The electrical properties of the nuclear envelope, and their possible role in the regulation of eukaryotic gene expression

Electromechanical interactions between cell membrane and nuclear envelope: Beyond the standard Schwan's model of biological cells

We investigate little-appreciated features of the hierarchical core-shell (CS) models of the electrical, mechanical, and electromechanical interactions between the cell membrane (CM) and nuclear envelope (NE). We first consider a simple model of an individual cell based on a coupled resistor-capacitor (Schwan model (SM)) network and show that the CM, when exposed to ac electric fields, acts as a low pass filter while the NE acts as a wide and asymmetric bandpass filter. We provide a simplified calculation for characteristic time associated with the capacitive charging of the NE and parameterize its range of behavior. We furthermore observe several new features dealing with mechanical analogs of the SM based on elementary spring-damper combinations. The chief merit of these models is that they can predict creep compliance responses of an individual cell under static stress and their effective retardation time constants. Next, we use an alternative and a more accurate CS physical model solved by finite element simulations for which geometrical cell reshaping under electromechanical stress (electrodeformation (ED)) is included in a continuum approach with spatial resolution. We show that under an electric field excitation, the elongated nucleus scales differently compared to the electrodeformed cell.

Evidence That Ion-Based Signaling Initiating at the Cell Surface Can Potentially Influence Chromatin Dynamics and Chromatin-Bound Proteins in the Nucleus

We have developed tools and performed pilot experiments to test the hypothesis that an intracellular ion-based signaling pathway, provoked by an extracellular stimulus acting at the cell surface, can influence interphase chromosome dynamics and chromatin-bound proteins in the nucleus. The experimental system employs chromosome-specific fluorescent tags and the genome-encoded fluorescent pH sensor SEpHluorinA227D, which has been targeted to various intracellular membranes and soluble compartments in root cells of Arabidopsis thaliana. We are using this system and three-dimensional live cell imaging to visualize whether fluorescent-tagged interphase chromosome sites undergo changes in constrained motion concurrently with reductions in membrane-associated pH elicited by extracellular ATP, which is known to trigger a cascade of events in plant cells including changes in calcium ion concentrations, pH, and membrane potential. To examine possible effects of the proposed ion-based signaling pathway directly at the chromatin level, we generated a pH-sensitive fluorescent DNA-binding protein that allows pH changes to be monitored at specific genomic sites. Results obtained using these tools support the existence of a rapid, ion-based signaling pathway that initiates at the cell surface and reaches the nucleus to induce alterations in interphase chromatin mobility and the surrounding pH of chromatin-bound proteins. Such a pathway could conceivably act under natural circumstances to allow external stimuli to swiftly influence gene expression by affecting interphase chromosome movement and the structures and/or activities of chromatin-associated proteins.

Monitoring voltage fluctuations of intracellular membranes

In eukaryotic cells, the endoplasmic reticulum (ER) is the largest continuous membrane-enclosed network which surrounds a single lumen. Using a new genetically encoded voltage indicator (GEVI), we applied the patch clamp technique to cultured HEK293 cells and neurons and found that there is a very fast electrical interaction between the plasma membrane and internal membrane(s). This discovery suggests a novel mechanism for interaction between the external membrane and internal membranes as well as mechanisms for interactions between the various internal membranes. The ER may transfer electrical signals between the plasma membrane and other internal organelles. The internal membrane optical signal is reversed in polarity but has a time course similar to that of the plasma membrane signal. The optical signal of the GEVI in the plasma membrane is consistent from trial to trial. However, the internal signal decreases in size with repeated trials suggesting that the electrical coupling is degrading and/or the resistance of the internal membrane is decaying.

