Organisation of Xenopus egg cytoplasm: response to simulated microgravity

The cytoplasm of Xenopus fertilised eggs appears to be organised into three major compartments based primarily on the uneven distribution of yolk platelets. There is a shift of these yolk compartments during the first cell cycle that is thought to be involved in the dorsal/ventral morphogenesis of the embryo. The involvement of gravity in Xenopus cytoplasmic organisation and in compartment shifts was addressed by examining, cytologically, the yolk compartments in embryos that developed under the simulated microgravity conditions of the horizontal clinostat. The cytoplasmic organisation into yolk compartments was found to be maintained, and the asymmetric movements of compartments still occurred in eggs that developed on the clinostat. It is suggested that the organisation of Xenopus egg cytoplasm into discrete compartments relies on forces other than those involving gravity (i.e., not density differences), and that the compartment shifts that take place during the first cell cycle are active movements. The variation in compartment size and composition observed from batch to batch of eggs, and to a lesser extent from egg to egg, during this study was addressed.     

Teloplasm formation in a leech, Helobdella triserialis, is a microtubule-dependent process

Fertilized eggs of the leech Helobdella triserialis undergo a cytoplasmic reorganization which generates domains of nonyolky cytoplasm, called teloplasm, at the animal and vegetal poles. The segregation of teloplasm to one cell of the eight-cell embryo is responsible for a unique developmental fate of that cell, i.e., to give rise to segmental ectoderm and mesoderm. We have studied the cytoplasmic movements that generate teloplasm using time-lapse video microscopy; the formation and migration of rings of nonyolky cytoplasm were visualized using transmitted light, while the movements of mitochondria into these rings were monitored with epifluorescence after labeling embryos with rhodamine 123, a fluorescent mitochondrial dye. To examine the likelihood that cytoskeletal elements play a role in the mechanism of teloplasm formation in Helobdella, we examined the distribution of microtubules and microfilaments during the first cell cycle by indirect immunofluorescence and rhodamine-phalloidin labeling, respectively. The cortex of the early embryo contained a network of microtubules many of which were oriented parallel to the cell surface. As teloplasm formation ensued, microtubule networks became concentrated in the animal and the vegetal cortex relative to the equatorial cortex. More extensive microtubule arrays were found within the rings of teloplasm. Actin filaments appeared in the form of narrow rings in the cortex, but these varied apparently randomly from embryo to embryo in terms of number, size, and position. The role of microtubules and microfilaments in teloplasm formation was tested using depolymerizing agents. Teloplasm formation was blocked by microtubule inhibitors, but not by microfilament inhibitors. These results differ significantly from those obtained in embryos of the oligochaete Tubifex hattai, suggesting that the presumably homologous cytoplasmic reorganizations seen in these two annelids have different cytoskeletal dependencies.     

CiYB1 is a major component of storage mRNPs in ascidian oocytes: implications in translational regulation of localized mRNAs

In ascidian eggs, the existence of several localized maternal cytoplasmic determinants has been proposed and the importance of localized mRNAs for tissue differentiation has been demonstrated. We previously identified the ascidian Y-box proteins (CiYB1, 2 and 3), homologues of which are known to be involved in the storage of maternal mRNA in oocytes of other organisms. In this study, we found that CiYB1 protein is abundant in the gonad, egg, and embryo. Purification of messenger ribonucleoprotein (mRNP) particles from the gonad revealed that CiYB1 was one of their major components. A significant change in the distribution of CiYB1 protein from stored mRNP particles in the gonad to the ribosome fraction in eggs and embryos was observed. This change correlates most likely with the shift of stored maternal mRNAs to polyribosomes. Moreover, we found that CiYB1 colocalized with Cipem and Ci-macho1 mRNAs, which are localized at the posterior end of the embryo at the cleavage stage. Cipem and Ci-macho1 mRNAs were co-immunoprecipitated with CiYB1 in the oocyte and embryo lysates. The formation of a complex between Cipem mRNA and CiYB1 protein resulted in translational repression in the in vitro translation system. Our results indicate that associating with CiYB1 protein contributes to the translational control of the localized mRNA in eggs and embryos.     

