

Initiation of cell volume regulation and unique cell volume regulatory mechanisms in mammalian oocytes and embryos

The period beginning with the signal for ovulation, when a fully-grown oocyte progresses through meiosis to become a mature egg that is fertilized and develops as a preimplantation embryo, is crucial for healthy development. The early preimplantation embryo is unusually sensitive to cell volume perturbations, with even moderate decreases in volume or dysregulation of volume-regulatory mechanisms resulting in developmental arrest. To prevent this, early embryos possess mechanisms of cell volume control that are apparently unique to them. These rely on the accumulation of glycine and betaine (N, N, N-trimethylglycine) as organic osmolytes-compounds that can provide intracellular osmotic support without the deleterious effects of inorganic ions. Preimplantation embryos also have the same mechanisms as somatic cells that mediate rapid responses to deviations in cell volume, which rely on inorganic ion transport. Both the unique, embryo-specific mechanisms that use glycine and betaine and the inorganic ion-dependent mechanisms undergo major changes during meiotic maturation and preimplantation development. The most profound changes occur immediately after ovulation is triggered. Before this, oocytes cannot regulate their volume, since they are strongly attached to their rigid extracellular matrix shell, the zona pellucida. After ovulation is triggered, the oocyte detaches from the zona pellucida and first becomes capable of independent volume regulation. A complex set of developmental changes in each cell volume-regulatory mechanism continues through egg maturation and preimplantation development. The unique cell volume-regulatory mechanisms in eggs and preimplantation embryos and the developmental changes they undergo appear critical for normal healthy embryo development.

Need for choosing the ideal pH value for IVF culture media

PURPOSE

Monitoring the pH of IVF culture media is a good practice, but the required pH levels have been "arbitrarily" set. Assisted reproductive technology centers around the world are spending time and money on pH monitoring without any consensus to date. The objective of this narrative review was to evaluate the importance of pH monitoring during IVF, discover how the oocyte and embryo regulate their intracellular pH and try to determine the optimal pH to be applied.

METHODS

A narrative literature review was performed on publications in the PubMed database reporting on the impact of pH on cellular function, oocyte and embryo development, IVF outcomes and pathophysiology, or on physiological pH in the female reproductive tract.

RESULTS

Intracellular pH regulates many cellular processes such as meiotic spindle stability of the oocyte, cell division and differentiation, embryo enzymatic activities, and blastocoel formation. The internal pH of the human embryo is maintained by regulatory mechanisms (mainly Na⁺/H⁺ and HCO₃^-/Cl^- exchangers) that can be exceeded, particularly in the oocyte and early-stage embryos. The opinion that the optimal pH for embryo culture is physiological pH is not correct since several physicochemical parameters specific to IVF culture conditions (temperature, medium composition, duration of culture, or implication of CO₂) can modify the intracellular pH of the embryo and change its needs and adaptability.

CONCLUSIONS

Because correct and stable extracellular pH is essential to embryo health and development, monitoring pH is imperative. However, there is a lack of clinical data on choosing the ideal pH for human IVF culture media.

Rapid Prototyping of a Cyclic Olefin Copolymer Microfluidic Device for Automated Oocyte Culturing

Assisted reproductive technology (ART) can benefit from the features of microfluidic technologies, such as the automation of time-consuming labor-intensive procedures, the possibility to mimic in vivo environments, and the miniaturization of the required equipment. To date, most of the proposed approaches are based on polydimethylsiloxane (PDMS) as platform substrate material due to its widespread use in academia, despite certain disadvantages, such as the elevated cost of mass production. Herein, we present a rapid fabrication process for a cyclic olefin copolymer (COC) monolithic microfluidic device combining hot embossing—using a low-temperature cofired ceramic (LTCC) master—and micromilling. The microfluidic device was suitable for trapping and maturation of bovine oocytes, which were further studied to determine their ability to be fertilized. Furthermore, another COC microfluidic device was fabricated to store sperm and assess its quality parameters over time. The study herein presented demonstrates a good biocompatibility of the COC when working with gametes, and it exhibits certain advantages, such as the nonabsorption of small molecules, gas impermeability, and low fabrication costs, all at the prototyping and mass production scale, thus taking a step further toward fully automated microfluidic devices in ART.

Rapid Prototyping of a Cyclic Olefin Copolymer Microfluidic Device for Automated Oocyte Culturing

Assisted reproductive technology (ART) can benefit from the features of microfluidic technologies, such as the automation of time-consuming labor-intensive procedures, the possibility to mimic in vivo environments, and the miniaturization of the required equipment. To date, most of the proposed approaches are based on polydimethylsiloxane (PDMS) as platform substrate material due to its widespread use in academia, despite certain disadvantages, such as the elevated cost of mass production. Herein, we present a rapid fabrication process for a cyclic olefin copolymer (COC) monolithic microfluidic device combining hot embossing-using a low-temperature cofired ceramic (LTCC) master-and micromilling. The microfluidic device was suitable for trapping and maturation of bovine oocytes, which were further studied to determine their ability to be fertilized. Furthermore, another COC microfluidic device was fabricated to store sperm and assess its quality parameters over time. The study herein presented demonstrates a good biocompatibility of the COC when working with gametes, and it exhibits certain advantages, such as the nonabsorption of small molecules, gas impermeability, and low fabrication costs, all at the prototyping and mass production scale, thus taking a step further toward fully automated microfluidic devices in ART.

