Nuclear sodium and potassium

Mutant analysis of Kcng4b reveals how the different functional states of the voltage-gated potassium channel regulate ear development

The voltage gated (Kv) slow-inactivating delayed rectifier channel regulates the development of hollow organs of the zebrafish. The functional channel consists of the tetramer of electrically active Kcnb1 (Kv2.1) subunits and Kcng4b (Kv6.4) modulatory or electrically silent subunits. The two mutations in zebrafish kcng4b gene - kcng4b-C1 and kcng4b-C2 (Gasanov et al., 2021) - have been studied during ear development using electrophysiology, developmental biology and in silico structural modelling. kcng4b-C1 mutation causes a C-terminal truncation characterized by mild Kcng4b loss-of-function (LOF) manifested by failure of kinocilia to extend and formation of ectopic otoliths. In contrast, the kcng4b-C2-/- mutation causes the C-terminal domain to elongate and the ectopic seventh transmembrane (TM) domain to form, converting the intracellular C-terminus to an extracellular one. Kcng4b-C2 acts as a Kcng4b gain-of-function (GOF) allele. Otoliths fail to develop and kinocilia are reduced in kcng4b-C2-/-. These results show that different mutations of the silent subunit Kcng4 can affect the activity of the Kv channel and cause a wide range of developmental defects.

Why Na+ has higher propensity than K+ to condense DNA in a crowded environment

Experimentally, in the presence of the crowding agent polyethylene glycol (PEG), sodium ions compact double-stranded DNA more readily than potassium ions. Here, we have used molecular dynamics simulations and the "ion binding shells model" of DNA condensation to provide an explanation for the observed variations in condensation of short DNA duplexes in solutions containing different monovalent cations and PEG; several predictions are made. According to the model we use, externally bound ions contribute the most to the ion-induced aggregation of DNA duplexes. The simulations reveal that for two adjacent DNA duplexes, the number of externally bound Na+ ions is larger than the number of K+ ions over a wide range of chloride concentrations in the presence of PEG, providing a qualitative explanation for the higher propensity of sodium ions to compact DNA under crowded conditions. The qualitative picture is confirmed by an estimate of the corresponding free energy of DNA aggregation that is at least 0.2kBT per base pair more favorable in solution with NaCl than with KCl at the same ion concentration. The estimated attraction free energy of DNA duplexes in the presence of Na+ depends noticeably on the DNA sequence; we predict that AT-rich DNA duplexes are more readily condensed than GC-rich ones in the presence of Na+. Counter-intuitively, the addition of a small amount of a crowding agent with high affinity for the specific condensing ion may lead to the weakening of the ion-mediated DNA-DNA attraction, shifting the equilibrium away from the DNA condensed phase.

Phosphorylation Alters the Properties of Pol η: Implications for Translesion Synthesis

There are significant ambiguities regarding how DNA polymerase η is recruited to DNA lesion sites in stressed cells while avoiding normal replication forks in non-stressed cells. Even less is known about the mechanisms responsible for Pol η-induced mutations in cancer genomes. We show that there are two safeguards to prevent Pol η from adventitious participation in normal DNA replication. These include sequestration by a partner protein and low basal activity. Upon cellular UV irradiation, phosphorylation enables Pol η to be released from sequestration by PDIP38 and activates its polymerase function through increased affinity toward monoubiquitinated proliferating cell nuclear antigen (Ub-PCNA). Moreover, the high-affinity binding of phosphorylated Pol η to Ub-PCNA limits its subsequent displacement by Pol δ. Consequently, activated Pol η replicates DNA beyond the lesion site and potentially introduces clusters of mutations due to its low fidelity. This mechanism could account for the prevalence of Pol η signatures in cancer genome.

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Pol η activation requires both ATR and PKC phosphorylation

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Phosphorylation directly enhances the affinity of Pol η toward Ub-PCNA

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PDIP38 sequesters Pol η away from normal replication fork

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Pol δ is not able to displace phosphorylated Pol η from Ub-PCNA complex

Novel genetically encoded fluorescent probes enable real-time detection of potassium in vitro and in vivo

Changes in intra- and extracellular potassium ion (K⁺) concentrations control many important cellular processes and related biological functions. However, our current understanding of the spatiotemporal patterns of physiological and pathological K⁺ changes is severely limited by the lack of practicable detection methods. We developed K⁺-sensitive genetically encoded, Förster resonance energy transfer-(FRET) based probes, called GEPIIs, which enable quantitative real-time imaging of K⁺ dynamics. GEPIIs as purified biosensors are suitable to directly and precisely quantify K⁺ levels in different body fluids and cell growth media. GEPIIs expressed in cells enable time-lapse and real-time recordings of global and local intracellular K⁺ signals. Hitherto unknown Ca²⁺-triggered, organelle-specific K⁺ changes were detected in pancreatic beta cells. Recombinant GEPIIs also enabled visualization of extracellular K⁺ fluctuations in vivo with 2-photon microscopy. Therefore, GEPIIs are relevant for diverse K⁺ assays and open new avenues for live-cell K⁺ imaging.

