Bicarbonate/chloride exchange regulates intracellular pH of embryos but not oocytes of the hamster

History and prospects of intracytoplasmic sperm injection (ICSI) and the development of golden hamster ICSI embryos

BACKGROUND

Golden (Syrian) hamsters have many advantages for the study of reproductive biology and developmental biology, including a consistent estrous cycle, a stable superovulation response, and a short gestation period. However, there are serious difficulties in doing in vitro manipulations of hamster embryos, because they are very sensitive to various environmental factors. Thus, biotechnological researches of hamster embryos should be performed with high-level skills of embryo manipulations.

METHODS

The authors summarized the history of hamster intracytoplasmic sperm injection (ICSI) and introduced key points for hamster ICSI, which were found in our previous studies on the production of embryos by ICSI and offspring by embryo transfer.

MAIN FINDINGS

The key points for hamster ICSI were in vitro manipulations under the light-controlled environment, injection of acrosome-less sperm heads into oocytes as soon as possible before spontaneous oocyte activation occurs, and determination of the optimal culture conditions.

CONCLUSION

To our knowledge, there are no available reports on production of offspring from ICSI embryos in hamsters except our reports. Moreover, success rates of hamster ICSI remain very low. For the purpose of spreading hamster ICSI, it is necessary to make further researches to improve manipulation techniques and to resolve experimental problems.

Saccharomyces cerevisiae: First Steps to a Suitable Model System To Study the Function and Intracellular Transport of Human Kidney Anion Exchanger 1

Distal renal tubular acidosis (dRTA) is a common kidney dysfunction characterized by impaired acid secretion via urine. Previous studies revealed that α-intercalated cells of dRTA patients express mutated forms of human kidney anion exchanger 1 (kAE1) which result in inefficient plasma membrane targeting or diminished expression levels of kAE1. However, the precise dRTA-causing processes are inadequately understood, and alternative model systems are helpful tools to address kAE1-related questions in a fast and inexpensive way. In contrast to a previous study, we successfully expressed full-length kAE1 in Saccharomyces cerevisiae. Using advanced microscopy techniques as well as different biochemical and functionality assays, plasma membrane localization and biological activity were confirmed for the heterologously expressed anion transporter. These findings represent first important steps to use the potential of yeast as a model organism for studying trafficking, activity, and degradation of kAE1 and its mutant variants in the future.

Saccharomyces cerevisiae has been frequently used to study biogenesis, functionality, and intracellular transport of various renal proteins, including ion channels, solute transporters, and aquaporins. Specific mutations in genes encoding most of these renal proteins affect kidney function in such a way that various disease phenotypes ultimately occur. In this context, human kidney anion exchanger 1 (kAE1) represents an important bicarbonate/chloride exchanger which maintains the acid-base homeostasis in the human body. Malfunctions in kAE1 lead to a pathological phenotype known as distal renal tubular acidosis (dRTA). Here, we evaluated the potential of baker's yeast as a model system to investigate different cellular aspects of kAE1 physiology. For the first time, we successfully expressed yeast codon-optimized full-length versions of tagged and untagged wild-type kAE1 and demonstrated their partial localization at the yeast plasma membrane (PM). Finally, pH and chloride measurements further suggest biological activity of full-length kAE1, emphasizing the potential of S. cerevisiae as a model system for studying trafficking, activity, and/or degradation of mammalian ion channels and transporters such as kAE1 in the future.

IMPORTANCE Distal renal tubular acidosis (dRTA) is a common kidney dysfunction characterized by impaired acid secretion via urine. Previous studies revealed that α-intercalated cells of dRTA patients express mutated forms of human kidney anion exchanger 1 (kAE1) which result in inefficient plasma membrane targeting or diminished expression levels of kAE1. However, the precise dRTA-causing processes are inadequately understood, and alternative model systems are helpful tools to address kAE1-related questions in a fast and inexpensive way. In contrast to a previous study, we successfully expressed full-length kAE1 in Saccharomyces cerevisiae. Using advanced microscopy techniques as well as different biochemical and functionality assays, plasma membrane localization and biological activity were confirmed for the heterologously expressed anion transporter. These findings represent first important steps to use the potential of yeast as a model organism for studying trafficking, activity, and degradation of kAE1 and its mutant variants in the future.

