Intracellular pH regulation in the early embryo

Haploid Parthenogenetic Embryos Exhibit Unique Stress Response to pH, Osmotic and Oxidative Stress

Preimplantation-stage embryos are susceptible to various types of stress when cultured in vitro. Parthenogenetic embryos that lack spermatozoa contribution exhibit aberrant developmental dynamics due to their uniparental origin. Herein, we assessed whether the absence of paternal genome affects the susceptibility of the embryos to pH, osmotic and oxidative stress. Haploid parthenogenetic embryos (HPE) (activated oocytes with 1 pronucleus and 2 polar bodies) were generated by incubating cumulus oocyte complexes of Swiss albino mice with 10 mM strontium chloride for 3 h. Normally fertilized embryos (NFE) (fertilized oocytes with 2 pronuclei and 2 polar bodies) were derived using in vitro fertilization. At 2-cell stage, both HPE and NFE were exposed to various stressors including pH (6.8 to 8.2), osmotic (isotonic, hypotonic, and hypertonic), and peroxidatic oxidative (H2O2, 25 µM) stress. Endoplasmic reticulum stress response, mitochondrial membrane potential, and the rate of blastocyst development were assessed. HPE were susceptible to alteration in the pH that was well tolerated by NFE. Similarly, HPE displayed remarkable difference in sensitivity to hypertonic stress and oxidative stress compared to NFE. The results clearly indicate that the oocytes that develop into embryos in the absence of paternal contribution are more vulnerable to environmental stressors, further highlighting the importance of spermatozoa contribution and/or the ploidy status in mitigating these stressors and towards healthy early embryo development.

Supplementing culture medium with the weak acid, 5,5-dimethyl-2,4-oxazolidinedione (DMO) limits the development of aneuploid mouse embryos through a Trp53-dependent mechanism

PURPOSE

This study was designed to determine if DMO limits in vitro development of aneuploid-enriched mouse embryos by activating a Trp53-dependent mechanism.

METHODS

Mouse cleavage-stage embryos were treated with reversine to induce aneuploidy or vehicle to generate controls, and then cultured in media supplemented with DMO to reduce the pH of the culture media. Embryo morphology was assessed by phase microscopy. Cell number, mitotic figures, and apoptotic bodies were revealed by staining fixed embryos with DAPI. mRNA levels of Trp53, Oct-4, and Cdx2 were monitored by quantitative polymerase chain reactions (qPCRs). The effect of Trp53 on the expression of Oct-4 and Cdx2 was assessed by depleting Trp53 using Trp53 siRNA.

RESULTS

Aneuploid-enriched late-stage blastocysts were morphologically indistinguishable from control blastocysts but had fewer cells and reduced mRNA levels of Oct-4 and Cdx2. Adding 1 mM DMO to the culture media during the 8-cell to blastocyst transition reduced the formation of aneuploid-enriched late-stage blastocysts but not control blastocysts and further suppressed the levels of Oct-4 and Cdx2 mRNA. Trp53 RNA levels in aneuploid-enriched embryos that were exposed to DMO were > twofold higher than controls, and Trp53 siRNA levels reduced the levels of Trp53 and increased levels of Oct-4 and Cdx2 mRNA by > twofold.

CONCLUSION

These studies suggest that the development of morphologically normal aneuploid-enriched mouse blastocysts can be inhibited by adding low amounts of DMO to the culture media, which results in elevated levels of Trp53 mRNA that suppresses Oct-4 and Cdx2 expression.