Elevated intracellular pH appears in aged oocytes and causes oocyte aneuploidy associated with the loss of cohesion in mice

Increases in the aneuploidy rate caused by the deterioration of cohesion with increasing maternal age have been well documented. However, the molecular mechanism for the loss of cohesion in aged oocytes remains unknown. In this study, we found that intracellular pH (pHi) was elevated in aged oocytes, which might disturb the structure of the cohesin ring to induce aneuploidy. We observed for the first time that full-grown germinal vesicle (GV) oocytes displayed an increase in pHi with advancing age in CD1 mice. Furthermore, during the in vitro oocyte maturation process, the pHi was maintained at a high level, up to ∼7.6, in 12-month-old mice. Normal pHi is necessary to maintain protein localization and function. Thus, we put forward a hypothesis that the elevated oocyte pHi might be related to the loss of cohesion and the increased aneuploidy in aged mice. Through the in vitro alkalinization treatment of young oocytes, we observed that the increased pHi caused an increase in the aneuploidy rate and the sister inter-kinetochore (iKT) distance associated with the strength of cohesion and caused a decline in the cohesin subunit SMC3 protein level. Young oocytes with elevated pHi exhibited substantially the increase in chromosome misalignment.

Cortical mechanics and myosin-II abnormalities associated with post-ovulatory aging: implications for functional defects in aged eggs

STUDY HYPOTHESIS

Cellular aging of the egg following ovulation, also known as post-ovulatory aging, is associated with aberrant cortical mechanics and actomyosin cytoskeleton functions.

STUDY FINDING

Post-ovulatory aging is associated with dysfunction of non-muscle myosin-II, and pharmacologically induced myosin-II dysfunction produces some of the same deficiencies observed in aged eggs.

WHAT IS KNOWN ALREADY

Reproductive success is reduced with delayed fertilization and when copulation or insemination occurs at increased times after ovulation. Post-ovulatory aged eggs have several abnormalities in the plasma membrane and cortex, including reduced egg membrane receptivity to sperm, aberrant sperm-induced cortical remodeling and formation of fertilization cones at the site of sperm entry, and reduced ability to establish a membrane block to prevent polyspermic fertilization.

STUDY DESIGN, SAMPLES/MATERIALS, METHODS

Ovulated mouse eggs were collected at 21-22 h post-human chorionic gonadotrophin (hCG) (aged eggs) or at 13-14 h post-hCG (young eggs), or young eggs were treated with the myosin light chain kinase (MLCK) inhibitor ML-7, to test the hypothesis that disruption of myosin-II function could mimic some of the effects of post-ovulatory aging. Eggs were subjected to various analyses. Cytoskeletal proteins in eggs and parthenogenesis were assessed using fluorescence microscopy, with further analysis of cytoskeletal proteins in immunoblotting experiments. Cortical tension was measured through micropipette aspiration assays. Egg membrane receptivity to sperm was assessed in in vitro fertilization (IVF) assays. Membrane topography was examined by low-vacuum scanning electron microscopy (SEM).

MAIN RESULTS AND THE ROLE OF CHANCE

Aged eggs have decreased levels and abnormal localizations of phosphorylated myosin-II regulatory light chain (pMRLC; P = 0.0062). Cortical tension, which is mediated in part by myosin-II, is reduced in aged mouse eggs when compared with young eggs, by ∼40% in the cortical region where the metaphase II spindle is sequestered and by ∼50% in the domain to which sperm bind and fuse (P < 0.0001). Aging-associated parthenogenesis is partly rescued by treating eggs with a zinc ionophore (P = 0.003), as is parthenogenesis induced by inhibition of mitogen-activated kinase (MAPK) 3/1 [also known as extracellular signal-regulated kinase (ERK)1/2] or MLCK. Inhibition of MLCK with ML-7 also results in effects that mimic those of post-ovulatory aging: fertilized ML-7-treated eggs show both impaired fertilization and increased extents of polyspermy, and ML-7-treated young eggs have several membrane abnormalities that are shared by post-ovulatory aged eggs.

LIMITATIONS, REASONS FOR CAUTION

These studies were done with mouse oocytes, and it remains to be fully determined how these findings from mouse oocytes would compare with other species. For studies using methods not amenable to analysis of large sample sizes and data are limited to what images one can capture (e.g. SEM), data should be interpreted conservatively.

WIDER IMPLICATIONS OF THE FINDINGS

These data provide insights into causes of reproductive failures at later post-copulatory times.