K⁺ plays an important role in physiology and disease, but the lack of high specificity K⁺ sensors limits our understanding of its spatiotemporal dynamics. Here the authors develop genetically-encoded FRET-based probes able to quantify K⁺ concentration in body fluids, cells and specific organelles.

Alternative DNA structure formation in the mutagenic human c-MYC promoter

Mutation 'hotspot' regions in the genome are susceptible to genetic instability, implicating them in diseases. These hotspots are not random and often co-localize with DNA sequences potentially capable of adopting alternative DNA structures (non-B DNA, e.g. H-DNA and G4-DNA), which have been identified as endogenous sources of genomic instability. There are regions that contain overlapping sequences that may form more than one non-B DNA structure. The extent to which one structure impacts the formation/stability of another, within the sequence, is not fully understood. To address this issue, we investigated the folding preferences of oligonucleotides from a chromosomal breakpoint hotspot in the human c-MYC oncogene containing both potential G4-forming and H-DNA-forming elements. We characterized the structures formed in the presence of G4-DNA-stabilizing K+ ions or H-DNA-stabilizing Mg2+ ions using multiple techniques. We found that under conditions favorable for H-DNA formation, a stable intramolecular triplex DNA structure predominated; whereas, under K+-rich, G4-DNA-forming conditions, a plurality of unfolded and folded species were present. Thus, within a limited region containing sequences with the potential to adopt multiple structures, only one structure predominates under a given condition. The predominance of H-DNA implicates this structure in the instability associated with the human c-MYC oncogene.

Dielectrophoresis for Biomedical Sciences Applications: A Review

Dielectrophoresis (DEP) is a label-free, accurate, fast, low-cost diagnostic technique that uses the principles of polarization and the motion of bioparticles in applied electric fields. This technique has been proven to be beneficial in various fields, including environmental research, polymer research, biosensors, microfluidics, medicine and diagnostics. Biomedical science research is one of the major research areas that could potentially benefit from DEP technology for diverse applications. Nevertheless, many medical science research investigations have yet to benefit from the possibilities offered by DEP. This paper critically reviews the fundamentals, recent progress, current challenges, future directions and potential applications of research investigations in the medical sciences utilizing DEP technique. This review will also act as a guide and reference for medical researchers and scientists to explore and utilize the DEP technique in their research fields.

Zn2+ blocks annealing of complementary single-stranded DNA in a sequence-selective manner

Zinc is the second most abundant trace element essential for all living organisms. In human body, 30–40% of the total zinc ion (Zn²⁺) is localized in the nucleus. Intranuclear free Zn²⁺ sparks caused by reactive oxygen species have been observed in eukaryotic cells, but question if these free Zn²⁺ outrages could have affected annealing of complementary single-stranded (ss) DNA, a crucial step in DNA synthesis, repair and recombination, has never been raised. Here the author reports that Zn²⁺ blocks annealing of complementary ssDNA in a sequence-selective manner under near-physiological conditions as demonstrated in vitro using a low-temperature EDTA-free agarose gel electrophoresis (LTEAGE) procedure. Specifically, it is shown that Zn²⁺ does not block annealing of repetitive DNA sequences lacking CG/GC sites that are the major components of junk DNA. It is also demonstrated that Zn²⁺ blocks end-joining of double-stranded (ds) DNA fragments with 3′ overhangs mimicking double-strand breaks, and prevents renaturation of long stretches (>1 kb) of denatured dsDNA, in which Zn²⁺-tolerant intronic DNA provides annealing protection on otherwise Zn²⁺-sensitive coding DNA. These findings raise a challenging hypothesis that Zn²⁺-ssDNA interaction might be among natural forces driving eukaryotic genomes to maintain the Zn²⁺-tolerant repetitive DNA for adapting to the Zn²⁺-rich nucleus.

Endogenous voltage gradients as mediators of cell-cell communication: strategies for investigating bioelectrical signals during pattern formation

Alongside the well-known chemical modes of cell-cell communication, we find an important and powerful system of bioelectrical signaling: changes in the resting voltage potential (Vmem) of the plasma membrane driven by ion channels, pumps and gap junctions. Slow Vmem changes in all cells serve as a highly conserved, information-bearing pathway that regulates cell proliferation, migration and differentiation. In embryonic and regenerative pattern formation and in the disorganization of neoplasia, bioelectrical cues serve as mediators of large-scale anatomical polarity, organ identity and positional information. Recent developments have resulted in tools that enable a high-resolution analysis of these biophysical signals and their linkage with upstream and downstream canonical genetic pathways. Here, we provide an overview for the study of bioelectric signaling, focusing on state-of-the-art approaches that use molecular physiology and developmental genetics to probe the roles of bioelectric events functionally. We highlight the logic, strategies and well-developed technologies that any group of researchers can employ to identify and dissect ionic signaling components in their own work and thus to help crack the bioelectric code. The dissection of bioelectric events as instructive signals enabling the orchestration of cell behaviors into large-scale coherent patterning programs will enrich on-going work in diverse areas of biology, as biophysical factors become incorporated into our systems-level understanding of cell interactions.