Membrane "potential-omics": toward voltage imaging at the cell population level in roots of living plants

Genetically encoded voltage-sensitive fluorescent proteins (VSFPs) are being used in neurobiology as non-invasive tools to study synchronous electrical activities in specific groups of nerve cells. Here we discuss our efforts to adapt this "light-based electrophysiology" for use in plant systems. We describe the production of transgenic plants engineered to express different versions of VSFPs that are targeted to the plasma membrane and internal membranes of root cells. The aim is to optically record concurrent changes in plasma membrane potential in populations of cells and at multiple membrane systems within single cells in response to various stimuli in living plants. Such coordinated electrical changes may globally orchestrate cell behavior to elicit successful reactions of the root as a whole to varying and unpredictable environments. Findings from membrane "potential-omics" can eventually be fused with data sets from other "omics" approaches to forge the integrated and comprehensive understanding that underpins the concept of systems biology.

Non-uniform velocity of homogeneous DNA in a uniform electric field: consequence of electric-field-induced slow dissociation of highly stable DNA-counterion complexes

Identical molecules move with identical velocities when placed in a uniform electric field within a uniform electrolyte. Here we report that homogeneous DNA does not obey this fundamental rule. While most DNA moves with similar velocities, a fraction of DNA moves with velocities that vary within a multiple-fold range. The size of this irregular fraction increases several orders of magnitude when exogenous counterions are added to DNA. The irregular fraction decreases several orders of magnitude when DNA counterions are removed by dialysis against deionized water in the presence of a strong electric field (0.6 kV/cm). Dialysis without the field is ineffective in decreasing the size of irregular fraction. These results suggest that (i) DNA can form very stable complexes with counterions, (ii) these complexes can be dissociated by an electric field, and (iii) the observed non-uniform velocity of DNA is caused by electric-field-induced slow dissociation of these stable complexes. Our findings help to better understand a fundamental property of DNA: its interaction with counterions. In addition, these findings suggest a practical way of making electromigration of DNA more uniform: removal of strongly bound DNA counterions by electro-dialysis against deionized water.

DNA electrophoretic migration patterns change after exposure of Jurkat cells to a single intense nanosecond electric pulse

Intense nanosecond pulsed electric fields (nsPEFs) interact with cellular membranes and intracellular structures. Investigating how cells respond to nanosecond pulses is essential for a) development of biomedical applications of nsPEFs, including cancer therapy, and b) better understanding of the mechanisms underlying such bioelectrical effects. In this work, we explored relatively mild exposure conditions to provide insight into weak, reversible effects, laying a foundation for a better understanding of the interaction mechanisms and kinetics underlying nsPEF bio-effects. In particular, we report changes in the nucleus of Jurkat cells (human lymphoblastoid T cells) exposed to single pulses of 60 ns duration and 1.0, 1.5 and 2.5 MV/m amplitudes, which do not affect cell growth and viability. A dose-dependent reduction in alkaline comet-assayed DNA migration is observed immediately after nsPEF exposure, accompanied by permeabilization of the plasma membrane (YO-PRO-1 uptake). Comet assay profiles return to normal within 60 minutes after pulse delivery at the highest pulse amplitude tested, indicating that our exposure protocol affects the nucleus, modifying DNA electrophoretic migration patterns.

Ion channels at the nucleus: electrophysiology meets the genome

The nuclear envelope is increasingly viewed from an electrophysiological perspective by researchers interested in signal transduction pathways that influence gene transcription and other processes in the nucleus. Here, we describe evidence for ion channels and transporters in the nuclear membranes and for possible ion gating by the nuclear pores. We argue that a systems-level understanding of cellular regulation is likely to require the assimilation of nuclear electrophysiology into molecular and biochemical signaling pathways.