LARGE SCALE DATA

Not applicable.

STUDY FUNDING AND COMPETING INTERESTS

This project was supported by R01 HD037696 and R01 HD045671 from the NIH to J.P.E. Cortical tension studies were supported by R01 GM66817 to D.N.R. The authors declare there are no financial conflicts of interest.

Optimizing the culture environment and embryo manipulation to help maintain embryo developmental potential

With increased use of comprehensive chromosome screening (CCS), the question remains as to why some practices do not experience the same high levels of clinical success after implementation of the approach. Indeed, the debate surrounding the efficacy and usefulness of blastocyst biopsy and CCS continues. Importantly, several variables impact the success of an assisted reproductive technology cycle. Transfer of a euploid embryo is but one factor in an intricate system that requires numerous steps to occur successfully. Certainly, the culture environment and the manipulations of the embryo during its time in the laboratory can impact its reproductive potential. Environmental stressors ranging from culture media to culture conditions and even culture platform can impact biochemical, metabolic, and epigenetic patterns that can affect the developing cell independent of chromosome number. Furthermore, accompanying procedures, such as biopsy and vitrification, are complex and, when performed improperly, can negatively impact embryo quality. These are areas that likely still carry room for improvement within the IVF laboratory.

Media composition: pH and buffers

The proper pH of media is a crucial parameter for optimizing efficacy of gamete and embryo culture. Selecting the right media pH and stabilizing this pH are important variables in minimizing intracellular stress and optimizing development. Regulation of intracellular (pHi) and extracellular pH (pHe) is discussed, as well as methods to prevent harmful pHe oscillations. Furthermore, proper approaches to ensure accurate measurement of media pHe are described.

K+ efflux through two-pore domain K+ channels is required for mouse embryonic development

Numerous studies have suggested that K+ channels regulate a wide range of physiological processes in mammalian cells. However, little is known about the specific function of K+ channels in germ cells. In this study, mouse zygotes were cultured in a medium containing K+ channel blockers to identify the functional role of K+ channels in mouse embryonic development. Voltage-dependent K+ channel blockers, such as tetraethylammonium and BaCl2, had no effect on embryonic development to the blastocyst stage, whereas K2P channel blockers, such as quinine, selective serotonin reuptake inhibitors (fluoxetine, paroxetine, and citalopram), gadolinium trichloride, anandamide, ruthenium red, and zinc chloride, significantly decreased blastocyst formation (P<0.05). RT-PCR data showed that members of the K2P channel family, specifically KCNK2, KCNK10, KCNK4, KCNK3, and KCNK9, were expressed in mouse oocytes and embryos. In addition, their mRNA expression levels, except Kcnk3, were up-regulated by above ninefold in morula-stage embryos compared with 2-cell stage embryos (2-cells). Immunocytochemical data showed that KCNK2, KCNK10, KCNK4, KCNK3, and KCNK9 channel proteins were expressed in the membrane of oocytes, 2-cells, and blastocysts. Each siRNA injection targeted at Kcnk2, Kcnk10, Kcnk4, Kcnk3, and Kcnk9 significantly decreased blastocyst formation by ∼38% compared with scrambled siRNA injection (P<0.05). The blockade of K2P channels acidified the intracellular pH and depolarized the membrane potential. These results suggest that K2P channels could improve mouse embryonic development through the modulation of gating by activators.

The physiology of bicarbonate transporters in mammalian reproduction

HCO(3)(-) plays critically important roles during virtually the entire process of reproduction in mammals, including spermatogenesis, sperm capacitation, fertilization, and development of early stage embryos. Therefore, the acid-base balance in the male and female reproductive tracts must be finely modulated. The fluid milieu in the epididymis is acidic, containing very low concentration of HCO(3)(-). In this acidic low HCO(3)(-) environment, mature sperm are rendered quiescent in the epididymis. In contrast, the luminal fluid in the female uterus and oviduct is alkaline, with very high concentration of HCO(3)(-) that is essential for sperm to fulfill fertilization. HCO(3)(-) transporter of solute carrier 4 (SLC4) and SLC26 families represent the major carriers for HCO(3)(-) transport across the plasma membrane. These transporters play critical roles in intracellular pH regulation and transepithelial HCO(3)(-) transport. The physiological roles of these transporters in mammalian reproduction are of fundamental interest to investigators. Here we review recent progress in understanding the expression of HCO(3)(-) transporters in reproductive tract tissues as well as the physiological roles of these transporters in mammalian reproduction.

Gatekeeper of pluripotency: a common Oct4 transcriptional network operates in mouse eggs and embryonic stem cells

BACKGROUND

Oct4 is a key factor of an expanded transcriptional network (Oct4-TN) that governs pluripotency and self-renewal in embryonic stem cells (ESCs) and in the inner cell mass from which ESCs are derived. A pending question is whether the establishment of the Oct4-TN initiates during oogenesis or after fertilisation. To this regard, recent evidence has shown that Oct4 controls a poorly known Oct4-TN central to the acquisition of the mouse egg developmental competence. The aim of this study was to investigate the identity and extension of this maternal Oct4-TN, as much as whether its presence is circumscribed to the egg or maintained beyond fertilisation.