Preimplantation embryo gene expression: 56 years of discovery, and counting

The biology of preimplantation embryo gene expression began 56 years ago with studies of the effects of protein synthesis inhibition and discovery of changes in embryo metabolism and related enzyme activities. The field accelerated rapidly with the emergence of embryo culture systems and progressively evolving methodologies that have allowed early questions to be re-addressed in new ways and in greater detail, leading to deeper understanding and progressively more targeted studies to discover ever more fine details. The advent of technologies for assisted reproduction, preimplantation genetic testing, stem cell manipulations, artificial gametes, and genetic manipulation, particularly in experimental animal models and livestock species, has further elevated the desire to understand preimplantation development in greater detail. The questions that drove enquiry from the earliest years of the field remain drivers of enquiry today. Our understanding of the crucial roles of oocyte-expressed RNA and proteins in early embryos, temporal patterns of embryonic gene expression, and mechanisms controlling embryonic gene expression has increased exponentially over the past five and a half decades as new analytical methods emerged. This review combines early and recent discoveries on gene regulation and expression in mature oocytes and preimplantation stage embryos to provide a comprehensive understanding of preimplantation embryo biology and to anticipate exciting future advances that will build upon and extend what has been discovered so far.

Amino Acids and the Early Mammalian Embryo: Origin, Fate, Function and Life-Long Legacy

Amino acids are now recognised as having multiple cellular functions in addition to their traditional role as constituents of proteins. This is well-illustrated in the early mammalian embryo where amino acids are now known to be involved in intermediary metabolism, as energy substrates, in signal transduction, osmoregulation and as intermediaries in numerous pathways which involve nitrogen metabolism, e.g., the biosynthesis of purines, pyrimidines, creatine and glutathione. The amino acid derivative S-adenosylmethionine has emerged as a universal methylating agent with a fundamental role in epigenetic regulation. Amino acids are now added routinely to preimplantation embryo culture media. This review examines the routes by which amino acids are supplied to the early embryo, focusing on the role of the oviduct epithelium, followed by an outline of their general fate and function within the embryo. Functions specific to individual amino acids are then considered. The importance of amino acids during the preimplantation period for maternal health and that of the conceptus long term, which has come from the developmental origins of health and disease concept of David Barker, is discussed and the review concludes by considering the potential utility of amino acid profiles as diagnostic of embryo health.

SGK regulates pH increase and cyclin B-Cdk1 activation to resume meiosis in starfish ovarian oocytes

Tight regulation of intracellular pH (pH_i) is essential for biological processes. Fully grown oocytes, having a large nucleus called the germinal vesicle, arrest at meiotic prophase I. Upon hormonal stimulus, oocytes resume meiosis to become fertilizable. At this time, the pH_i increases via Na⁺/H⁺ exchanger activity, although the regulation and function of this change remain obscure. Here, we show that in starfish oocytes, serum- and glucocorticoid-regulated kinase (SGK) is activated via PI3K/TORC2/PDK1 signaling after hormonal stimulus and that SGK is required for this pH_i increase and cyclin B-Cdk1 activation. When we clamped the pH_i at 6.7, corresponding to the pH_i of unstimulated ovarian oocytes, hormonal stimulation induced cyclin B-Cdk1 activation; thereafter, oocytes failed in actin-dependent chromosome transport and spindle assembly after germinal vesicle breakdown. Thus, this SGK-dependent pH_i increase is likely a prerequisite for these events in ovarian oocytes. We propose a model that SGK drives meiotic resumption via concomitant regulation of the pH_i and cell cycle machinery.

Changes in extra- and intracellular pH in hepatocytes exposed to gabexate mesilate

Gabexate mesilate (GM) is a synthetic inhibitor of plasmatic and pancreatic serine proteases licensed for the treatment of pancreatitis. Here we show that in suspensions of isolated hepatocytes, profound changes in extracellular, cytoplasmic, and vesicular pH occur after addition of GM. Isolated hepatocytes obtained by collagenase perfusion of rat liver were pre-incubated with 1, 2, and 4 mM GM. Extracellular pH (pH in the incubation medium) was measured by a conventional pH electrode, cytosolic and vesicular pH were measured by fluorescence changes of 2',7'-biscarboxyethyl-5,6-carboxyfluorescein acetoxymethyl ester (BCECF-AM) and fluorescein dextran, respectively. Incubation of hepatocytes with GM resulted in a dose-dependent decrease of extracellular pH. Cytosolic pH decreased rapidly and markedly in a dose-dependent manner during the first minutes and gradually returned towards baseline. Simultaneously, GM induced a rapid alkalinization of acidic vesicles. The presence of bis-(p-nitrophelyl) phosphate (BNPP), an esterase inhibitor, reduced the extent of extracellular acidification. Incubation of hepatocytes in the presence of dimethylamiloride, an Na+/H+ exchanger inhibitor, or in a sodium-free medium, did not modify the rate and extent of extracellular acidification. GM, a commercially available pharmacological agent, could be useful to manipulate extra- and intracellular pH.