In vivo oxygen, temperature and pH dynamics in the female reproductive tract and their importance in human conception: a systematic review

BACKGROUND

Despite advances in ART, implantation and pregnancy rates per embryo transfer still remain low. IVF laboratories strive to ensure that the process of handling gametes in vitro closely mimics the in vivo environment. However, there remains a lack of knowledge regarding the in vivo regulation and dynamic variation in biophysical parameters such as oxygen concentration, pH and temperature within the reproductive tract.

OBJECTIVE AND RATIONALE

To undertake a systematic review of the current understanding of the physico-chemical parameters of oxygen tension (pO2), pH and temperature within the female reproductive tract, and their potential implications in clinical and pathological processes related to fertility and those pertaining to limited reproductive capacity.

SEARCH METHODS

A comprehensive literature search was performed using electronic databases including Medline, Embase, Cochrane Library and Pubmed to identify original and review articles addressing the biophysical parameters (pO2, pH and temperature) in the female reproductive tract of any species. The search included all studies published between 1946 and November 2015. Search terms included 'oxygen', 'pH', 'hydrogen ion concentration', 'acid base' and others terms. We also used special features and truncations to identify synonyms and broaden the search. Studies were excluded if they only assessed embryo culture conditions, fetal acid-base status, oxidative stress, outcomes of pregnancy and measurements of these parameters in non-reproductive organs.

OUTCOMES

Our search generated 18 685 records and 60 articles were included. pO2 within the female reproductive tract shows cyclical variation and minute-to-minute oscillations, which may be influenced by uterine contractility, hormones, the autonomic system, cardiac pulsatility, and myometrial and smooth muscle integrity. Fine balanced control of pO2 and avoidance of overwhelming oxidative stress is crucial for embryogenesis and implantation. The pH in the female reproductive tract is graduated, with lowest pH in the vagina (~pH 4.42) increasing toward the Fallopian tubes (FTs) (~pH 7.94), reflecting variation in the site-specific microbiome and acid-base buffering at the tissue/cellular level. The temperature variation in humans is cyclical by day and month. In humans, it is biphasic, increasing in the luteal phase; with the caudal region of the oviduct 1-2 degrees cooler than the cranial portion. Temperature variation is influenced by hormones, density of pelvic/uterine vascular beds and effectiveness of heat exchange locally, crucial for sperm motility and embryo development. We have identified significant deficiencies and inconsistencies in the methods used to assess these biophysical factors within the reproductive tract. We have suggested that the technological solutions including the development of methods and models for real time, in vivo recordings of biophysical parameters.

WIDER IMPLICATIONS

The notion of 'back to nature' in assisted conception suggested 20 years ago has yet to be translated into clinical practice. While the findings from this systematic review do not provide evidence to change current in vitro protocols, it highlights our current inability to assess the in vivo reproductive tract environment in real time. Data made available through future development of sensing technology in utero may help to provide new insights into how best to optimize the in vitro embryo environment and allow for more precise and personalized fertility treatment.

Preferential intracellular pH regulation: hypotheses and perspectives

The regulation of vertebrate acid-base balance during acute episodes of elevated internal PCO2  is typically characterized by extracellular pH (pHe) regulation. Changes in pHe are associated with qualitatively similar changes in intracellular tissue pH (pHi) as the two are typically coupled, referred to as 'coupled pH regulation'. However, not all vertebrates rely on coupled pH regulation; instead, some preferentially regulate pHi against severe and maintained reductions in pHe Preferential pHi regulation has been identified in several adult fish species and an aquatic amphibian, but never in adult amniotes. Recently, common snapping turtles were observed to preferentially regulate pHi during development; the pattern of acid-base regulation in these species shifts from preferential pHi regulation in embryos to coupled pH regulation in adults. In this Commentary, we discuss the hypothesis that preferential pHi regulation may be a general strategy employed by vertebrate embryos in order to maintain acid-base homeostasis during severe acute acid-base disturbances. In adult vertebrates, the retention or loss of preferential pHi regulation may depend on selection pressures associated with the environment inhabited and/or the severity of acid-base regulatory challenges to which they are exposed. We also consider the idea that the retention of preferential pHi regulation into adulthood may have been a key event in vertebrate evolution, with implications for the invasion of freshwater habitats, the evolution of air breathing and the transition of vertebrates from water to land.