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.     

Mitochondrial dysfunction in reproduction

The mitochondrial genome passes from one generation to the next by way of the egg's cytoplasm, so ordinarily an individual's mitochondrial DNA (mtDNA) is entirely derived from his or her mother. A potential mother has a finite number of eggs, or oocytes, all of which were formed when she herself was still a fetus, many years before she can conceive. The eggs are progressively depleted through childhood and her reproductive years at a much faster rate than is accounted for by ovulation. Up to a decade before the ultimate depletion of ovarian follicles (and hence oocytes) at or soon after menopause, cytoplasmic senility of the remaining eggs leads to physiological sterility; a phenomenon that is suspected of being mitochondrially based and has been termed the oopause. When ovulation and conception occur, oxidative phosphorylation and other mitochondrial functions of the fertilized oocyte are thought to be essential to the early embryo well before it implants in the uterus. The competition between follicles to deliver the oocyte that will be fertilized and which will found a new generation could also be mitochondrially based, but the mechanism remains to be elucidated. Increasing experience with the culture of human embryos in vitro is highlighting the importance of mitochondrial metabolism generally, and the avoidance of excessive generation of reactive oxygen species in particular. Paradoxes abound in the experimental data, however. Although natural selection operates on mitochondria only in females (and in extreme cases through the survival of their offspring), reproductive disturbance from mitochondrial mutations is most obvious in males, who typically have reduced sperm motility. mtDNA point mutations such as T8993G, which is serious enough to cause the death of infants from Leigh disease in the first few years of life, can carry through the female germ line apparently unhindered; yet mtDNA deletions that cause a less severe phenotype, and which typically manifest at a later age, are effectively blocked from transmission to offspring--a phenomenon in accord with early experimental observations that deleted mtDNA species are less common in cleaving embryos than in unselected preovulatory oocytes. A mitochondrial basis for ooplasmic aging has not been convincingly established, but the novel IVF-based practice of micro-aspiration and transfer of ooplasm from younger eggs to older eggs, which includes the transfer of mitochondria, appears in preliminary studies to have some clinical efficacy in rejuvenating fertility in older women.     

Mitochondrial function in the human oocyte and embryo and their role in developmental competence

The role of mitochondria as a nexus of developmental regulation in mammalian oogenesis and early embryogenesis is emerging from basic research in model species and from clinical studies in infertility treatments that require in vitro fertilization and embryo culture. Here, mitochondrial bioenergetic activities and roles in calcium homeostasis, regulation of cytoplasmic redox state, and signal transduction are discussed with respect to outcome in general, and as possible etiologies of chromosomal defects, maturation and fertilization failure in human oocytes, and as causative factors in early human embryo demise. At present, the ability of mitochondria to balance ATP supply and demand is considered the most critical factor with respect to fertilization competence for the oocyte and developmental competence for the embryo. mtDNA copy number, the timing of mtDNA replication during oocyte maturation, and the numerical size of the mitochondrial complement in the oocyte are evaluated with respect to their relative contribution to the establishment of developmental competence. Rather than net cytoplasmic bioenergetic capacity, the notion of functional compartmentalization of mitochondria is presented as a means by which ATP may be differentially supplied and localized within the cytoplasm by virtue of stage-specific changes in mitochondrial density and potential (ΔΨm). Abnormal patterns of calcium release and sequestration detected at fertilization in the human appear to have coincident effects on levels of mitochondrial ATP generation. These aberrations are not uncommon in oocytes obtained after ovarian hyperstimulation for in vitro fertilization. The possibility that defects in mitochondrial calcium regulation or bioenergetic homeostasis could have negative downstream development consequences, including imprinting disorders, is discussed in the context of signaling pathways and cytoplasmic redox state.     