Effects of systemic progesterone during the early luteal phase on the availabilities of amino acids and glucose in the bovine uterine lumen

The uterine histotroph provides essential nutrition to the developing conceptus during the preimplantation period of pregnancy. The objective of the present study was to examine the effects of cycle stage and progesterone (P4) concentrations in the blood on the recoverable quantities of amino acids and glucose in the histotroph during the preimplantaion period of conceptus development. Following oestrus, dairy heifers were assigned to low, control or high P4 groups (n=6 heifers per treatment and time point). The uterine horn ipsilateral to the corpus luteum was flushed on either Day 7 or Day 13. The present study quantified 24 amino acids and glucose in the uterine flushings using HPLC and fluorometry, respectively. Heifers in the low P4 group had lower plasma concentrations of P4 throughout the cycle, whereas heifers in the high group had higher plasma concentrations of P4 between Days 3 and 7 compared with the control group (P<0.05). Total recoverable neutral (Ser, Gln, Gly, Thr, Cit, β-Ala, Tau, Ala, Tyr, Trp, Met, Val, Phe, Ile, Leu, Pro and Cys), acidic (Glu) and basic (His, Arg, Orn and Lys) amino acids were greater (P<0.05) on Day 13 than on Day 7. There was no significant difference in the amount of Asp or Asn between Day 7 and Day 13. The amount of amino acids recovered on Day 7 was similar across treatment groups. On Day 13, the amount of Asn, His and Thr was lower (P<0.05) in the low P4 heifers compared with the controls and/or high P4 heifers. Quantities of glucose were not altered by cycle stage or P4 treatment. In conclusion, the stage of oestrous cycle and P4 play important roles in modulating amino acids in the histotroph, a potentially critical factor for early embryonic and/or conceptus survival.

Glucose and glycine synergistically enhance the in vitro development of porcine blastocysts in a chemically defined medium

In the present study, the effects of glucose and/or glycine on the in vitro development of Day 5 (Day 0=IVF) porcine blastocysts were determined. The addition of 2.5-10 mM glucose to the chemically defined culture medium porcine zygote medium (PZM)-5 significantly increased blastocyst survival rates compared with those of blastocysts cultured in the absence of glucose. The addition of 5 and 10 mM glycine to PZM-5 containing 5 mM glucose significantly enhanced the development to hatching and the number of hatched blastocysts compared with no addition of glycine. However, the addition of glycine to PZM-5 with no glucose did not improve blastocyst development. The ATP content of Day 6 blastocysts cultured with glucose was significantly higher than that of blastocysts cultured in the absence of glucose, regardless of glycine supplementation. The diameter and total cell numbers were significantly greater, and the apoptotic index was significantly lower, in Day 6 blastocysts cultured with both glucose and glycine. These results indicate that glucose is an important energy source for the porcine blastocyst and that glucose and glycine act synergistically to enhance development to the hatching and hatched blastocyst stage in vitro.

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.

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.