Involvement of Cl(-)/HCO3(-) exchanger SLC26A3 and SLC26A6 in preimplantation embryo cleavage

Bicarbonate (HCO3(-)) is essential for preimplantation embryo development. However, the mechanism underlying the HCO3(-) transport into the embryo remains elusive. In the present study, we examined the possible involvement of Cl(-)/HCO3(-) exchanger in mediating HCO3(-) transport into the embryo. Our results showed that depletion of extracellular Cl(-), even in the presence of HCO3(-), suppressed embryo cleavage in a concentration-dependent manner. Cleavage-associated HCO3(-)-dependent events, including increase of intracellular pH, upregulation of miR-125b and downregulation of p53, also required Cl(-). We further showed that Cl(-)/HCO3(-) exchanger solute carrier family 26 (SLC26) A3 and A6 were expressed at 2-cell through blastocyst stage. Blocking individual exchanger's activity by inhibitors or gene knockdown differentially decreased embryo cleavage and inhibited HCO3(-)-dependent events, while inhibiting/knocking down both produced an additive effect to an extent similar to that observed when CFTR was inhibited. These results indicate the involvement of SLC26A3 and A6 in transporting HCO3(-) essential for embryo cleavage, possibly working in concert with CFTR through a Cl(-) recycling pathway. The present study sheds light into our understanding of molecular mechanisms regulating embryo cleavage by the female reproductive tract.

Embryonic common snapping turtles (Chelydra serpentina) preferentially regulate intracellular tissue pH during acid-base challenges

The nests of embryonic turtles naturally experience elevated CO2 (hypercarbia), which leads to increased blood PCO2 and a respiratory acidosis resulting in reduced blood pH [extracellular pH (pHe)]. Some fishes preferentially regulate tissue pH [intracellular pH (pHi)] against changes in pHe; this has been proposed to be associated with exceptional CO2 tolerance and has never been identified in amniotes. As embryonic turtles may be CO2 tolerant based on nesting strategy, we hypothesized that they preferentially regulate pHi, conferring tolerance to severe acute acid-base challenges. This hypothesis was tested by investigating pH regulation in common snapping turtles (Chelydra serpentina) reared in normoxia then exposed to hypercarbia (13kPa PCO2) for 1h at three developmental ages, 70 and 90% of incubation, and in yearlings. Hypercarbia reduced pHe but not pHi, at all developmental ages. At 70% of incubation, pHe was depressed by 0.324 pH units while pHi of brain, white muscle, and lung increased; heart, liver, and kidney pHi remained unchanged. At 90% of incubation, pHe was depressed by 0.352 pH units but heart pHi increased with no change in pHi of other tissues. Yearling exhibited a pHe reduction of 0.235 pH units but had no changes in pHi of any tissues. The results indicate common snapping turtles preferentially regulate pHi during development, but the degree of the response is reduced throughout development. This is the first time preferential pHi regulation has been identified in an amniote. These findings may provide insight into the evolution of acid-base homeostasis during development of amniotes, and vertebrates in general.

Mechanisms of guanylin action on water and ion absorption at different regions of seawater eel intestine

Guanylin (GN) inhibited water absorption and short-circuit current (Isc) in seawater eel intestine. Similar inhibition was observed after bumetanide, and the effect of bumetanide was abolished by GN or vice versa, suggesting that both act on the same target, Na(+)-K(+)-2Cl(-) cotransporter (NKCC), which is a key player for the Na(+)-K(+)-Cl(-) transport system responsible for water absorption in marine teleost intestine. However, effect of GN was always greater than that of bumetanide: 10% greater in middle intestine (MI) and 40% in posterior intestine (PI) for Isc, and 25% greater in MI and 34% in PI for water absorption. After treatment with GN, Isc decreased to zero, but 20-30% water absorption still remained. The remainder may be due to the Cl(-)/HCO3 (-) exchanger and Na(+)-Cl(-) cotransporter (NCC), since inhibitors for these transporters almost nullified the remaining water absorption. Quantitative PCR analysis revealed the presence of major proteins involved in water absorption; the NKCC2β and AQP1 genes whose expression was markedly upregulated after seawater acclimation. The SLC26A6 (anion exchanger) and NCCβ genes were also expressed in small amounts. Consistent with the inhibitors' effect, expression of NKCC2β was MI > PI, and that of NCCβ was MI << PI. The present study showed that GN not only inhibits the bumetanide-sensitive Na(+)-K(+)-Cl(-) transport system governed by NKCC2β, but also regulates unknown ion transporters different from GN-insensitive SLC26A6 and NCC. A candidate is cystic fibrosis transmembrane conductance regulator Cl(-) channel, as demonstrated in mammals, but its expression is low in eel intestine, and its role may be minor, as indicated by the small effect of its inhibitors.