Regulation of cell behavior and tissue patterning by bioelectrical signals: challenges and opportunities for biomedical engineering

Achieving control over cell behavior and pattern formation requires molecular-level understanding of regulatory mechanisms. Alongside transcriptional networks and biochemical gradients, there functions an important system of cellular communication and control: transmembrane voltage gradients (V(mem)). Bioelectrical signals encoded in spatiotemporal changes of V(mem) control cell proliferation, migration, and differentiation. Moreover, endogenous bioelectrical gradients serve as instructive cues mediating anatomical polarity and other organ-level aspects of morphogenesis. In the past decade, significant advances in molecular physiology have enabled the development of new genetic and biophysical tools for the investigation and functional manipulation of bioelectric cues. Recent data implicate V(mem) as a crucial epigenetic regulator of patterning events in embryogenesis, regeneration, and cancer. We review new conceptual and methodological developments in this fascinating field. Bioelectricity offers a novel way of quantitatively understanding regulation of growth and form in vivo, and it reveals tractable, powerful control points that will enable truly transformative applications in bioengineering, regenerative medicine, and synthetic biology.

A marked animal-vegetal polarity in the localization of Na(+),K(+) -ATPase activity and its down-regulation following progesterone-induced maturation

The stage-VI Xenopus oocyte has a very distinct animal-vegetal polarity with structural and functional asymmetry. In this study, we show the expression and distribution pattern of Na(+),K(+) -ATPase in stage-VI oocytes, and its changes following progesterone-induced maturation. Using enzyme-specific electron microscopy phosphatase histochemistry, [(3) H]-ouabain autoradiography, and immunofluorescence cytochemistry at light microscopic level, we find that Na(+),K(+) -ATPase activity is mainly confined to the animal hemisphere. Electron microscopy histochemical results also suggest that polarized distribution of Na(+),K(+) -ATPase activity persists following progesterone-induced maturation, and it becomes gradually more polarized towards the animal pole. The time course following progesterone-induced maturation suggests that there is an initial up-regulation and then gradual down-regulation of Na(+),K(+) -ATPase activity leading to germinal vesicle breakdown (GVBD). By GVBD, the Na(+),K(+) -ATPase activity is completely down-regulated due to endocytotic removal of pump molecules from the plasma membrane into the sub-cortical region of the oocyte. This study provides the first direct evidence for a marked asymmetric localization of Na(+),K(+) -ATPase activity in any vertebrate oocyte. Here, we propose that such asymmetry in Na(+),K(+) -ATPase activity in stage-VI oocytes, and their down-regulation following progesterone-induced maturation, is likely to have a role in the active state of the germinal vesicle in stage-VI oocytes and chromosomal condensation after GVBD.

DNA binding properties of the ecdysteroid receptor in the salivary gland of the female ixodid tick, Amblyomma hebraeum

Salivary gland degeneration in the female tick, Amblyomma hebraeum Koch (Acari: Ixodidae) is controlled by an ecdysteroid hormone. In an earlier study (Mao, H., McBlain, W.A., Kaufman, W.R., 1995. Some properties of the ecdysteroid receptor in the salivary gland of the ixodid tick, Amblyomma hebraeum. Gen. Comp. Endocrinol. 99, 340-348), we demonstrated that a protein component of a salivary gland extract binds to ponasterone A (Pon A) with high affinity (Kd-1 nM), suggesting a tick ecdysteroid receptor (EcR). In this study, the Pon A binding protein bound to calf thymus DNA; this binding could be dissociated by Drosophila hsp27 EcRE. The binding protein shifted the [32P]hsp27 EcRE band on a gel mobility shift assay; formation of the complex with hsp27 EcRE required KCl (optimal concentration was approximately 75 mM). A number of physiologically effective ecdysteroids enhanced the binding with the following order of potency: Pon A > Mur A > Mak A > 20E > ecdysone, whereas vertebrate steroids (estradiol, cholesterol, corticosterone, progesterone, testosterone) had no such effect. Using monoclonal antibodies against Drosophila EcR and USP, we found that AG 10.2 recognized three bands (90.5, 87.3 and 84 kDa for EcR) and AB11 recognized at least two major bands (50.3 and 47.1 kDa for USP) in the salivary gland extract by western blot analysis. In addition, AB11 supershifted the tick EcR-hsp27 EcRE band on a gel mobility shift assay, indicating that the tick EcR heterodimerized with a USP-like protein for DNA binding. Furthermore, selective mutations to the 15-basepair palindrome of hsp27 EcRE at positions-5, + 2, or adding a base to the spacer, resulted in considerably reduced affinity to the tick EcR/USP. We thus propose a sequence similarity of EcREs between A. hebraeum and its insect counterpart.