The Effect of Energy Substrate Concentration and Amino Acids on the In Vitro Development of Preimplantation Porcine Embryos

As the pig becomes increasingly used for biomedical research, an effective and efficient in vitro culture system is essential. This study aimed to improve the commonly used porcine embryo culture medium, NCSU23, by altering the energy substrates and adding amino acids, using electrically activated diploid parthenotes from oocytes obtained from the ovaries of prepubertal and adult animals. Morphological development to day 6 and blastocyst cell number were examined. Glucose (5.56 mM) was replaced by pyruvate and lactate (0.2 mM and 5.7 mM, respectively) for either the entire culture period or for the first 48 h only. Blastocyst rates were not different between any of the treatments, and were similar for prepubertal and adult oocytes. When the embryos were cultured with pyruvate and lactate for the first 48 h and then glucose, there was a significant increase in blastocyst cell number compared to glucose only. Blastocysts produced using pyruvate and lactate for the entire time tended to have more cells than those exposed to glucose only and less than those who were cultured in pyruvate and lactate for the first 48 h and then glucose. Nonessential amino acids added for the first 48 h and nonessential and essential amino acids added for the remaining time significantly increased blastocyst cell number only when the embryos were grown in pyruvate and lactate followed by glucose. Blastocyst rates were not different between any of the treatments, and this result was the same when using sow or gilt oocytes. The modified medium was then tested using in vitro matured and fertilized embryos from sow oocytes. Blastocyst rates and cell number were significantly increased in the modified medium compared to those grown in unmodified NCSU23. This shows that altering energy substrates and adding amino acids can increase the quantity and cell number of IVP blastocysts compared with NCSU23.

Amino acid metabolism of the porcine blastocyst

The pattern of depletion and appearance of a mixture of amino acids by single porcine blastocysts incubated in two different media has been determined non-invasively using high performance liquid chromatography. Zygotes were produced by the in vitro fertilisation of in vitro-matured, abattoir-derived immature oocytes and cultured in medium NCSU 23 with or without amino acids. Embryos grown in the absence of amino acids up to the blastocyst stage were transferred to amino acid-containing culture medium for measurement of turnover (Experiment 1). Blastocysts grown in NCSU 23+amino acids were transferred into fresh droplets of the same medium (Experiment 2). Although the specific pattern of amino acid production and depletion varied between experiments, a general pattern emerged, with arginine being significantly depleted (p<0.001) and alanine consistently appearing in the media, in quantities that varied depending with culture conditions. The data suggest that arginine is important during porcine blastocyst development, most likely contributing to the formation of nitric oxide and polyamines and that alanine is produced as a means of disposing of excess amino groups. A model for the interactions of amino acids during porcine early embryo development is proposed. The profile of amino acid metabolism by porcine blastocysts is qualitatively and quantitatively similar to that given by human embryos during the morula:blastocyst transition suggesting that the porcine blastocyst is a good model for the human.

The oxidative burst at fertilization is dependent upon activation of the dual oxidase Udx1

The sea urchin egg is a quiescent cell...until fertilization, when the egg is activated. The classic respiratory burst at fertilization is the result of prodigious hydrogen peroxide production, but the mechanism for this synthesis is not known. Here we quantitate the kinetics of hydrogen peroxide synthesis at a single-cell level using an imaging photon detector, showing that 60 nM hydrogen peroxide accumulates within the perivitelline space of each zygote. We find that the NADPH oxidation activity is enriched at the cell surface and is sensitive to a pharmacological inhibitor of NADPH oxidase enzymes. Finally, we show that a sea urchin dual oxidase homolog, Udx1, is responsible for generating the hydrogen peroxide necessary for the physical block to polyspermy. Phylogenetic analysis of the enzymatic modules in Udx1 suggests a potentially conserved role for the dual oxidase family in hydrogen peroxide production and regulation during fertilization.