Changing expression of chloride channels during preimplantation mouse development

Plasma membrane chloride channels (ClCs) play important roles in a broad range of cellular processes including cell volume regulation, proliferation, and transepithelial transport, all of which are critical during preimplantation embryonic development. In this study, the molecular and functional expression of voltage-gated ClCs was analyzed throughout preimplantation development of the mouse conceptus. mRNA transcripts for allClcngenes were detected. OnlyClcn1mRNA showed differential expression in the blastocyst, being detected in the trophectoderm but not in the inner cell mass. CLCN3 protein was detected at low levels in the cytoplasm and plasma membrane in 4-cell embryos and was localized to the apical plasma membrane of the trophoblasts in the blastocyst. Whole-cell patch-clamp recordings demonstrated the presence of a DIDS-sensitive, outwardly rectifying Cl−current throughout development, with this conductance being large at the 1-cell, morula and blastocyst stages. A second DIDS-insensitive Cl−current, which was inactivated by membrane depolarization, was present in cells differentiating into the trophoblast lineage and during blastocyst expansion. Inhibition of the DIDS-sensitive current and the DIDS-insensitive current, with 9-AC, prevented blastocyst expansion.

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.

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.

A novel isolator-based system promotes viability of human embryos during laboratory processing

In vitro fertilisation (IVF) and related technologies are arguably the most challenging of all cell culture applications. The starting material is a single cell from which one aims to produce an embryo capable of establishing a pregnancy eventually leading to a live birth. Laboratory processing during IVF treatment requires open manipulations of gametes and embryos, which typically involves exposure to ambient conditions. To reduce the risk of cellular stress, we have developed a totally enclosed system of interlinked isolator-based workstations designed to maintain oocytes and embryos in a physiological environment throughout the IVF process. Comparison of clinical and laboratory data before and after the introduction of the new system revealed that significantly more embryos developed to the blastocyst stage in the enclosed isolator-based system compared with conventional open-fronted laminar flow hoods. Moreover, blastocysts produced in the isolator-based system contained significantly more cells and their development was accelerated. Consistent with this, the introduction of the enclosed system was accompanied by a significant increase in the clinical pregnancy rate and in the proportion of embryos implanting following transfer to the uterus. The data indicate that protection from ambient conditions promotes improved development of human embryos. Importantly, we found that it was entirely feasible to conduct all IVF-related procedures in the isolator-based workstations.

Agonist-dependent potentiation of vanilloid receptor transient receptor potential vanilloid type 1 function by stilbene derivatives

Transient receptor potential vanilloid type 1 (TRPV1) is a nonselective cation channel activated by capsaicin, low pH, and noxious heat and plays a key role in nociception. Understanding mechanisms for functional modulation of TRPV1 has important implications. One characteristic of TRPV1 is that channel activity induced by either capsaicin or other activators can be sensitized or modulated by factors involving different cell signaling mechanisms. In this study, we describe a novel mechanism for the modulation of TRPV1 function: TRPV1 function is modulated by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) and its analogs. We found that, in rat dorsal root ganglion neurons, although DIDS did not induce the activation of TRPV1 per se but drastically increased the TRPV1 currents induced by either capsaicin or low pH. DIDS also blocked the tachyphylaxis of the low pH-induced TRPV1 currents. 4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS), a DIDS analog, failed to enhance the capsaicin-evoked TRPV1 current but increased the low pH-evoked TRPV1 currents, with an effect comparable with that of DIDS. SITS also blocked the low pH-induced tachyphylaxis. DIDS also potentiated the currents of TRPV1 channels expressed in human embryonic kidney 293 cells, with an effect of left-shifting the concentration-response curve of the capsaicin-induced TRPV1 currents. This study demonstrates that DIDS and SITS, traditionally used chloride channel blockers, can modify TRPV1 channel function in an agonist-dependent manner. The results provide new input for understanding TRPV1 modulation and developing new modulators of TRPV1 function.