Salt concentration determines 1,25-dihydroxyvitamin D3 dependency of vitamin D receptor-retinoid X receptor--vitamin D-responsive element complex formation

The electrophoretic mobility shift assay was used to determine in vitro formation of the vitamin D receptor-retinoid X receptor beta (VDR-RXR beta) heterodimer complex on vitamin D-response elements (VDREs) from rat osteocalcin, mouse osteopontin, rat 25-hydroxyvitamin D3 24-hydroxylase, and human parathyroid hormone (PTH) genes. Baculovirus-expressed rat VDR was used as VDR and the binding reactions were performed at salt concentrations ranging from 50 to 170 mM KCI. Without ligand, optimum complex formation was observed at 50 mM KCI and markedly decreased with increasing KCI for all VDREs. In the presence of 1,25-dihydroxyvitamin D3, optimum complex formation occurred between 110 and 130 mM KCI for positive (enhancer) VDREs. At low salt concentrations (50-70 mM KCI), 1,25-dihydroxyvitamin D3 did not increase complex formation and actually caused a slight decrease. However, above 90 mM KCI, 1,25-dihydroxy-vitamin D3 markedly increased complex formation and at 150-170 mM KCI, a concentration that presumably mimics physiologic nuclear levels, 1,25-dihydroxy-vitamin D3 appeared to be required for complex formation. With the suppressive cis-acting sequence, i.e., PTH-VDRE, optimum detection of VDR complexes in the presence of 1,25-dihydroxyvitamin D3 occurred at a lower salt concentration (90-110 mM KCI). Moreover, no specific complexes were formed at high salt concentrations, even when 1,25-dihydroxyvitamin D3 was added. Thus, when analyzing an effect of ligand on VDR-RXR-VDRE complex formation, it is essential that the reaction be carried out with a range of salt concentrations. Further, 1,25-dihydroxyvitamin D3 appears to be required for formation of the VDR-RXR beta-VDRE complex at salt concentrations approaching physiological.

The content of glutathione and glutathione S-transferases and the glutathione peroxidase activity in rat liver nuclei determined by a non-aqueous technique of cell fractionation

Hepatocellular nuclei require glutathione, glutathione S-transferases (GSTs) and Se-dependent glutathione peroxidase (GPx) for intranuclear protection against damage from electrophiles or products of active oxygen. Data so far available from the literature on nuclei isolated in aqueous systems range from glutathione, GSTs and GPx either being absent altogether to being present in quantities in excess of those in the cytoplasm. This paper describes a small-scale preparation of a nuclear fraction from rat liver by a non-aqueous technique, designed to retain nuclear water-soluble molecules in situ, since low-molecular-mass compounds can diffuse freely into other compartments during aqueous separation. This non-aqueous procedure shows the nucleus to contain glutathione at 8.4 mM and soluble GSTs at 38 micrograms/mg of protein, the enrichment over the homogenate being 1.2-1.4-fold. Se-dependent GPx activity was also present in the nucleus (56 m-units/mg), although with slightly lower activity than in the homogenate (0.7-fold).

The pivotal role of the plasma membrane-cytoskeletal complex and of epithelium formation in differentiation in animals

1. If a few exceptions are disregarded, the several somatic cell types of a differentiated organism all have an identical genome. They all differ in their plasma membrane-cytoskeletal complex. 2. Differences in plasma membrane properties usually result in differences in ionic concentrations/activities in the cytoplasm and nucleoplasm. A basic question therefore is whether there exists a causal relationship between the ionic environment of the nucleus and differential gene expression/protein synthesis. 3. Development is switched on by a "Ca2+ explosion", often accompanied by pH changes and plasma membrane depolarisation. The penetration of the spermatozoon in the plasma membrane acts as a trigger. 4. All animal species develop from a blastula. At this stage they organise themselves as an epithelium enclosing an inner (fluid) compartment. This suggests that epithelium formation is absolutely essential in animal development. 5. As development proceeds, more and more compartments, lined by different epithelia, are formed. Differentiated organisms largely consist of folded epithelia. Some cells leave their original epithelial environment and become free floating (e.g. blood cells) or engage in other types of organisation. 6. Epithelial cells have the ability to segregate some membrane proteins, e.g. receptors, ion pumps, ion channels etc., so as to make selective transcellular transport possible. The cytoskeleton plays an important role in this segregation and in the interconnection of epithelial cells. 7. Transembryonic electric currents which have been measured by the vibrating probe technique, are due to electrogenic ion transport by epithelia. 8. Segregation of membrane proteins is not an exclusive property of epithelial cells but it is probably a property of all animal cell types, single cells inclusive; asymmetry is the rule, symmetry--if it exists at all--the exception. 9. Differences in several plasma membrane proteins (receptors, ion transporting molecules, cell adhesion molecules and signal transducing systems) are not only causally related to differential transcellular transport but also indirectly to differential protein synthesis and hence to differentiation. There are already a few well documented examples of "electrical" control of gene expression. 10. The major "strategy" which applies in differentiation seems to be to keep the genome constant but to change over and over its ionic and macromolecular environment, both acting in a complementary way. The first one may be considered as the coarse tuning mechanism of gene expression-protein synthesis, the second as the fine one. In our opinion this might be a principle universal to differentiation processes in all animal species.