Fertilization in Nicotiana tabacum: ultrastructural organization of propane-jet-frozen embryo sacs in vivo

Ovules of Nicotiana tabacum L. were cryofixed with a propane-jet freezer and freeze-substituted in acetone to examine technique-dependent changes in pre- and post-fertilization embryo sacs using rapidly frozen material. Freezing quality was acceptable in 10% of the embryo sacs in the partially dissected ovules, with ice-crystal damage frequently evident in vacuoles and nuclei. One of the two synergids begins to degenerate before pollen-tube arrival in cryofixed material, with breakdown of the plasma membrane and large chalazal vacuole delayed until the penetration of the pollen tube. Early synergid degeneration involved characteristic increases in cytoplasmic electron density and the generation of cytoplasmic bodies to the intercellular space through “pinching-off”. Upon pollen-tube arrival, the male gametes are released through a terminal aperture into the degenerate synergid. Sperm cells undergo morphological alteration before gametic fusion: their mitochondrial electron density increases, the endoplasmic reticulum dilates, cytoplasm becomes finely vacuolated and the surrounding pollen plasma membrane is lost, causing the sperm cells and vegetative nucleus to dissociate. Discharge of the pollen tube results in the formation of numerous enucleated cytoplasmic bodies which are either stripped or shed from sperm cells and pollen-tube cytoplasm. Two so-called X-bodies are found in the degenerate synergid after pollen-tube penetration: the presumed vegetative nucleus occurs at the chalazal end and the presumed synergid nucleus near the micropylar end.     

The Meganuclease I-SceI Containing Nuclear Localization Signal (NLS-I-SceI) Efficiently Mediated Mammalian Germline Transgenesis via Embryo Cytoplasmic Microinjection

The meganuclease I-SceI has been effectively used to facilitate transgenesis in fish eggs for nearly a decade. I-SceI-mediated transgenesis is simply via embryo cytoplasmic microinjection and only involves plasmid vectors containing I-SceI recognition sequences, therefore regarding the transgenesis process and application of resulted transgenic organisms, I-SceI-mediated transgenesis is of minimal bio-safety concerns. However, currently no transgenic mammals derived from I-SceI-mediated transgenesis have been reported. In this work, we found that the native I-SceI molecule was not capable of facilitating transgenesis in mammalian embryos via cytoplasmic microinjection as it did in fish eggs. In contrast, the I-SceI molecule containing mammalian nuclear localization signal (NLS-I-SceI) was shown to be capable of transferring DNA fragments from cytoplasm into nuclear in porcine embryos, and cytoplasmic microinjection with NLS-I-SceI mRNA and circular I-SceI recognition sequence-containing transgene plasmids resulted in transgene expression in both mouse and porcine embryos. Besides, transfer of the cytoplasmically microinjected mouse and porcine embryos into synchronized recipient females both efficiently resulted in transgenic founders with germline transmission competence. These results provided a novel method to facilitate mammalian transgenesis using I-SceI, and using the NLS-I-SceI molecule, a simple, efficient and species-neutral transgenesis technology based on embryo cytoplasmic microinjection with minimal bio-safety concerns can be established for mammalian species. As far as we know, this is the first report for transgenic mammals derived from I-SceI-mediated transgenesis via embryo cytoplasmic microinjection.     