Alterations in mouse embryo intracellular pH by DMO during culture impair implantation and fetal growth

The preimplantation embryo is highly susceptible to in-vitro stress, and although this does not necessarily perturb blastocyst development, it can significantly affect embryo physiology and the ability to form a viable pregnancy. This study determined that the preimplantation mouse embryo is highly sensitive to a small decrease in intracellular pH (<0.2 pH units). Embryos cultured in media containing a weak acid (5,5-dimethyl-2,4-oxazolidinedione; DMO) formed blastocysts with decreased cell number and inner cell mass number, as well as increased apoptosis, even though blastocyst development and morphology were unchanged. Interestingly, the effects were similar regardless of whether the pH stress was present for a short-term 'acute' exposure (during the zygote to 2-cell, or 2-cell to 8-cell division) or an extended 'chronic' period of time (continually from the zygote to the blastocyst stage). Exposure to DMO during the first cleavage division did not alter implantation; however, fetal weight and crown-rump length were significantly decreased (P<0.05). In contrast, continuous exposure to DMO throughout preimplantation development reduced not only implantation but also fetal weight and crown-rump length. This study highlights the importance of correct intracellular pH and demonstrates that slight deviations can significantly impact embryo development and viability.

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.

The role of co-culture systems on developmental competence of preimplantation mouse embryos against pH fluctuations

PURPOSE

To determine the effect of pH fluctuations of culture media, and the role of co-culture systems on embryo development.

METHODS

Mouse embryos were incubated in phosphate buffered solutions (PBSs) with different pH for various lengths of time. After 3 h incubation of embryos at various pH, the embryos were transferred into four media with human (HEF) and mouse (MEF) embryonic fibroblast cells, and without feeder cells; HTF and MEM-alpha. Developmental rate at day three (morula), four (expanded blastocyst) and five (hatching or hatched blastocyst) was evaluated.

RESULTS

Developmental rate at day three, four and five decreased when the incubation time at pH 6.2 and 8 increased to 3 h and more. In addition, significantly less embryos incubated at pH 6.2 and 8 developed to hatching and hatched blastocysts compared with pH 7.35. Embryos incubated at pH 6.2, co-cultured with MEF or HEF showed a significant improvement (P < 0.05) at day three in HEF compared to HTF, and at day five in MEF compared to HTF. At pH 8, a significant improvement (P < 0.05) was observed at day five in HEF and MEF compared to MEM-alpha.

CONCLUSIONS

Mouse 2-cell embryos could tolerate minor pH fluctuations, but that major pH changes affect subsequent development. Besides, feeder cells could improve embryo development, especially when embryos are prone to rise or fall in pH.

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.

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.

Glycosidase determination in bovine oviducal fluid at the follicular and luteal phases of the oestrous cycle

Gamete recognition and binding of spermatozoa to the oviduct are carbohydrate-mediated processes in which several glycosidases are thought to have a role, although this has not been demonstrated unequivocally. Oviducal fluid is the biological milieu in which fertilisation and early embryo development take place, but the enzyme composition of oviducal fluid is largely unknown. The aim of the present study was to determine glycosidase activity and protein content in bovine oviducal fluid (bOF) and the volume of fluid collected per oviduct. Oviducts obtained from a slaughterhouse were classified as either in the follicular or luteal phase on the basis of ovarian luteal morphology. Oviducal fluid was aspirated, centrifuged and the volume determined. Samples were then frozen until assay. Substrates conjugated to 4-methylumbelliferyl were used to screen for the activity of seven glycosidases at pH 7.2. The results indicate that bOF has alpha-l-fucosidase, beta-N-acetyl-glucosaminidase, beta-d-galactosidase, alpha-D-mannosidase and beta-N-acetyl-galactosaminidase activity during both phases of the cycle, with the specific activity of the latter two enzymes being higher during the follicular phase. There was no N-acetyl-neuraminidase or alpha-d-galactosidase activity detected in bOF at either phase of the oestrous cycle at pH 7.2, but activity for both glycosidases was detected at pH 4.4. There were no differences in protein concentration or the volume of bOF collected between the two phases of the cycle. These findings indicate that oviducal fluid exhibits glycosidase activity, with specific variations throughout the oestrous cycle, suggesting that these enzymes play a role in carbohydrate-mediated events.