II. Model building: an electrical theory of control of growth and development in animals, prompted by studies of exogenous magnetic field effects (paper I), and evidence of DNA current conduction, in vitro

A theory of control of cellular proliferation and differentiation in the early development of metazoan systems, postulating a system of electrical controls "parallel" to the processes of molecular biochemistry, is presented. It is argued that the processes of molecular biochemistry alone cannot explain how a developing organism defies a stochastic universe. The demonstration of current flow (charge transfer) along the long axis of DNA through the base-pairs (the "pi-way) in vitro raises the question of whether nature may employ such current flows for biological purposes. Such currents might be too small to be accessible to direct measurement in vivo but conduction has been measured in vitro, and the methods might well be extended to living systems. This has not been done because there is no reasonable model which could stimulate experimentation. We suggest several related, but detachable or independent, models for the biological utility of charge transfer, whose scope admittedly outruns current concepts of thinking about organization, growth, and development in eukaryotic, metazoan systems. The ideas are related to explanations proposed to explain the effects demonstrated on tumors and normal tissues described in Article I (this issue). Microscopic and mesoscopic potential fields and currents are well known at sub-cellular, cellular, and organ systems levels. Not only are such phenomena associated with internal cellular membranes in bioenergetics and information flow, but remarkable long-range fields over tissue interfaces and organs appear to play a role in embryonic development (Nuccitelli, 1992 ). The origin of the fields remains unclear and is the subject of active investigation. We are proposing that similar processes could play a vital role at a "sub-microscopic level," at the level of the chromosomes themselves, and could play a role in organizing and directing fundamental processes of growth and development, in parallel with the more discernible fields and currents described.

Nuclear membrane ion channels mediate root nodule development

Previous work has implicated two predicted ion channels in mediating perinuclear calcium spiking, which is essential for rhizobia-induced root nodule formation in legumes. A new study demonstrates that these ion channels are preferentially permeable to cations, such as potassium, and are located in the nuclear envelope. Here, we consider ways in which the ion channels influence perinuclear calcium spiking and discuss a potentially broader role for nuclear membrane ion channels in signal transduction in plants.

Developmental control in animals and a biological role for DNA charge transfer

A model is developed based on data from diverse lines of inquiry, describing the possible role of charge transfer in the timing and coordination of DNA replication and gene expression in eukaryotic cells. Emphasis is placed on a possible electrical process which participates in the simultaneous base-pair opening in many regions of DNA preceding replication, and a similar coordination of gene expression by simultaneous generation of base-pair openings around promoter and transcriptional start sites. This process could have the character of an ultradian biological clock. There is no specific evidence for such a process, but the argument is presented that such a process is neither unphysical nor unbiological, and its postulation would act as a first-iteration road map for explaining the coordinated nature of eukaryotic organism development.

Electrorelease of Escherichia coli nucleoids

Bacterial chromosome is assembled and folded into one or several nucleoids, depending on the metabolic status of the cell. Development of reliable nucleoid isolation protocols has always been an objective for researchers. A rapid and reproducible procedure for isolation of E. coli nucleoids is described here, while the cell envelope is maintained. Membrane dispersions and vesicles were prepared by lysozyme-EDTA treatment with subsequent rupture of the spheroplasts by electric field. Under these conditions the yield of electroreleased nucleoids was around 90%. The extent of DNA-envelope contacts was determined by light microscopy employing phase contrast and fluorescence modes.

Electrical dimension of the nuclear envelope

Eukaryotic chromosomes are confined to the nucleus, which is separated from the rest of the cell by two concentric membranes known as the nuclear envelope (NE). The NE is punctuated by holes known as nuclear pore complexes (NPCs), which provide the main pathway for transport of cellular material across the nuclear-cytoplasmic boundary. The single NPC is a complicated octameric structure containing more than 100 proteins called nucleoporins. NPCs function as transport machineries for inorganic ions and macromolecules. The most prominent feature of an individual NPC is a large central channel, ~7 nm in width and 50 nm in length. NPCs exhibit high morphological and functional plasticity, adjusting shape to function. Macromolecules ranging from 1 to >100 kDa travel through the central channel into (and out of) the nucleoplasm. Inorganic ions have additional pathways for communication between cytosol and nucleus. NE can turn from a simple sieve that separates two compartments by a given pore size to a smart barrier that adjusts its permeabiltiy to the metabolic demands of the cell. Early microelectrode work characterizes the NE as a membrane barrier of highly variable permeability, indicating that NPCs are under regulatory control. Electrical voltage across the NE is explained as the result of electrical charge separation due to selective barrier permeability and unequal distribution of charged macromolecules across the NE. Patch-clamp work discovers NE ion channel activity associated with NPC function. From comparison of early microelectrode work with patch-clamp data and late results obtained by the nuclear hourglass technique, it is concluded that NPCs are well-controlled supramolecular structures that mediate transport of macromolecules and small ions by separate physical pathways, the large central channel and the small peripheral channels, respectively. Electrical properties of the two pathways are still unclear but could have great impact on the understanding of signal transfer across NE and gene expression.