A CYTOLOGICAL STUDY OF ARTIFICIAL PARTHENOGENESIS IN THE SEA URCHIN ARBACIA PUNCTULATA

Eggs of the sea urchin Arbacia punctulata were artificially activated with hypertonic seawater. The artificially activated eggs undergo the cortical reaction which is not distinguished by a wavelike progression as in the case of inseminated eggs. The cortical granules are released at random loci at the surface of the egg and result in spaces separated by large cytoplasmic projections. Unreacted cortical granules and ribosomes are found within the matrix comprising the large cytoplasmic projections. No "fertilization cone" is formed. The subsequent release of additional cortical granules results in the formation of a continuous perivitelline space, 15 min following activation. 85 min postactivation, an organization of annulate lamellae, endoplasmic reticulum of the smooth variety, and microtubules around a centriole is observed prior to nuclear division. Before the breakdown of the nuclear envelope a streak stage is formed. The streak is composed of a central core of annulate lamellae and is encompassed by endoplasmic reticulum and vesicular components. Condensation of chromatin is followed by the establishment of the mitotic apparatus. Centrioles were not found in the mature egg; however, they are present after activation prior to the first nuclear division, in the four-cell embryo, multicellular embryo, and at blastula. Artificially activated eggs have been observed to develop to the pluteus stage in more than 50% of the eggs treated.     

Maternal information and localized maternal mRNAs in eggs and early embryos of the ascidian Halocynthia roretzi

Summary

The mosaic behavior of blastomeres isolated from ascidian embryos has been taken as evidence that localized ooplasmic factors (cytoplasmic determinants) specify tissue precursor cells during embryogenesis. Experiments involving the transfer of egg cytoplasm have revealed the presence and localization of various kinds of cytoplasmic determinants in eggs of Halocynthia roretzi. Three cell fates, epidermis, muscle and endoderm, are fixed by cytoplasmic determinants. The three kinds of tissue determinants move in different directions during ooplasmic segregation. Prior to the onset of the first cleavage the three kinds of determinants reside in egg regions that correspond to the future fate map of the embryo and then they are differentially partitioned into specific blastomeres. In addition to tissue-specific determinants, there is evidence suggesting that ascidian eggs contain localized cytoplasmic factors that are responsible for controlling the cleavage pattern and morphogenetic movements. Transplantation of posterior-vegetal egg cytoplasm to an anterior-vegetal position causes a reversal of the anterior-posterior polarity of the cleavage pattern. Localized cytoplasmic factors in the posterior-vegetal region are involved in the generation of a unique cleavage pattern. When vegetal pole cytoplasm is transplanted to the animal pole or equatorial position of the egg, ectopic gastrulation occurs at the site of transplantation. This finding supports the idea that vegetal pole cytoplasm specifies the site of gastrulation. Recently, we started a cDNA project to analyze maternal mRNAs. An arrayed cDNA library of fertilized eggs of H. roretzi was constructed, and more than 2000 clones have been partially sequenced so far. To estimate the proportion of the maternal mRNAs that are localized in the egg and embryo, 150 randomly selected clones were examined by in situ hybridization. We found eight mRNAs that are localized in the eight-cell embryo, of which three were localized to the myoplasm (a specific region of the egg cytoplasm that is partitioned into muscle-lineage blastomeres) of the egg, and then to the postplasm of cleavage-stage embryos. These results indicate that the proportion of localized messages is much higher than we expected. These localized maternal messages may be involved in the regulation of various developmental processes.     

A transient array of parallel microtubules in frog eggs: potential tracks for a cytoplasmic rotation that specifies the dorso-ventral axis

The dorsoventral axis of the frog embryo is specified by a rotation of the egg cytoplasm relative to the cortex. When eggs undergoing the cortical/cytoplasmic rotation were examined by immunocytochemistry and electron microscopy, an extensive array of parallel microtubules was found covering the vegetal hemisphere of the egg. The microtubules were 1-3 microns deep from the plasma membrane and were aligned parallel to the direction of rotation. They formed at the start of rotation and disappeared at its completion. Colchicine and uv irradiation, inhibitors of the rotation, prevented the formation of the parallel microtubules. Based on these properties, we suggest that the parallel microtubules serve as tracks for the cortical/cytoplasmic rotation which specifies the dorsoventral axis of the embryo.     