Understanding cellular disruptions during early embryo development that perturb viability and fetal development

An inability to regulate ionic and metabolic homeostasis is related to a reduction in the developmental capacity of the embryo. The early embryo soon after fertilisation and up until compaction appears to have a reduced capacity to regulate its homeostasis. The reduced ability to regulate homeostasis, such as intracellular pH and calcium levels, by the precompaction-stage embryo appears to impact on the ability to regulate mitochondrial function and maintain adequate levels of energy production. This reduction in ATP production causes a cascade of events leading to disrupted cellular function and, perhaps ultimately, disrupted epigenetic regulation and aberrant placental and fetal development. In contrast, after compaction the embryo takes on a more somatic cell-like physiology and is better able to regulate its physiology and therefore appears less vulnerable to stress. Therefore, for human IVF it would seem important for the establishment of healthy pregnancies that the embryos are maintained in systems that are designed to minimise homeostatic stress, particularly for the cleavage-stage embryos, as exposure to stress is likely to culminate in impaired embryo function.

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.

Ammonium induces aberrant blastocyst differentiation, metabolism, pH regulation, gene expression and subsequently alters fetal development in the mouse

The presence of ammonium in the culture medium has significant detrimental effects on the regulation of embryo physiology and genetics. Ammonium levels build up linearly over time in the culture medium when media containing amino acids are incubated at 37 degrees C. Ammonium in the culture media significantly reduces blastocyst cell number, decreases inner cell mass development, increases apoptosis, perturbs metabolism, impairs the ability of embryos to regulate intracellular pH, and alters the expression of the imprinted gene H19. In contrast, the rate of blastocyst development and blastocyst morphology appear to be normal. The transfer of blastocysts exposed to ammonium results in a significant reduction in the ability to establish a pregnancy. Furthermore, of those embryos that manage to implant, fetal growth is significantly impaired. Embryos exposed to 300 microM ammonium are retarded by 1.5 days developmentally at Day 15 of pregnancy. It is therefore essential that culture conditions for mammalian embryos are designed to minimize the buildup of ammonium to prevent abnormalities in embryo physiology, genetic regulation, pregnancy, and fetal development.

Developmental changes in the management of acid loads during preimplantation mouse development

Intracellular pH recovery in Quackenbush Swiss mouse preimplantation embryos following acid loading was investigated under conditions of H+-monocarboxylate cotransporter inactivity. Isoform-sensitive inhibitors of Na+-H+ exchange (NHE) were used to block the Na+-dependent component of the response. A biphasic dose-response curve for HOE-694 and N-methylisopropylamiloride (MIA) suggested that two isoforms (putatively NHE1 and NHE3) are active in the oocyte, 1-cell, and 2-cell stages. By the blastocyst stage, loss of one of the MIA-sensitive NHE activities (putatively NHE3) was observed in isolated inner cell masses, and an MIA-resistant component of the recovery was identified. The MIA-resistant component was inhibited by 2 mM amiloride and enhanced by external K+ and by 4,4'-diisothiocyanostilbene-2,2'-disulfonate, suggesting NHE4 activity. However, unlike NHE4 in other tissues, the MIA-resistant component did not transport Li+ in exchange for H+, and reverse transcription-polymerase chain reaction detected NHE4 mRNA in the oocyte but not in later stages. Trophoblast, whether in intact or collapsed blastocysts, did not show measurable NHE activity or MIA-sensitive activity during recovery from acid load. Both trophoblast and pluriblast manifested an H+ conductance in response to acid load. This H+ conductance was first detected at the 8-cell stage and was blocked by zinc in the isolated inner cell mass but not in trophoblast. No other effective inhibitors of its activity were found.