The 1.4-MDa apoptosome is a critical intermediate in apoptosome maturation

Previously, we demonstrated that both 150 mM KCl and alkaline pH inhibit cytochrome c-mediated activation of procaspase-3 in a unique manner. To determine the mechanism of inhibition, we analyzed the effect of KCl and alkaline pH on the formation of apoptosomes (a large complex consisting of cytochrome c, Apaf-1, and procaspase-9/caspase-9) in vitro. Our results suggest that an initial ∼700-kDa apoptosome matures through a 1.4-MDa intermediate before a ∼700-kDa apoptosome is reformed and procaspase-3 is activated. We further demonstrate that 150 mM KCl interferes with the conversion of the initial ∼700-kDa apoptosome to the 1.4-MDa intermediate, while alkaline pH “traps” the apoptosome in the 1.4-MDa intermediate. Analysis of the cleaved state of procaspase-9 and procaspase-3 suggests that the 1.4-MDa intermediate may be required for cleavage of procaspase-9. Consistent with these results, in vivo data suggest that blocking acidification during the induction of apoptosis inhibits activation of procaspase-3. On the basis of these results, we propose a model of apoptosome maturation.

Effects of pH on human bone marrow stromal cells in vitro: implications for tissue engineering of bone

The objective of this study was to address the hypothesis that changes in extracellular pH alter collagen gene expression, collagen synthesis, and alkaline phosphatase activity in bone marrow stromal cells (BMSCs). Potential effects of pH on cell function are of particular importance for tissue engineering because considerable effort is being placed on engineering biodegradable polymers that may generate a local acidic microenvironment on degradation. Human and murine single-cell marrow suspensions were plated at a density of 2 x 10(4) cells/cm(2). After 7 days in culture, the pH of the culture medium was adjusted to one of six ranges: > or = 7.8, 7.5.-7.7, 7.2-7.4, 6.9-7.1, 6.6-6.8, or < or = 6.5. After 48 h of exposure to an altered pH, alkaline phosphatase activity and collagen synthesis decreased significantly with decreasing pH. This decrease was two-to threefold as pH decreased from 7.5 to 6.6. In contrast, alpha1(I) procollagen mRNA levels increased two- to threefold as pH was decreased. The trend in osteocalcin mRNA expression was opposite to that of collagen. Small shifts in extracellular pH led to significant changes in the ability of BMSCs to express markers of the osteoblast phenotype. These pH effects potentially relate to the microenvironment supplied by a tissue-engineering scaffold and suggest that degrading polymer scaffolds may influence the biologic activity of the cells in the immediate environment.

A spliced variant of AE1 gene encodes a truncated form of Band 3 in heart: the predominant anion exchanger in ventricular myocytes

The anion exchangers (AE) are encoded by a multigenic family that comprises at least three genes, AE1, AE2 and AE3, and numerous splicoforms. Besides regulating intracellular pH (pHi) via the Cl-/HCO3- exchange, the AEs exert various cellular functions including generation of a senescent antigen, anchorage of the cytoskeleton to the membrane and regulation of metabolism. Most cells express several AE isoforms. Despite the key role of this family of proteins, little is known about the function of specific AE isoforms in any tissue, including the heart. We therefore chose isolated cardiac cells, in which a tight control of pHi is mandatory for the excitation-contraction coupling process, to thoroughly investigate the expression of the AE genes at both the mRNA and protein levels. RT-PCR revealed the presence of AE1, AE2 and AE3 mRNAs in both neonatal and adult rat cardiomyocytes. AE1 is expressed both as the erythroid form (Band 3 or eAE1) and a novel alternate transcript (nAE1), which was more specifically characterized using a PCR mapping strategy. Two variants of AE2 (AE2a and AE2c) were found at the mRNA level. Cardiac as well as brain AE3 mRNAs were expressed in both neonatal and adult rat cardiomyocytes. Several AE protein isoforms were found, including a truncated form of AE1 and two AE3s, but there was no evidence of AE2 protein in adult rat cardiomyocytes. In cardiomyocytes transfected with an AE3 oligodeoxynucleotide antisense, AE3 immunoreactivity was dramatically decreased but the activity of the Cl-/HCO3- exchange was unchanged. In contrast, intracellular microinjection of blocking anti-AE1 antibodies inhibited the AE activity. Altogether, our findings suggest that a specific and novel AE1 splicoform (nAE1) mediates the cardiac Cl-/HCO3- exchange. The multiple gene and protein expression within the same cell type suggest numerous functions for this protein family.