RESULTS

By comparing the genome-wide transcriptional profile of developmentally competent eggs that express the OCT4 protein to that of developmentally incompetent eggs in which OCT4 is down-regulated, we unveiled a maternal Oct4-TN of 182 genes. Eighty of these transcripts escape post-fertilisation degradation and represent the maternal Oct4-TN inheritance that is passed on to the 2-cell embryo. Most of these 80 genes are expressed in cancer cells and 37 are notable companions of the Oct4 transcriptome in ESCs.

CONCLUSIONS

These results provide, for the first time, a developmental link between eggs, early preimplantation embryos and ESCs, indicating that the molecular signature that characterises the ESCs identity is rooted in oogenesis. Also, they contribute a useful resource to further study the mechanisms of Oct4 function and regulation during the maternal-to-embryo transition and to explore the link between the regulation of pluripotency and the acquisition of de-differentiation in cancer cells.

Mouse embryos stressed by physiological levels of osmolarity become arrested in the late 2-cell stage before entry into M phase

Preimplantation mouse embryos of many strains become arrested at the 2-cell stage if the osmolarity of culture medium that normally supports development to blastocysts is raised to approximately that of their normal physiological environment in the oviduct. Arrest can be prevented if molecules that serve as "organic osmolytes" are present in the medium, because organic osmolytes, principally glycine, are accumulated by embryos to provide intracellular osmotic support and regulate cell volume. Medium with an osmolarity of 310 mOsM induced arrest of approximately 80% of CF1 mouse embryos at the 2-cell stage, in contrast to the approximately 100% that progressed beyond the 2-cell stage at 250 or 301 mOsM with glycine. The nature of this arrest induced by physiological levels of osmolarity is unknown. Arrest was reversible by transfer to lower-osmolarity medium at any point during the 2-cell stage, but not after embryos would normally have progressed to the 4-cell stage. Cessation of development likely was not due to apoptosis, as shown by lack of external annexin V binding, detectable cytochrome c release from mitochondria, or nuclear DNA fragmentation. Two-cell embryos cultured at 310 mOsM progressed through the S phase, and zygotic genome activation markers were expressed. However, most embryos failed to initiate the M phase, as evidenced by intact nuclei with decondensed chromosomes, low M-phase promoting factor activity, and an inactive form of CDK1, although a few blastomeres were arrested in metaphase. Thus, embryos become arrested late in the G(2) stage of the second embryonic cell cycle when stressed by physiological osmolarity in the absence of organic osmolytes.

Essential role of protein phosphatase 2A in metaphase II arrest and activation of mouse eggs shown by okadaic acid, dominant negative protein phosphatase 2A, and FTY720

Vertebrate eggs arrest at second meiotic metaphase. The fertilizing sperm causes meiotic exit through Ca(2+)-mediated activation of the anaphase-promoting complex/cyclosome (APC/C). Although the loss in activity of the M-phase kinase CDK1 is known to be an essential downstream event of this process, the contribution of phosphatases to arrest and meiotic resumption is less apparent, especially in mammals. Therefore, we explored the role of protein phosphatase 2A (PP2A) in mouse eggs using pharmacological inhibition and activation as well as a functionally dominant-negative catalytic PP2A subunit (dn-PP2Ac-L199P) coupled with live cell imaging. We observed that PP2A inhibition using okadaic acid induced events normally observed at fertilization: degradation of the APC/C substrates cyclin B1 and securin resulting from loss of the APC/C inhibitor Emi2. Although sister chromatids separated, chromatin remained condensed, and polar body extrusion was blocked as a result of a rapid spindle disruption, which could be ameliorated by non-degradable cyclin B1, suggesting that spindle integrity was affected by CDK1 loss. Similar cell cycle effects to okadaic acid were also observed using dominant-negative PP2Ac. Preincubation of eggs with the PP2A activator FTY720 could block many of the actions of okadaic acid, including Emi2, cyclin B1, and securin degradation and sister chromatid separation. Therefore, in conclusion, we used okadaic acid, dn-PP2Ac-L199P, and FTY720 on mouse eggs to demonstrate that PP2A is needed to for both continued metaphase arrest and successful exit from meiosis.

Optimizing the culture environment in the IVF laboratory: impact of pH and buffer capacity on gamete and embryo quality

Supplying and maintaining appropriate culture conditions is critical to minimize stress imposed upon gametes and embryos and to optimize the in-vitro environment. One parameter that requires close scrutiny in this endeavour is pH. Though embryos have a limited ability to regulate their internal pH (pH(i)), oocytes lack robust mechanisms. Thus, careful attention to external pH (pH(e)) of culture media is imperative in IVF. Ability to withstand deviations in hydrogen ion concentration varies depending on culture conditions, as well as laboratory procedures. Cryopreserved--thaw--thawed embryos, as well as denuded oocytes, are especially susceptible to perturbations in pH(e). Therefore, proper setting, monitoring and stabilizing of pH(e) during IVF laboratory procedures is a crucial component of a rigorous quality control programme. Here, importance of both pH(i) and pH(e) in respect to gamete and embryo quality are discussed. Furthermore, factors influencing selection of pH(e), as well as emerging methods to stabilize pH(e) in the IVF laboratory are detailed.