Oral-aboral axis specification in the sea urchin embryo II. Mitochondrial distribution and redox state contribute to establishing polarity in Strongylocentrotus purpuratus

The initial asymmetry that specifies the oral-aboral (OA) axis of the sea urchin embryo has long been a mystery. It was shown previously that OA polarity can be entrained in embryos by imposing a respiratory asymmetry, with the most oxidizing side of the embryo tending to develop as the oral pole. This suggests that one of the earliest observable asymmetries along the incipient OA axis, a redox gradient established by a higher density and/or activity of mitochondria on the prospective oral side of the embryo, might play a causal role in establishing the axis. Here, we examine the origin and functional significance of this early redox gradient. Using MitoTracker Green, we show that mitochondria are asymmetrically distributed in the unfertilized egg of Strongylocentrotus purpuratus, and that the polarity of the maternal asymmetry is maintained in the zygote. Vital staining indicates that the side of the embryo that inherits the highest density of mitochondria tends to develop into the oral pole. This correlation holds when mitochondria are redistributed by centrifugation of eggs or by transfer of purified mitochondria into zygotes, indicating that an asymmetric mitochondrial distribution can entrain OA polarity, possibly through effects on intracellular redox state. In support of this possibility, we find that specification of oral ectoderm is suppressed when embryos are cultured under hypoxic conditions that enforce a relatively reducing redox state. This effect is reversed by overexpression of nodal, an early zygotic marker of oral specification whose localized expression suffices to organize the entire OA axis, indicating that redox state is upstream of nodal expression. We therefore propose that a threshold level of intracellular oxidation is required to effectively activate nodal, and that precocious attainment of this threshold within the blastomeres containing the highest density of mitochondria results in asymmetric nodal activity and consequent specification of the OA axis.     

Subcellular localization of DNA polymerase gamma and changes in its activity in sea urchin embryos

1. Subcellular localization and changes in the activity of DNA polymerase gamma were examined in sea urchin eggs and embryos. 2. The enzyme was shown to be localized predominantly in mitochondria by differential and isopycnic centrifugation. 3. During embryogenesis, the enzyme activity per embryo remained constant until blastula stage, and thereafter increased. 4. Similarly mitochondrial DNA per embryo increased, indicating that mitochondrial DNA replication starts during embryogenesis. 5. The gamma-activity per mitochondrial DNA remained constant during embryogenesis. 6. These results suggest that mitochondria contain a constant amount of replicative enzyme (DNA polymerase gamma) regardless of mitochondrial DNA replication, which differs from the case of nuclear DNA replication.     

Initial observation of potential factors involved in the specification process of oral-aboral axis in the sand dollar Scaphechinus mirabilis

To elucidate factors involved in the oral-aboral axis specification, several observations and experiments were undertaken using the sand dollar Scaphechinus mirabilis. Unlike in Strongylcentrotus purpuratus, localization of mitochondria was not detected in unfertilized eggs. After fertilization, however, the bulk of mitochondria became localized to the opposite side of sperm entry. The first cleavage divided this mitochondrial cluster into daughter blastomeres. On the other hand, a second cleavage produced daughter blastomeres containing quite different amounts of mitochondria. To know whether such mitochondrial localization affects the oral-aboral axis specification, 4-cell-stage embryos were separated along the second cleavage plane. Although both half embryos developed into morphologically normal plutei, some differences, such as the number of pigment cells, were noticed between the siblings. In contrast, cell tracing revealed that the first cleavage separated the oral from the aboral part in most cases, indicating that the unequal distribution of mitochondria is not critical for the oral-aboral axis specification. Further, stained and non-stained half embryo fragments were combined. Such combined embryos developed into normal plutei with a single oral-aboral axis. The plane dividing labeled and non-labeled parts were incident, oblique or perpendicular to the median plane of the combined embryo, and the appearance frequencies of those labeling patterns were similar to those obtained by cell tracing in intact embryos. Interestingly, the half fragments derived from embryos inseminated earlier showed a tendency to form the oral part. These suggest that several factors as well as the localized cytoplasmic components would be involved in the specification process of oral-aboral axis.     