Ca2+ is required for phosphorylation of clam p82/CPEB in vitro: implications for dual and independent roles of MAP and Cdc2 kinases

During early development gene expression is controlled principally at the translational level. Oocytes of the surf clam Spisula solidissima contain large stockpiles of maternal mRNAs which are translationally dormant or masked until meiotic maturation. Fertilisation of the oocyte leads to rapid polysomal recruitment of the abundant cyclin and ribonucleotide reductase mRNAs at about the time they undergo cytoplasmic polyadenylation. Clam p82, a 3' UTR RNA-binding protein, and a member of the CPEB (cytoplasmic polyadenylation element binding protein) family, functions as a translational masking factor in oocytes and as a polyadenylation factor in fertilised eggs. In meiotically maturing clam oocytes, p82/CPEB is rapidly phosphorylated on multiple residues to a 92-kDa apparent size, prior to its degradation during the first cell cleavage. Here we examine the protein kinase(s) that phosphorylates clam p82/CPEB using a clam oocyte activation cell-free system that responds to elevated pH, mirroring the pH rise that accompanies fertilisation. We show that p82/CPEB phosphorylation requires Ca2+ (<100 microM) in addition to raised pH. Examination of the calcium dependency combined with the use of specific inhibitors implicates the combined and independent actions of cdc2 and MAP kinases in p82/CPEB phosphorylation. Calcium is necessary for both the activation and the maintenance of MAP kinase, whose activity is transient in vitro, as in vivo. While cdc2 kinase plays a role in the maintenance of MAP kinase activity, it is not required for the activation of MAP kinase. We propose a model of clam p82/CPEB phosphorylation in which MAP kinase initially phosphorylates clam p82/CPEB, at a minor subset of sites that does not alter its migration, and cdc2 kinase is necessary for the second wave of phosphorylation that results in the large mobility size shift of clam p82/CPEB. The possible roles of phosphorylation for the function and regulation of p82/CPEB are discussed.

Connexin diversity and gap junction regulation by pHi

The molecular mechanisms controlling pH-sensitivity of gap junctions formed of two different connexins are yet to be determined. We used a proton-sensitive fluorophore and electrophysiological techniques to correlate changes in intracellular pH (pHi) with electrical coupling between connexin-expressing Xenopus oocytes. The pH sensitivities of alpha 3 (connexin46), alpha 2 (connexin38), and alpha 1 (connexin43) were studied when these proteins were expressed as: 1) nonjunctional hemichannels (for alpha 3 and alpha 2), 2) homotypic gap junctions, and 3) heterotypic gap junctions. We found that alpha 3 hemichannels are sensitive to changes in pHi within a physiological range (pKa = 7.13 +/- 0.03; Hill coefficient = 3.25 +/- 1.73; n = 8; mean +/- SEM); an even more alkaline pKa was obtained for alpha 2 hemichannels (pKa = 7.50 +/- 0.03; Hill coefficient = 3.22 +/- 0.66; n = 13). The pH sensitivity curves of alpha 2 and alpha 3 homotypic junctions were indistinguishable from those recorded from hemichannels of the same connexin. Based on a comparison of pKa values, both alpha 3 and alpha 2 gap junctions were more pHi-dependent than alpha 1. The pH sensitivity of alpha 2-containing heterotypic junctions could not be predicted from the behavior of the two connexons in the pair. When alpha 2 was paired with alpha 3, the pH sensitivity curve was similar to that obtained from alpha 2 homotypic pairs. Yet, pairing alpha 2 with alpha 1 shifted the curve similar to homotypic alpha 1 channels. Pairing alpha 2 with a less pH sensitive mutant of alpha 1 (M257) yielded the same curve as when alpha 1 was used. However, the pH sensitivity curve of alpha 3/alpha 1 channels was similar to alpha 3/alpha 3, while alpha 3/M257 was indistinguishable from alpha 3/alpha 1. Our results could not be consistently predicted by a probabilistic model of two independent gates in series. The data show that dissimilarities in the pH regulation of gap junctions are due to differences in the primary sequence of connexins. Moreover, we found that pH regulation is an intrinsic property of the hemichannels, but pH sensitivity is modified by the interactions between connexons. These interactions should provide a higher level of functional diversity to gap junctions that are formed by more than one connexin.