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

The HCO(3)(-)/Cl(-) exchanger is quiescent in the unfertilized mouse egg but is highly active in regulating intracellular pH in the early embryo and required for normal development. We show here that the HCO(3)(-)/Cl(-) exchanger is active in first meiotic prophase (GV) oocyte but inactivated during meiotic metaphase before the MI to MII transition. Reactivation does not occur until the activated egg enters interphase. A quiescent HCO(3)(-)/Cl(-) exchanger is not simply a general feature of metaphase, because activity did not decrease during first mitotic metaphase. Inactivation of the HCO(3)(-)/Cl(-) exchanger during MI coincided with the activation of MAP kinase (MAPK), whereas its reactivation coincided with the loss of MAPK activity after egg activation. Maintaining high MAPK activity after egg activation prevented the normal reactivation of the HCO(3)(-)/Cl(-) exchanger. Inactivating MAPK in unfertilized MII eggs resulted in HCO(3)(-)/Cl(-) exchanger activation. Preventing MAPK activation during first meiotic metaphase prevented the inactivation of HCO(3)(-)/Cl(-) exchange. Conversely, activating MAPK in the GV oocyte resulted in inactivation of HCO(3)(-)/Cl(-) exchange. These results imply that the HCO(3)(-)/Cl(-) exchanger in mouse oocytes is negatively regulated by MAPK. Thus, suppression of pH-regulatory mechanisms during meiosis is a novel function of MAPK and cytostatic factor activity in the oocyte.

Osmoregulation and cell volume regulation in the preimplantation embryo

The early preimplantation mammalian embryo possesses mechanisms that regulate intracellular osmolarity and cell volume. While transport of osmotically active inorganic ions might play a role in this process in embryos, the major mechanisms that have been identified and studied are those that employ organic osmolytes. Organic osmolytes provide a substantial portion of intracellular osmotic support in embryos and are required for their development under in vivo conditions. The main osmolytes that have been identified in cleavage stage embryos are accumulated via two transport systems of the neurotransmitter transporter family active in early preimplantation embryos--the glycine transport system (GLY) and the beta-amino acid transport system (system beta). While system beta has been established to have a similar role in many other cells, this is a novel function for the GLY transport system. The intracellular concentration of organic osmolytes such as glycine in early preimplantation embryos is regulated by tonicity, allowing the embryo to regulate its volume against shrinkage and to control its internal osmolarity. In addition, the cells of the embryo can regulate against an increase in volume via controlled release of osmolytes from the cytoplasm. This is mediated by a swelling-activated anion channel that is also highly permeable to a range of organic osmolytes, and which closely resembles similar channels found in many other cell types (VSOAC channels). Together, these mechanisms appear to regulate cell volume in the egg through the early cleavage stages of embryogenesis, after which there are indications that the mechanisms of osmoregulation change.

Altering intracellular pH disrupts development and cellular organization in preimplantation hamster embryos

In early cleavage stage hamster embryos, the inability to regulate intracellular pH (pHi) properly is associated with reduced developmental competence in vitro. The disruption of mitochondrial organization is also correlated with reduced development in vitro. To determine the relationship between pHi and the disruption of cytoplasmic organization, we examined the effects of altering pHi on hamster embryo development, mitochondrial distribution, and cytoskeletal organization. The weak base trimethylamine was used to increase pHi and was found to reduce embryo development and disrupt the perinuclear organization of mitochondria. The weak acid 5,5-dimethyl-2,4-oxazolinedione was used to decrease pH(i) and was also found to reduce development and disrupt the perinuclear organization of mitochondria. With either treatment, the microfilament organization was perturbed, but the microtubule cytoskeleton was not. However, the temporal progression of the disruption of mitochondrial distribution was more rapid in alkalinized embryos than acidified embryos, as revealed by two-photon imaging of living embryos. Additionally, the disruption of the microfilament network by the two treatments was not identical. The cytoplasmic disruptions observed were not due to acute toxicity of the compounds because embryos recovered developmentally when the treatment compounds were removed. These observations link ionic homeostasis, structural integrity and developmental competence in preimplantation hamster embryos.

Differences in intracellular pH regulation by Na(+)/H(+) antiporter among two-cell mouse embryos derived from females of different strains