HCO3(-)/Cl(-) exchange inactivation and reactivation during mouse oocyte meiosis correlates with MEK/MAPK-regulated Ae2 plasma membrane localization

Background

Germinal Vesicle (GV) stage mouse oocytes in first meiotic prophase exhibit highly active HCO₃⁻/Cl⁻ exchange—a class of transport nearly ubiquitously involved in regulation of intracellular pH and cell volume. During meiosis, however, oocyte HCO₃⁻/Cl⁻ exchange becomes inactivated during first metaphase (MI), remains inactive in second metaphase (MII), and is reactivated only after egg activation. Previous work using pharmacological manipulations had indicated that activity of the MEK/MAPK signaling pathway was negatively correlated with HCO₃⁻/Cl⁻ exchange activity during meiosis. However, the mechanism by which the exchanger is inactivated during meiotic progression had not been determined, nor had the role of MEK/MAPK been directly established.

Methodology/Principal Findings

Expression of a constitutively active form of MEK (MAP kinase kinase), which prevented the normal downregulation of MAPK after egg activation, also prevented reactivation of HCO₃⁻/Cl⁻ exchange. Conversely, suppression of endogenous MAPK activity with dominant negative MEK activated the normally quiescent HCO₃⁻/Cl⁻ exchange in mature MII eggs. A GFP-tagged form of the HCO₃⁻/Cl⁻ exchanger isoform Ae2 (Slc4a2) was strongly expressed at the GV oocyte plasma membrane, but membrane localization decreased markedly during meiotic progression. A similar pattern for endogenous Ae2 was confirmed by immunocytochemistry. The loss of membrane-localized Ae2 appeared selective, since membrane localization of a GFP-tagged human dopamine D1 receptor did not change during meiotic maturation.

Conclusions

Direct manipulation of MAPK activity indicated that GFP-tagged Ae2 localization depended upon MAPK activity. Inactivation of HCO₃⁻/Cl⁻ exchange during the meiotic cell cycle may therefore reflect the loss of Ae2 from the oocyte plasma membrane, downstream of MEK/MAPK signaling. This identifies a novel role for MEK/MAPK-mediated cytostatic factor (CSF) activity during meiosis in membrane protein trafficking in mouse oocytes, and shows for the first time that selective retrieval of membrane proteins is a feature of meiosis in mammalian oocytes.

New pH-buffering system for media utilized during gamete and embryo manipulations for assisted reproduction

Maintenance of stable pH is important for optimizing gamete and embryo culture. One method to stabilize pH entails using zwitterionic buffers in IVF handling media used outside the laboratory incubator. Current handling media utilize single buffers, such as MOPS or HEPES. However, the use of a single buffer limits the ability to adjust the range of buffering capacity. Furthermore, changes in temperature alter buffering of these compounds. Therefore, traditional IVF handling media utilizing a single buffer may not provide ideal pH buffering. This study reports that combining multiple buffers, such as HEPES, MOPS and DIPSO, into a single medium in various ratios gives the ability to shift the effective buffering range to cover a specific pH. Additionally, by combining various buffers, it is possible to expand pH buffering over a range of temperatures, while simultaneously reducing the absolute concentration of individual buffers, thereby reducing or alleviating toxicity concerns. This report verifies that DIPSO, MOPS and HEPES, and their combinations, support embryo development. Therefore, utilization of bi- and tri-buffered media, containing a mixture of HEPES, MOPS or DIPSO, offers advantages compared with media containing HEPES or MOPS alone, and may be used for procedures such as oocyte retrieval, intracytoplasmic sperm injection, embryo transfer and cryopreservation.

Cell volume regulation is initiated in mouse oocytes after ovulation

Fertilized mouse eggs regulate their size principally by accumulating glycine as an intracellular osmolyte using the GLYT1 (SLC6A9) transporter, a mechanism of cell volume homeostasis apparently unique to early embryos before the morula stage. However, nothing was known of cell volume regulation in oocytes before fertilization. We show here that GLYT1 is quiescent in mouse germinal-vesicle-stage oocytes but becomes fully activated within hours after ovulation is triggered. This initiates accumulation of substantial amounts of intracellular glycine in oocytes during meiotic progression, reaching a maximal level in mature eggs. Measurements of endogenous free glycine showed that there were nearly undetectable levels in ovarian germinal-vesicle-stage oocytes, but high levels were present in mature ovulated eggs and in preimplantation embryos through the two-cell stage, but not in morulae. Furthermore, intracellular glycine was regulated in response to changes in external tonicity in eggs and embryos through the two-cell stage, but not in oocytes or embryos after the two-cell stage. Before activation of GLYT1,oocytes were unable to independently regulate their volume. As GLYT1 became active, however, oocyte volume decreased substantially and oocytes gained the ability to regulate their size, which required GLYT1 activity. Before ovulation, oocyte size was instead determined by a strong adhesion to the rigid extracellular matrix of the oocyte, the zona pellucida, which was released coincident with GLYT1 activation. The ability to acutely regulate cell size is thus acquired by the oocyte only after ovulation, when it first develops glycine-dependent cell volume regulation.