Characterization of the cytoplasm of Escherichia coli K-12 as a function of external osmolarity. Implications for protein-DNA interactions in vivo

The water-accessible volumes, the amounts of all significant osmolytes, and the protein concentration in the cytoplasm of aerobically grown Escherichia coli K-12 have been determined as a function of the osmolarity of the minimal growth medium. The volume of cytoplasmic water (Vcyto) decreases linearly with increasing osmolarity from 2.23(+/- 0.12) microliters/mg dry weight in cells grown at 0.10 OSM to 1.18(+/- 0.06) microliters/mg dry weight at 1.02 OSM. Above 0.28 OSM, growth rate decreases linearly with increasing osmolarity. The growth rate extrapolates to zero at an osmolarity of approximately 1.8, corresponding to an estimated Vcyto of 0.5(+/- 0.2) microliters/mg dry weight. Measurements of Vcyto in titrations of non-growing cells with the plasmolyzing agent NaCl were used to obtain volumes of "bound" water (presumably water of macromolecular hydration) and cytoplasmic osmotic coefficients for cells grown in medium of low (0.10 OSM) and moderate (0.28 OSM) osmolarity. The volume of bound water Vb is similar in the two osmotic conditions (Vb = 0.40(+/- 0.04) microliters/mg dry wt), and corresponds to approximately 0.5 g H2O/g cytoplasmic macromolecule. Since Vcyto decreases with increasing osmolarity, whereas Vb appears to be independent of osmolarity, water of hydration becomes a larger fraction of Vcyto as the osmolarity of the growth medium increases. Growth appears to cease at the osmolarity where Vcyto is approximately equal to Vb. K+ and glutamate (Glu-) are the only significant cytoplasmic osmolytes in cells grown in medium of low osmolarity. The amount of K+ greatly exceeds that of Glu-. Analysis of cytoplasmic electroneutrality indicates that the cytoplasm behaves like a concentrated solution of the K+ salt of cytoplasmic polyanions, in which the amount of additional electrolyte (K+ Glu-) increases with increasing osmolarity. As the osmolarity of the growth medium becomes very low, the cytoplasm approaches an electrolyte-free K+-polyanion solution. In vivo osmotic coefficients were determined from the variation of Vcyto with external osmolarity in plasmolysis titrations of non-growing cells. The values obtained (phi = 0.54(+/- 0.06) for cells grown at 0.10 OSM and phi = 0.71(+/- 0.11) at 0.28 OSM) indicate a high degree of non-ideality of intracellular ions arising from coulombic interactions between K+ and cytoplasmic polyanions. Analysis of these osmotic coefficients using polyelectrolyte theory indicates that the thermodynamic activity of cytoplasmic K+ increases from approximately 0.14 M in cells grown at an external osmolarity of 0.10 OSM to approximately 0.76 M at 1.02 OSM.(ABSTRACT TRUNCATED AT 400 WORDS)

Detection of a large cation-selective channel in nuclear envelopes of avian erythrocytes

To determine whether the nuclear envelope of eukaryotic cells has the capability to regulate ion fluxes, we have used the patch-clamp technique to detect ion channels in this membrane system. Since possible sites for ion channels in the nuclear envelope include not only the nuclear pores, but also both the inner and outer nuclear membranes, we have patched giant liposomes composed of phosphatidylcholine and nuclear envelope fragments isolated from mature avian erythrocytes. A large, cation-selective channel with a maximum conductance of approximately 800 pS in symmetrical 100 mM KCl was detected. This channel is a possible candidate for a nuclear pore.

Preparation and characterization of nuclear-envelope vesicles from rat liver nuclei

We describe a procedure for the preparation of sealed nuclear-envelope vesicles from rat liver nuclei. These vesicles are strikingly similar in their polypeptide composition when compared with those of nuclear envelopes prepared conventionally using deoxyribonuclease I. Subfractionation analysis by means of extraction with high salt and urea show that the components of the nuclear envelope, e.g. the pore-complex/lamina fraction, are present. The residual DNA content is only 1.5%, and typical preparations consist of about 80% vesicles, with the vesicular character of these envelopes shown by microscopic and biochemical studies. The vesicles can be obtained in high yield, are tight and stable for at least two days and are enriched in a nucleoside triphosphatase thought to be involved in nucleocytoplasmic transport processes. Because the vesicles are largely free of components of the nuclear interior, but retain properties of intact nuclei, we believe that they are a valuable model system to study nucleocytoplasmic transport. Although in transport studies with isolated nuclei interference from intranuclear events has to be considered, the nuclear-envelope vesicles provide the possibility of studying translocation alone. Furthermore, the less complex nature of these vesicles compared with whole nuclei should facilitate investigation of the components involved in the regulation of nuclear transport processes.