Metabolic and ultrastructural responses of lupine embryo axes to sugar starvation

Embryo axes isolated from germinating lupine seeds were cultivated in vitro for 24-96 h over media containing either 60 mmol/L sucrose or no sucrose. Ultrastructural studies showed that large vacuoles were accumulating in a central region of primary parenchyma cells in sucrose starved lupine embryo axes, whereas cytoplasm along with organelles were forced to a periphery of the cells. We suggest that the autolysis of cytoplasmic proteins contributes to the accumulation of the vacuoles and this suggestion is consistent with the results of the characterisation of protein content. The level of cytosolic proteins was reduced by 50% and the activity of cytosolic marker enzyme, PEP carboxylase, was reduced by 46% in starved embryos as compared to control. The mitochondria from starved tissues were not degraded. The level of mitochondrial proteins was reduced by only 10% and the activity of mitochondrial NAD-isocitrate dehydrogenase decreased by 8% as a result of starvation. As demonstrated by the results of Percoll density gradient centrifugation, sucrose starvation caused an increase of 49% in many of the higher density mitochondria fractions, whereas many of the lower density mitochondria fractions were decreased by 33%. The samples of mitochondria from starved embryo axes were determined to have higher respiration activity in the presence of glutamate and malate as compared to control samples. EPR-based analyses of free radicals showed the presence of free radicals with a signal at g = 2.0060 in embryo axes. The level of the radical was two times higher in sucrose-starved embryo axes than in control (the level of this radical increased in senescing plant tissues as well). The results of EPR-based quantitation of Mn2+ ions revealed that the level was a few times higher in starved material than in control. Starved embryo axes, however, do possess a number of adaptive mechanisms protecting them from oxidative damage. Densitometric analyses of gels revealed an increase in the activity of SOD in sugar-starved embryos, whereas CAT and POX activities were lower in axes grown without sucrose as compared to control. Superoxide dismutase, catalase and peroxidase zymogram analyses showed that synthesis of new isoforms was not induced by sugar starvation. An accumulation of phytoferritin was found in plastids of sucrose starved embryos. These results are discussed in relation to the metabolic changes observed in senescing plant tissues.     

Distribution of cytoplasmic determinants in unfertilized eggs of the ascidian Halocynthia roretzi

 Cytoplasmic determinants that specify the fate of endoderm, muscle and epidermis cells are known to be localized in specific areas of fertilized eggs of ascidians. The presence of such cytoplasmic determinants in unfertilized eggs was demonstrated in previous studies, but no information has yet been proved about their distribution. To investigate the distribution of cytoplasmic determinants in unfertilized eggs, we devised a method for distinguishing the polarity of unfertilized eggs using vital staining and we performed cytoplasmic-transfer experiments by fusing blastomeres and cytoplasmic fragments from various identified regions of unfertilized eggs. Cytoplasmic fragments, that contained cortical and subcortical material, from five different positions along the animal-vegetal axis were prepared, and they were fused with a4.2 (presumptive-epidermis) or A4.1 (non-epidermis) blastomeres. The ectopic development of endoderm, muscle and epidermis cells that was promoted by the transplanted cytoplasm was assessed by examining the expression of alkaline phosphatase (ALP), myosin and epidermis-specific antigen, respectively. Differentiation of endoderm and muscle was observed at higher frequencies as cytoplasmic fragments closer to the vegetal pole were transplanted. Conversely, formation of epidermis was observed at higher frequencies as cytoplasmic fragments closer to the animal pole were transplanted. The results suggest that, in cortical and subcortical regions of unfertilized ascidian eggs, endoderm and muscle determinants are widely distributed along a gradient, with maximum activity at the vegetal pole, whilst epidermis determinants are also distributed along a gradient but with maximum activity at the animal pole. Recieved: 10 June 1996 / Accepted: 12 September 1996     

Mitochondrial inhibitor sodium azide inhibits the reorganization of mitochondria‐rich cytoplasm and the establishment of the anteroposterior axis in ascidian embryo