Regulation of intracellular pH during oocyte growth and maturation in mammals

Regulation of intracellular pH (pH(i)) is a fundamental homeostatic process essential for the survival and proliferation of virtually all cell types. The mammalian preimplantation embryo, for example, possesses Na(+)/H(+) and HCO(3)(-)/Cl(-) exchangers that robustly regulate against acidosis and alkalosis respectively. Inhibition of these transporters prevents pH corrections and, perhaps unsurprisingly, leads to impaired embryogenesis. However, recent studies have revealed that the role and regulation of pH(i) is somewhat more complex in the case of the developing and maturing oocyte. Small meiotically incompetent growing oocytes are apparently incapable of regulating their own pH(i), and instead rely upon the surrounding granulosa cells to correct ooplasmic pH, until such a time that the oocyte has developed the capacity to regulate its own pH(i). Later, during meiotic maturation, pH(i)-regulating activities that were developed during growth are inactivated, apparently under the control of MAPK signalling, until the oocyte is successfully fertilized. Here, we will discuss pH homeostasis in early mammalian development, focussing on recent developments highlighting the unusual and unexpected scenario of pH regulation during oocyte growth and maturation.

SIT1 is a betaine/proline transporter that is activated in mouse eggs after fertilization and functions until the 2-cell stage

Betaine (N,N,N-trimethylglycine) added to culture media is known to substantially improve the development of preimplantation mouse embryos in vitro, and to be imported into 1-cell embryos by a transporter that also accepts proline. Here, we found that the betaine/proline transporter is active in preimplantation mouse embryos only for a short period of development, between the 1- and 2-cell stages. Betaine/proline transport was activated after fertilization, beginning approximately 4 hours post-egg activation and reaching a maximum by approximately 10 hours. One- and 2-cell embryos contained endogenous betaine, indicating that a likely function for the transporter in vivo is the accumulation or retention of intracellular betaine. The appearance of transport activity after egg activation was independent of protein synthesis, but was reversibly blocked by disruption of the Golgi with brefeldin A. We assessed two candidates for the betaine/proline transporter: SIT1 (IMINO; encoded by Slc6a20a) and PROT (Slc6a7). mRNA from both genes was present in eggs and 1-cell embryos. However, when exogenously expressed in Xenopus oocytes, mouse PROT did not transport betaine and had an inhibition profile different from that of the embryonic transporter. By contrast, exogenously expressed mouse SIT1 transported both betaine and proline and closely resembled the embryonic transporter. A morpholino oligonucleotide designed to block translation of SIT1, when present from the germinal vesicle stage, blocked the appearance of betaine transport activity in parthenogenotes. Thus, SIT1 is likely to be a developmentally restricted betaine transporter in mouse preimplantation embryos that is activated by fertilization.

Bicarbonate transport in cell physiology and disease

The family of mammalian bicarbonate transport proteins are involved in a wide-range of physiological processes. The importance of bicarbonate transport follows from the biochemistry of HCO(3)(-) itself. Bicarbonate is the waste product of mitochondrial respiration. HCO(3)(-) undergoes pH-dependent conversion into CO(2) and in doing so converts from a membrane impermeant anion into a gas that can diffuse across membranes. The CO(2)-HCO(3)(-) equilibrium forms the most important pH buffering system of our bodies. Bicarbonate transport proteins facilitate the movement of membrane-impermeant HCO(3)(-) across membranes to accelerate disposal of waste CO(2), control cellular and whole-body pH, and to regulate fluid movement and acid/base secretion. Defects of bicarbonate transport proteins manifest in diseases of most organ systems. Fourteen gene products facilitate mammalian bicarbonate transport, whose physiology and pathophysiology is discussed in the present review.