Calcium gradients in single smooth muscle cells revealed by the digital imaging microscope using Fura-2

Calcium is believed to control a variety of cellular processes, often with a high degree of spatial and temporal precision. For a cell to use Ca2+ in this manner, mechanisms must exist for controlling the ion in a localized fashion. We have now gained insight into such mechanisms from studies which measured Ca2+ in single living cells with high resolution using a digital imaging microscope and the highly fluorescent Ca2+-sensitive dye, Fura-2. Levels of Ca2+ in the cytoplasm, nucleus and sarcoplasmic reticulum (SR) are clearly different. Free [Ca2+] in the nucleus and SR was greater than in the cytoplasm and these gradients were abolished by Ca2+ ionophores. When external Ca2+ was raised above normal in the absence of ionophores, free cytoplasmic Ca2+ increased but nuclear Ca2+ did not. Thus, nuclear [Ca2+] appears to be regulated independently of cytoplasmic [Ca2+] by gating mechanisms in the nuclear envelope. The observed regulation of intranuclear Ca2+ in these contractile cells may thus be seen as a way to prevent fluctuation in Ca2+-linked nuclear processes during the rise in cytoplasmic [Ca2+] which triggers contraction. The approach described here offers the opportunity of following changes in Ca2+ in cellular compartments in response to a wide range of stimuli, allowing new insights into the role of local changes in Ca2+ in the regulation of cell function.

Mini-review. On the possible importance of an intracellular circulation

Since ultrastructural, biophysical and other studies continue to demonstrate that the internum of the cell is highly structured, the question raised and discussed in this review is whether an intracellular circulatory system is essential for the maintenance of active metabolism. Although cytoplasmic streaming is evident in large animal and plant cells, it is argued that it probably occurs in all cells irrespective of size, and is of particular importance in bringing together interacting molecules fast enough for metabolic processes to occur which would otherwise be far too slow if diffusion were the only form of motion. A common intracellular system would suffice for most metabolic processes and would also help to dissipate waste products. Interruption of this internal circulation would result in the inhibition of metabolic functioning, including for example protein turnover, for which evidence is presented to substantiate this hypothesis.

Changes in membrane hydrogen and sodium conductances during progesterone-induced maturation of Ambystoma oocytes

A voltage-gated hydrogen ion-selective conductance has been previously described in the immature oocyte of the urodele amphibian Ambystoma. The present study was prompted by reports that changes in membrane voltage and internal pH, as well as in internal sodium ion concentration, occur during the hormone-induced maturation of oocytes from other amphibians. As activation of membrane currents might mediate changes in internal ion concentrations in addition to altering the membrane voltage, microelectrode recording techniques have been employed to examine changes in membrane conductances which occur during maturation of Ambystoma oocytes. It was observed that during the first 5 hr of maturation the magnitude of the hydrogen ion conductance gradually decreased, and that subsequently there was an increase in the amplitude of a voltage-dependent noninactivating sodium conductance. After 6 to 7 hr, after the loss of the hydrogen conductance and at about the time of germinal vesicle breakdown, the resting potential of the oocyte spontaneously shifted from approximately -10 mV to approximately +30 mV, where it remained until at least 24 hr after the initiation of maturation. This voltage transition was due to the appearance of mechanisms generating inward current in the oocyte membrane; part of this inward current was due to the tonic activation of the sodium conductance. Changes in internal pH and internal sodium ion concentration occurred during maturation, as judged from shifts in the reversal potentials of both hydrogen and sodium currents. A gradual decrease in internal hydrogen ion concentration was observed up until the time of disappearance of the hydrogen conductance (change in internal pH from about 7.15 in immature oocytes to about 7.40 by 3 hr after application of progesterone). This was followed, as sodium conductance increased, by an apparent rise in the internal sodium ion concentration (from about 6 mM to about 17 mM by 10 hr postprogesterone).

Binding of benzo[a]pyrene to different chromatin domains following activation at the nuclear membrane

When isolated liver nuclei from methylcholanthrene-treated rats are incubated with benzopyrene, covalent adducts are formed between DNA and the ultimate carcinogen, benzopyrene diol epoxide. Brief digestion with DNaseI, or micrococcal nuclease has been used to demonstrate that benzopyrene metabolites bind more readily to DNA in chromatin regions with a more open, active conformation than to inactive chromatin.