In ascidian eggs, cytoplasmic and cortical reorganization, previously called ooplasmic segregation, occurs in two phases during the first cell cycle. In the second phase of reorganization, the mitochondria‐rich cytoplasm (myoplasm) moves to the future posterior side, concurrent with sperm aster migration along the egg cortex. Although this reorganization is the critical step for establishing the anteroposterior axis, its molecular mechanism is not fully understood. In this study, we showed that low concentrations of the mitochondrial inhibitor sodium azide (NaN₃), which showed the low toxicity in sperm, inhibited the second phase of reorganization without the microtubule depolymerization. In the NaN₃‐treated embryo, the sperm aster was not attracted to the cortex and altered its migration pathway; therefore, the myoplasm remained at the vegetal pole. Consequently, the anteroposterior axis was not established. Another mitochondrial inhibitor, oligomycin, did not affect these processes. These results suggest that NaN₃ inhibits unknown molecules that are important for the second phase of reorganization. Identifying the target molecule of NaN₃ will lead to a molecular understanding of cytoplasmic and cortical reorganization.     

Identity of mitochondrial and cytosolic glycerate kinases in rat liver and regulation of their intracellular localization by dietary protein

Glycerate kinase (ATP : D-glycerate 2-phosphotransferase EC 2.7.1.31) is a key enzyme of gluconeogenesis from serine via hydroxypyruvate. A differential centrifugation of rat liver homogenate and an analysis of the particle fraction by sucrose density gradient centrifugation indicated that 72% and 26% of glycerate kinase are present in mitochondria and cytosol, respectively. A study on the intramitochondrial localization of the enzyme suggested that the mitochondrial glycerate kinase was present in inner membrane and/or matrix. It was found that dietary protein selectively induced mitochondrial glycerate kinase. This result suggested that mitochondrial glycerate kinase had a physiological function for gluconeogenesis from serine. However, the metabolic significance of the cytoplasmic enzyme was still unclear. The properties of solubilized-mitochondrial and cytosolic glycerate kinases were compared. However, no difference between the two enzymes could be found in the kinetic properties, thermal stability, molecular size or electrochemical properties. These results suggested that both enzymes originate from common genetic information. In order to elucidate the regulatory mechanism of the intracellular distribution of glycerate kinase in rat liver, the responses of mitochondrial and cytosolic glycerate kinases to an alteration of dietary protein were studied. The result suggested that an alteration of dietary protein content may regulate the distribution and the translocation of glycerate kinase to mitochondria and cytosol as well as the total amount of glycerate kinase.     

Identity of mitochondrial and cytosolic glycerate kinases in rat liver and regulation of their intracellular localization by dietary protein.

Glycerate kinase (ATP: D-glycerate 2-phosphotransferase EC 2.7.1.31) is a key enzyme of glyconeogenesis from serine via hydroxypyruvate. A differential centrifugation of rat liver homogenate and an analysis of the particle fraction by sucrose density gradient centrifugation indicated that 72% and 26% of glycerate kinase are present in mitochondria and cytosol, respectively. A study on the intramitochondrial localization of the enzyme suggested that the mitochondrial glycerate kinase was present in inner membrane and/or matrix. It was found that dietary protein selectively induced mitochondrial glycerate kinase. This result suggested that mitochondrial glycerate kinase had a physiological function for gluconeogenesis from serin. However, the metabolic significance of the cytoplasmic enzyme was still unclear. The properties of solubilized-mitochondrial and cytosolic glycerate kinases were compared. However, no difference between the two enzymes could be found in the kinetic properties, thermal stability, molecular size or electrochemical properties. These results suggested that both enzymes originate from common genetic information. In order to elucidate the regulatory mechanism of the intracellular distribution of glycerate kinase in rat liver, the responses of mitochondrial and cytosolic glycerate kinases to an alteration of dietary protein were studied. The result suggested that an alteration of dietary protein content may regulate the distribution and the translocation of glycerate kinase to mitochondria and cytosol as well as the total amount of glycerate kinase.