Second meiotic spindle integrity requires MEK/MAP kinase activity in mouse eggs

ERK-type MAP kinase activity is required for normal first meiotic (MI) metaphase spindle dynamics and first polar body formation at the MI/MII transition, and for MII arrest until egg activation. MEK and MAPK, however, remain active until meiosis is completed and pronuclei form, but whether MEK/MAPK activity affects MII spindle function during egg activation has been unknown. Polarized light microscopy revealed that the MII spindle rapidly (within approximately 15 min) lost birefringence upon treatment of the egg with U0126, indicating decreased organization at the molecular level upon MEK inhibition. In contrast, birefringence rapidly increased when MPF was inhibited with roscovitine, and this was similar to the increased birefringence previously shown after fertilization or parthenogenetic activation with Sr(2+). Confocal microscopy indicated that many spindles in U0126-activated eggs had failed to rotate or were dissociated from the egg cortex. Subsequently, abnormally-located midbodies were evident in U0126-induced parthenogenotes. Thus, MEK/MAPK activity is required to maintain the ordered structure of the MII spindle and for normal spindle dynamics during second polar body formation.

Inositol 1,4,5-trisphosphate receptor 1, a widespread Ca2+ channel, is a novel substrate of polo-like kinase 1 in eggs

To initiate embryo development, the sperm induces in the egg release of intracellular calcium ([Ca2+](i)). During oocyte maturation, the inositol 1,4,5-trisphosphate receptor (IP(3)R1), the channel implicated, undergoes modifications that enhance its function. We found that IP(3)R1 becomes phosphorylated during maturation at an MPM-2 epitope and that this persists until the fertilization-associated [Ca2+](i) responses cease. We also reported that maturation without ERK activity diminishes IP(3)R1 MPM-2 reactivity and [Ca2+](i) responses. Here, we show that IP(3)R1 is a novel target for Polo-like kinase1 (Plk1), a conserved M-phase kinase, which phosphorylates it at an MPM-2 epitope. Plk1 and IP(3)R1 interact in an M-phase preferential manner, and they exhibit close co-localization in the spindle/spindle poles area. This co-localization is reduced in the absence of ERK activity, as the ERK pathway regulates spindle organization and IP(3)R1 cortical re-distribution. We propose that IP(3)R1 phosphorylation by Plk1, and possibly by other M-phase kinases, underlies the delivery of spatially and temporally regulated [Ca2+](i) signals during meiosis/mitosis and cytokinesis.

Granulosa cells regulate oocyte intracellular pH against acidosis in preantral follicles by multiple mechanisms

Mammalian oocytes grow within ovarian follicles in which the oocyte is coupled to surrounding granulosa cells by gap junctions. We report here that growing oocytes isolated from mouse preantral follicles are incapable of recovering from an experimentally induced acidosis, and that oocytes acquire the ability to manage acid loads by activating Na(+)/H(+) exchange during growth. By contrast, granulosa cells from similar preantral follicles possess substantial Na(+)/H(+) exchange capacity, which is attributable to the simultaneous action of two Na(+)/H(+) exchanger isoforms: NHE1 and NHE3. Granulosa cells were also found to possess a V-type H(+)-ATPase that drives partial acidosis recovery when Na(+)/H(+) exchange is inactivated. By monitoring intracellular pH (pH(i)) in small follicle-enclosed oocytes, we found that the oocyte has access to each of these acidosis-correcting activities, such that small follicle-enclosed oocytes readily recover from acidosis in a manner resembling granulosa cells. However, follicle-enclosed oocytes are unable to access these activities if gap-junction communication within the follicle is inhibited. Together, these experiments identify the NHE isoforms involved in regulating oocyte pH(i), indicate that gap junctions allow granulosa cells to exogenously regulate oocyte pH(i) against acidosis until the oocyte has acquired endogenous pH(i) regulation, and reveal that granulosa cells possess multiple mechanisms for carrying out this function.

Granulosa cells regulate intracellular pH of the murine growing oocyte via gap junctions: development of independent homeostasis during oocyte growth

Oocytes grow within ovarian follicles in which the oocyte is coupled to the surrounding granulosa cells by gap junctions. It was previously found that small growing oocytes isolated from juvenile mice and freed of their surrounding granulosa cells (denuded) lacked the ability to regulate their intracellular pH (pH(i)), did not exhibit the pH(i)-regulatory HCO(3)(-)/Cl(-) and Na(+)/H(+) exchange activities found in fully-grown oocytes, and had low pH(i). However, both exchangers became active as oocytes grew near to full size, and, simultaneously, oocyte pH(i) increased by approximately 0.25 pH units. Here, we show that, in the more physiological setting of the intact follicle, oocyte pH(i) is instead maintained at approximately 7.2 throughout oocyte development, and the growing oocyte exhibits HCO(3)(-)/Cl(-) exchange, which it lacks when denuded. This activity in the oocyte requires functional gap junctions, as gap junction inhibitors eliminated HCO(3)(-)/Cl(-) exchange activity from follicle-enclosed growing oocytes and substantially impeded the recovery of the oocyte from an induced alkalosis, implying that oocyte pH(i) may be regulated by pH-regulatory exchangers in granulosa cells via gap junctions. This would require robust HCO(3)(-)/Cl(-) exchange activity in the granulosa cells, which was confirmed using oocytectomized (OOX) cumulus-oocyte complexes. Moreover, in cumulus-oocyte complexes with granulosa cells coupled to fully-grown oocytes, HCO(3)(-)/Cl(-) exchange activity was identical in both compartments and faster than in denuded oocytes. Taken together, these results indicate that growing oocyte pH(i) is controlled by pH-regulatory mechanisms residing in the granulosa cells until the oocyte reaches a developmental stage where it becomes capable of carrying out its own homeostasis.