Contributions of Na+/H+ exchanger isoforms to preimplantation development of the mouse

Previous work provided evidence of Na+/H+ exchanger activity in the apical domain of mouse trophectodermal plasma membranes that provides a route for entry of extracellular Na+ (Manejwala et al., 1989). This activity was hypothesized to contribute to the trans-trophectodermal Na+ flux that is required for blastocoel expansion. In the present work, we have used reverse transcriptase-polymerase chain reaction (RT-PCR) and immunocytochemistry to identify members of the Na+/H+ exchanger (NHE) family that are likely to participate in this process. When cDNA preparations from ovulated oocytes and several stages of preimplantation development were tested with PCR primers specific for the NHE-1, -2, -3, and -4 isoforms of the exchanger, only amplicons representing the NHE-1 and NHE-3 isoforms were detected. The identity of these amplicons was confirmed by direct sequencing. NHE-1 mRNA is present in oocytes and in all preimplantation stages, increasing threefold on a per embryo basis between the 4-cell and blastocyst stages. NHE-3 mRNA, on the other hand, was only detected in oocytes. Immunocytochemical analysis of blastocysts revealed that NHE-1 is localized in the basolateral domain of the trophectoderm, whereas NHE-3 is localized in the apical domain, a situation like that in epithelia of adult organs. We conclude that NHE-3, an oogenetic product that persists into the blastocyst stage, is the Na+/H+ exchanger isoform most likely to be involved in blastocoel expansion.

Intracellular pH increase driven by an Na+/H+ exchanger upon activation of surf clam oocytes

Intracellular pH (pHi) measurements were performed in surf clam (Spisula solidissima) oocytes before and after artificial activation or fertilization [evidenced by germinal vesicle breakdown (GVBD)] by the dimethyloxazolidinedione (DMO) and 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF) methods. Results using both methods showed increases of pHi of 0.3 pH unit after activation by excess K+. Using BCECF, we found an increase of similar magnitude after fertilization or after the addition of serotonin. By contrast, GVBD did not occur when the pHi was increased to similar or even higher levels by exposing the oocytes to ammonia. In sodium-free seawater, excess K+ induced GVBD but the pHi of K+-activated oocytes decreased significantly below the resting level of unactivated oocytes. The pHi increases in K+-activated oocytes were otherwise proportional to the external Na+ concentration. The amiloride derivatives dimethylamiloride and hexamethylene amiloride (at 10-50 microM) efficiently inhibited the K+-induced increase of pHi but did not block GVBD. These two derivatives were able, however, to retard K+-induced GVBD, hexamethylene amiloride being the more efficient. This retardation of K+-induced GVBD could be abolished by the simultaneous addition of ammonia. Taken altogether, these results show that a pHi increase, driven by a typical Na+/H+ exchanger, follows activation of surf clam oocytes but that this pHi increase is neither sufficient nor required for GVBD, though it does allow its progression at an optimal rate.

Control of cytosolic pH in two-cell mouse embryos: roles of H(+)-lactate cotransport and Na+/H+ exchange

In this study we used imaging techniques with the fluorescent pH-sensitive dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein to investigate the control of cytosolic pH (pHi) in two-cell mouse embryos in nominally HCO3(-)-free conditions. We found that the resting pHi of two-cell embryos (40-50 h after human chorionic gonadotropin) in HCO3(-)-free M2 was 7.31 +/- 0.01 (n = 172 embryos), which is significantly above the level predicted if H+ is at electrochemical equilibrium. We showed that two-cell embryos contain a H(+)-monocarboxylate cotransport system with apparent Michaelis constants for D-lactate, L-lactate, and pyruvate of 11.5, 3.7, and 3.5 mM, respectively. It is inhibited by p-chloromercuribenzoic acid (300 microM), p-chloromercuriphenylsulfonic acid (300 microM), and alpha-cyano-4-hydroxycinnamate (1 mM) and is insensitive to 4,4'-diisothiocyanodihydrostilbene-2,2'-disulfonic acid (500 microM). We also showed that the pHi response to the acid load produced by an NH4Cl pulse has two components, one due to H(+)-monocarboxylate cotransport and the other due to Na+/H+ exchange. We found no evidence that a H+ conductance was responsible in these cells for the recovery in pHi after an acid load.