Solvent properties of ground substance studied by cryomicrodissection and intracellular reference-phase techniques

Water, sodium, potassium, ATP, amino acids, and sugars are not uniformly distributed in Rana pipiens oocytes. Concentration differences exist between nucleus (germinal vesicle) and ooplasm and between animal and vegetal ooplasmic regions. The mechanisms responsible for these differences were investigated using intracellular reference-phase (iRP) analysis. The iRP is an artificial "organelle" that has the solvent properties of a dilute salt solution and is in diffusional equilibrium with water and solutes present in other cellular compartments. Ooplasm/iRP solute distributions show that ooplasm differs from ordinary aqueous solutions--exhibiting both solute exclusion and solute binding. Yolk platelets are an important cause of this behavior, largely because their proteins are present as hydrate crystals, which are rich in anionic sites and which interact intensely with associated water. Because of yolk's abundance, it obscures the solvent and binding properties of ooplasmic ground substance. The oocyte nucleus is yolk and organelle free and the nuclear envelope is readily permeable. Consequently, nucleus/iRP solute concentration differences reflect the binding and solvent properties of nuclear ground substance. Nucleoplasm binds approximately 19 meq of potassium. Furthermore, the monosaccharides, 3-O-methylglucose, L-glucose, and D-xylose, are selectively excluded, their nucleus/iRP concentration ratios averaging about 0.7; ratios for other solutes studied are unity. We interpret monosaccharide exclusion to mean that nuclear ground substance water is different in its "instantaneous" structure from ordinary saline water. Because of this difference, hydrogen bond interaction between nuclear water and certain sterically restricted solutes, of which ringed monosaccharides are examples, is reduced. Some implications of modified ground substance water and selective solute exclusion are discussed.

Water, potassium, and sodium during amphibian oocyte development

Water, K+, and Na+ were measured in Rana pipiens oocytes during growth and prematurational development using low-temperature microdissection. Whole oocytes were analyzed during previtellogenic and vitellogenic growth. Ooplasm and germinal vesicle (nucleus) were analyzed at the onset and conclusion of vitellogenic growth. In previtellogenic oocytes (less than 40 micrograms), water, K+, and Na+ concentrations resembled those in somatic cells and were independent of cell size. With the onset of yolk deposition, water and K+ concentrations progressively decreased and Na+ progressively increased. These changes were restricted to ooplasm, the site of yolk deposition. In full-grown oocytes, vegetal ooplasm, with greater yolk density than animal ooplasm, contained less water and K+ and more Na+ than animal ooplasm. Collectively, the data indicate that yolk is poorer in water and K+ and richer in Na+ than yolk-free ooplasm (cytoplasm) or nucleoplasm. Yolk concentrations were estimated to be approximately 32%, water, approximately 69 meq K+/liter H2O, and approximately 94 meq Na+/liter H2O. Several nonyolk parameters, such as cation activities and nucleoplasmic binding, also appear to change during oogenesis.

Artifacts caused by cell microinjection

The effects of microinjection on Rana pipiens oocytes were determined using cryomicrodissection to measure Na, K, water, and injected radiolabeled sucrose (in gelatin) in the nucleus, animal, and vegetal ooplasm and injected bolus (reference phase, RP). The results point to potential problems in the interpretation of microinjection experiments. When oocytes were injected and incubated in Ringer's solution, nucleus, ooplasm, and RP lost K and sucrose and gained Na. Patterns of loss and gain were complex but were consistent with continuous solute leakage at the injection site causing artifactual intracellular diffusion gradients. In spite of leakage, oocytes completed scheduled meiotic maturation when exposed to progesterone. When oocytes were microinjected and incubated in paraffin oil (a medium in which polar solutes cannot exchange), nuclear and ooplasmic Na, K, and water concentrations remained identical to those in uninjected cells. Neither microinjection per se nor the injected bolus affected intraoocytic solute distributions. These findings imply that, after microinjection in aqueous media, metabolites are lost from and redistribute in cells, and that these artifactual changes are inadequately reflected in the ability of the cell to carry out a complex process. They also show that injection artifacts can be avoided by injecting and incubating cells under paraffin oil.

Cellular and molecular consequences of reduced cell water content

Cell volumes, viability, ultrastructure, and metabolism have been studied in mouse L cells at reduced water contents. Dehydration was achieved by addition of sorbitol to the incubation medium at concentrations of 0.15, 0.3, and 0.6 m. In 0.3 m sorbitol, cell volume was reduced to about 35% of the initial control level and cell ultrastructure was markedly altered. Nevertheless, such cells retained viability and their metabolism was much the same as that of control cells, evaluated by [U-14C]glucose. Except for a lesser volume reduction, cells in 0.15 m sorbitol behaved similarly. However, when placed in 0.6 m sorbitol the cells exhibited quantitative and qualitative differences in their metabolism compared with controls, and grew very poorly or not at all. Their ultrastructure was not obviously different from cells in 0.3 m sorbitol although cell volume was slightly reduced. These results are considered in the context of different conceptions of the properties and metabolic activities of the aqueous intracellular compartments (cytosol, nucleoplasm, and interiors of cytoplasmic membrane-bound-organelles). We interpret the data as additional evidence for the existence of extensive spatial organization of enzyme and the pathways they catalyze in the aqueous compartments. On this basis it is also suggested that at least part of the damage that occurs in severely dehydrated mammalian cells could result from the irreversible disruption of this organization and loss of metabolic control. We speculate about the evolutionary involvement of intracellular water and how it might have participated in the development of structure and function in contemporary cells.