Mechanisms regulating intracellular pH are activated during growth of the mouse oocyte coincident with acquisition of meiotic competence

Oocytes grow within ovarian follicles, and only gain the ability to complete meiosis when they are nearly fully grown. We have found that both of the major types of intracellular pH regulatory mechanisms in the mammal-the Na+/H+ and HCO3-/Cl- exchangers-were essentially inactive in mouse oocytes over most of the course of their growth. However, as oocytes approached full size, Na+/H+ and HCO3-/Cl- exchangers became simultaneously active, and, at the same time, the intracellular pH of isolated oocytes increased sharply by about 0.25 pH unit. This activation of intracellular pH regulatory mechanisms and increase in pH occurred coincident with the acquisition of meiotic competence. The activation of pH regulatory mechanisms during oocyte growth represents a previously unknown milestone in the development of the capacity of the oocyte to function independently upon ovulation.

Effects of MAP kinase pathway and other factors on meiosis ofUrechis unicinctus eggs

The eggs of Urechis unicinctus Von Drasche, an echiuroid, are arrested at P-I stage in meiosis. The meiosis is reinitiated by fertilization. Immunoblotting analysis using anti-ERK2 and anti-phospho-MAPK antibodies revealed a 44 kDa MAP kinase species that was constantly expressed in U. unicinctus eggs, quickly phosphorylated after fertilization, and dephosphorylated slowly before the completion of meiosis I. Phosphorylation of the protein was not depressed by protein synthesis inhibitor Cycloheximide (CHX), but was depressed by the MEK1 inhibitor PD98059. Under PD98059 treatment, polar body extrusion was suppressed and the function of centrosome and spindle was abnormal though GVBD was not affected, indicating that MAP kinase cascade was important for meiotic division of U. unicinctus eggs. Other discovery includes: A23187 and OA could parthenogenetically activate U. unicinctus eggs and phosphorylated 44 kDa MAP kinase species, indicating that the effect of fertilization on reinitiating meiosis and phosphorylation of 44 kDa MAP kinase specie is mediated by raising intracellular free calcium and by phosphorylation of some proteins, and that phosphotase(s) sensitive to OA is responsible for arresting U. unicinctus eggs in prophase I. diC8, an activator of PKC, accelerated the process of U. unicinctus egg meiotic division after fertilization and accelerated the dephosphorylation of 44 kDa MAP kinase specie, which implied that the acceleration effect of PKC on meiotic division was mediated by inactivation of MAP kinase cascade. Elevating cAMP/PKA level in U. unicinctus eggs had no effect on meiotic division of the eggs.

Entire mitogen activated protein kinase (MAPK) pathway is present in preimplantation mouse embryos

To understand how mitogenic signals are transduced into the trophoblasts in preimplantation embryos, the expression of mitogen-activated protein kinase (MAPK) pathway molecules was tested. We used immunocytochemical means and reverse transcriptase-polymerase chain reaction to test whether MAPK pathway molecule gene products exist at the protein and phosphoprotein level in the zygote and the RNA level in the egg and zygote. In addition, all antibodies detected the correct-sized major band in Westerns of placental cell lines representing the most prevalent cell type in preimplantation embryos. A majority of mRNA transcripts of MAPK pathway genes were detected in unfertilized eggs, and all were expressed in the zygote. We found that the MAPK pathway protein set consisting of the following gene products was present: FRS2 alpha, GRB2, GAB1, SOS1, Ha-ras, Raf1/RafB, MEK1,2,5, MAPK/ERK1,2, MAPK/ERK5, and RSK1,2,3 (see abbreviations). These proteins were detected in trophoblasts in embryonic day (E) 3.5 embryos when they could mediate mitogenic fibroblast growth factor signals from the embryo or colony stimulating factor-1 signals from the uterus. The phosphorylation state and position of the phosphoproteins in the cells suggested that they might function in mediating mitogenic signals. Interestingly, a subtle transition from maternal MAPK function to zygotic function was suggested by the localization for three MAPK pathway enzymes between E2.5 and E3.5, Raf1 phospho is largely cell membrane-localized at E2.5 and E3.5, and MEK1,2 phospho accumulates in the nucleus on E2.5 and E3.5. However, MAPK phospho shifts from nuclear accumulation at E2.5 to cytoplasmic accumulation at E3.5. This finding is similar to the cytoplasmic MAPK phospho localization reported in fibroblast growth factor signaling fields in postimplantation embryos (Corson et al. [2003] Development 130:4527-4537). This spatial and temporal expression study lays a foundation to plan and analyze perturbation studies aimed at understanding the role of the major mitogenic pathway in preimplantation mouse embryos.