pH regulation and proton signalling by glial cells

The regulation of H+ in nervous systems is a function of several processes, including H+ buffering, intracellular H+ sequestering, CO2 diffusion, carbonic anhydrase activity and membrane transport of acid/base equivalents across the cell membrane. Glial cells participate in all these processes and therefore play a prominent role in shaping acid/base shifts in nervous systems. Apart from a homeostatic function of H(+)-regulating mechanisms, pH transients occur in all three compartments of nervous tissue, neurones, glial cells and extracellular spaces (ECS), in response to neuronal stimulation, to neurotransmitters and hormones as well as secondary to metabolic activity and ionic membrane transport. A pivotal role for H+ regulation and shaping these pH transients must be assigned to the electrogenic and reversible Na(+)-HCO3-membrane cotransport, which appears to be unique to glial cells in nervous systems. Activation of this cotransporter results in the release and uptake of base equivalents by glial cells, processes which are dependent on the glial membrane potential. Na+/H+ and Cl-/HCO3-exchange, and possibly other membrane carriers, accomplish the set of tools in both glial cells and neurones to regulate their intracellular pH. Due to the pH dependence of a great variety of processes, including ion channel gating and conductances, synaptic transmission, intercellular communication via gap junctions, metabolite exchange and neuronal excitability, rapid and local pH transients may have signalling character for the information processing in nervous tissue. The impact of H+ signalling under both physiological and pathophysiological conditions will be discussed for a variety of nervous system functions.

Osteoclast ATP receptor activation leads to a transient decrease in intracellular pH

Application of extracellular adenosine triphosphate (ATP) induces a pulsed decrease in osteoclast intracellular pH (pHi), as measured with seminaphthofluorescein (SNAFL)-calcein on a laser scanning confocal microscope. Adenosine diphosphate also produces a pHi decrease, but adenosine monophosphate, uridine triphosphate, 2-methylthio-ATP, and beta, gamma-methylene-ATP have little effect on pHi. The ATP-induced pHi decrease is largely inhibited by suramin, a P2 purinergic receptor blocker. Clamping intracellular free [Ca2+] ([Ca2+]i) with BAPTA/AM does not affect the ATP-induced pHi change, showing that this pHi decrease is not caused by the increased intracellular [Ca2+]i that is produced by activation of osteoclast purinergic receptors. We show that an increase in [Ca2+]i by itself will produce a pHi increase. The ATP effect is not blocked by inhibition of Na+/H+ exchange by either Na(+)-free bathing medium or amiloride. Two inhibitors of the osteoclast cell membrane proton pump, N-ethylmaleimide and vanadate, produce partial inhibition of the ATP-induced pHi decrease. Two other proton pump inhibitors, bafilomycin and N,N'-dicyclohexylcarbodiimide, have no influence on the ATP effect. None of the proton pump inhibitors but vanadate has a direct effect on pHi. Vanadate produces a transient pHi increase upon application to the bathing medium, possibly as a result of its known effect of stimulating the Na+/H+ exchanger. Inhibition of Cl-/HCO3- exchange by decreasing extracellular Cl- gives a pronounced long-term pHi increase, supporting the hypothesis that this exchange has an important role in osteoclast pHi homeostasis. In Cl(-)-free extracellular medium, there is a greatly reduced effect of extracellular ATP on pHi.(ABSTRACT TRUNCATED AT 250 WORDS)