The distribution of sodium and potassium in toad (Bufo bufo) oocytes estimated by a stereological method

As the toad oocyte grows, the intracellular Na concentration rises and the K concentration falls. By stereological analysis using a point counting method in a projection apparatus, it is shown that as the oocyte grows the volume density of the nucleus, endoplasmic reticulum and mitochondria falls, while that of the yolk platelets and cytoplasmic vesicles rises. A multiple regression analysis applied to the sodium and potassium concentrations and to the volume densities of the subcellular components indicates that sodium is most concentrated in cytoplasmic vesicles while potassium is most concentrated in a component composed of the cell cytoplasmic ground substance, the mitochondria and the cell nucleus.

Fractionation of chromatin, released by nuclease digestion, on ECTHAM-cellulose. Separation of active and inactive chromatin

Chromatin released by two nucleases under various ionic conditions has been fractionated by chromatography on ECTHAM-cellulose. Mg2+ -soluble chromatin, which according to Gottesfeld and Partington is enriched in transcribed DNA sequences (Gottesfeld, J.M. and Partington, G.A., (1977) Cell 12, 953-962) and produced by DNAase II digestion at intermediate ionic strength, comprises material eluting from ECTHAM-cellulose at 80-100 mM Cl-, pH 6.8-7.0, whereas bulk, Mg2+ -insoluble chromatin comprises more tightly binding material. Free hnRNP particles elute at 30 mM Cl-, pH 6.8. Oligonucleosomes, which according to Dimitriadis and Tata are enriched in transcribed sequences (Dimitriadis, G.J. and Tata, J.R. (1980) Biochem. J. 187, 467-477) and produced by micrococcal nuclease digestion at physiological ionic strength, also elute predominantly at 80-100 mM Cl-, pH 6.8-7.0. When liver nuclei are digested with micrococcal nuclease at low ionic strength, the most rapidly released chromatin is enriched in nascent RNA and hnRNP particles, and binds weakly to ECTHAM-cellulose. More slowly solubilised chromatin, containing fewer hnRNP particles, binds much more strongly to ECTHAM-cellulose. In confirmation of results with mechanically sheared chromatin, the affinity of particular chromatin fractions is not dependent on the size of chromatin particles, rather it reflects the differing composition, and in particular the non-histone protein and hnRNP content, which, we propose, determines the conformation adopted by different chromatin fractions in the cation conditions used for elution from ECTHAM-cellulose.

Structure of the active nucleolar chromatin of Xenopus laevis Oocytes

Active nucleolar chromatin of Xenopus laevis oocytes was prepared for electron microscopy by a step gradient method, which separates the chromatin from proteins and other constituents that might nonspecifically bind at low ionic strength. Between putative RNA polymerases and within the nontranscribed spacer region, the chromatin appears as smooth, thin filaments. For the first time, it is shown here that these filaments are indistinguishable from pure DNA absorbed to the same specimen, even when the ionic strength is raised up to 100 mM NaCl. Bulk rat liver chromatin, however, which was coprepared as a biochemically well-characterized standard with the active nucleolar chromatin, shows nucleosomes containing fibers, which condense into supranucleosomal structures with increasing ionic strength. Since the appearance and the behavior of active nucleolar chromatin at different ionic strengths and pHs resembles tht of pure DNA, but not of any known type of chromatin, it is suggested that, except for the transcription apparatus, very few macromolecular constituents are associated with ribosomal DNA during transcription. The observations described in this paper explain most of the published and partly conflicting results obtained by electron microscopy of nucleolar chromatin.

The x-ray microanalysis of frozen-hydrated sections in scanning electron microscopy: an evaluation

The present status of the technique to measure concentrations of electrolyte elements and dry mass in or approximately 1 mum thick frozen-hydrated sections of soft biological tissues with electron probe X-ray microanalysis in a scanning electron microscope is critically reviewed. The technique is to quench-freeze fresh specimens to less than -180 degrees C, cut approximately 1 mum thick hydrated cryosections ( less than or equal to -70 degrees C), transfer on to a cold stage (less than -170 degrees C) of a suitable microanalytical arrangement, obtain scanning transmission images to identify the cell and tissue compartments, locate an electron probe (several mum2 to 100 nm) on the areas of interest and collect X-ray quanta. The X-ray quanta collected with suitable spectrometers (WDS and EDS) and processed with a computer using a comprehensive programme based on continuum normalization procedures ('Hall' programme). The cryosections are analysed first in a hydrated state and second after dehydration within the microanalyser column to obtain directly elemental concentrations in muM kg-1 wet wt and muM kg-1 dry wt of the compartments identified under the beam. The local water-fractions are estimated and the elemental concentrations converted into muM 1(-1) water. In the past 7 years the technique has been applied to obtain fully quantitative information on Na, K, Cl, P, S, Ca and H2O in more than ten types of tissue.