Regulation of intracellular pH (pH(i)) by two-cell-stage embryos derived from female mice of three different strains (CF-1, Balb/c, and BDF) was investigated. Embryos recovered at a slow rate from intracellular acidosis produced by a pulse of NH(4)Cl; the rate did not differ significantly among strains. Recovery was reversibly inhibited by amiloride or the absence of Na(+), implicating Na(+)/H(+) antiporter activity. The threshold pH(i) (setpoint) below which Na(+)/H(+) antiporter activity was elicited was approximately 7.15 for each strain. No recovery from induced acidosis occurred in the absence of external Na(+) in any strain, and thus embryos could be maintained in acidosis for an extended period. Upon reintroduction of Na(+), embryos derived from either CF-1 or BDF females recovered at a slow rate comparable to that measured in embryos not maintained for a period in Na(+)-free medium, but embryos derived from Balb/c females consistently recovered at a highly accelerated rate. This accelerated recovery appeared to be due, in part, to an activation of the Na(+)/H(+) antiporter in Balb/c-derived embryos, which did not occur in CF-1- or BDF-derived embryos. Thus, embryos derived from different strains of female mice differ in their control of mechanisms for pH(i) regulation.

Mechanisms for managing cellular and homeostatic stress in vitro

The ability to maintain embryo development in culture depends upon the ability of the embryo to maintain cellular homeostasis. Disruptions in the ability to regulate cellular homeostasis such as pH, calcium levels and osmotic pressure result in perturbed development and a reduced ability to establish and maintain a pregnancy following transfer. Therefore, it is important that in vitro conditions are designed to minimize stress on the embryo and maximize the ability of the embryo to maintain cellular homeostasis. While embryos do exhibit a degree of plasticity and can adapt to their environment, this requires expenditures of extra energy which negatively impacts viability. Therefore, reducing stress by taking into account the physiology of the embryo is essential for the maintenance of developmental competence in vitro.

Interactions between embryos and the culture milieu

Although in vitro production of embryos up to the blastocyst stage is now possible in numerous species, the quality and quantity of embryos are still not satisfactory. Clearly, culture conditions do not yet replace all of the benefits of development within the female reproductive tract. Analysis of the interactions between embryos and the components of culture media provides insights into regulatory mechanisms and how they are perturbed in vitro, and also offers some clues about the nature of the support provided to early embryos by the female tract. Further elucidation of these events and their underlying regulation will be helpful for improving culture media formulations to support normal embryo development in vitro.

Vitrification of mouse and human blastocysts using a novel cryoloop container-less technique

OBJECTIVE

To vitrify mouse and human blastocysts with use of the cryoloop procedure and to assess subsequent development.

DESIGN

Controlled study of vitrification of mouse and human blastocysts.

SETTING

Research department of a private assisted reproductive technology unit.

PATIENT(S)

Blastocysts that were not suitable to be frozen were donated from patients.

INTERVENTION(S)

Culture of pronucleate embryos in sequential media to the blastocyst stage.

MAIN OUTCOME MEASURE(S)

Survival of the vitrification procedure was assessed by reexpansion, hatching, and outgrowth in culture. In addition, the viability of mouse blastocysts was assessed after transfer to pseudopregnant recipients.

RESULT(S)

Vitrification of mouse blastocysts did not affect the ability to reexpand, hatch, or outgrow in culture. Furthermore, implantation rates and fetal development were equivalent for nonfrozen and vitrified blastocysts. Vitrified human blastocysts were able to hatch and outgrow in culture at rates similar to nonfrozen controls.

CONCLUSION(S)

Cryoloop vitrification was able to cryopreserve mouse and human blastocysts without any reduction in the ability to reexpand and hatch in culture. Furthermore, viability was not reduced by the cryoloop vitrification of mouse blastocysts.

Regulation of intracellular pH in bovine oocytes and cleavage stage embryos

This study investigated the mechanisms for the regulation of intracellular pH in bovine oocytes and embryos. Na(+)/H(+) antiporter activity for the regulation of intracellular pH in the acid to neutral range was detected in both in vitro matured bovine oocytes and in vitro produced embryos. However, the activity of the antiporter was significantly reduced in oocytes compared to 2-cell, 4-cell, and 8-cell embryos. HCO(3)(-)/Cl(-) exchanger activity could be detected in oocytes and embryos using the chloride removal method, however the ability of this transporter to regulate intracellular pH against an alkaline load was poor and intracellular pH could not be re-established. The inability of the HCO(3)(-)/Cl(-) exchanger to adequately regulate intracellular pH was further highlighted by the arrest of embryos at the 8-16 cell stage when challenged with a small alkaline load. Therefore, bovine embryos are extremely sensitive to alterations in intracellular pH above the resting value of around 7.2. This sensitivity could account in part for impaired development and viability of bovine embryos produced in vitro.