Time domain dielectric spectroscopy study of human cells. II. Normal and malignant white blood cells

The dielectric properties of human lymphocyte suspensions were studied by time domain dielectric spectroscopy (TDDS). Nine populations of malignant and normal lymphocytes were investigated. Analysis of the dielectric parameters of cell structural parts were performed in the framework of Maxwell-Wagner mixture formula and the double-shell model of cell. The specific capacitance of the cell membranes was estimated by the Hanai-Asami-Koisumi formula. It was shown that the dielectric permittivity, capacitance and conductivity values of cell membranes are higher for normal lymphocytes than for the malignant ones. The difference of the same parameters for normal B- and T-cells is also discussed.

Aldosterone activates the nuclear pore transporter in cultured kidney cells imaged with atomic force microscopy

Nuclear pore complexes (NPC), located in the nuclear envelope, functionally connect the cell nucleus with the cytoplasm and serve as a crucial pathway for macromolecule exchange. A Madin-Darby canine kidney (MDCK) clone that resembles principal cells of the collecting duct was shown recently to respond to sustained aldosterone exposure with a significant increase in the NPC number per nucleus. The present study elucidates the molecular nature of the NPC pathway and its regulation by aldosterone applying atomic force microscopy. We imaged individual NPC in situ and searched for a putative so-called transporter in the NPC centre. In aldosterone-depleted cells we found numerous macromolecules docked to individual NPC waiting for translocation into the nucleoplasm (standby mode=inactive pore). In contrast, in aldosterone-treated cells NPC were frequently found free of macromolecules, indicating that the translocation process kept pace with docking under hormone-stimulated conditions (transport mode=active pore). In the NPC centre we detected a ring-like structure with a central invagination. We assume that the ring is the putative transporter and that the invagination is the channel entrance used for translocation of macromolecules. Transporters were found in open and closed configurations. In conclusion, the results provide evidence for the existence of a nuclear transporter as part of the translocation machinery of an individual NPC. Aldosterone increases the activity of the nuclear transporter and thus facilitates steroid-mediated gene expression.

Nuclear pore complex ion channels (review)

It is currently thought that nuclear pore complexes (NPCs) primarily govern nucleocytoplasmic interactions via selective recognition and active transport of macromolecules. However, in various nuclear preparations, patch-clamp and fluorescence, luminiscence and ion microscopy support classical microelectrode measurements indicating that monoatomic ion flow across the nuclear envelope (NE) is strictly regulated. Gating of large conductance nuclear envelope ion channels (NICs) somewhat resembles that of gap junctional channels. In other respects, NICs are distinct in that they require cytosolic factors, are blocked by wheat germ agglutinin and are blocked and/or modified by antibodies to epitopes of NPC glycoproteins. Therefore, NIC activity, recorded as electrical current/conductance is likely to be intrinsic to NPCs. This observation suggests a potential use for the patch-clamp technique in establishing the mechanisms underlying nuclear pore gating in response to cytosolic and nucleosolic factors such as transcription and growth factors, oncogene and proto-oncogene products and receptors for retinoids, steroids and thyroid hormone. NIC activity may also be useful in evaluating the mechanisms of nuclear import of foreign nucleic acid material such as that contained in virons and viroids. Finally, in consideration to the electrophysiological data accumulated so far, the study of nuclear pore ion channel activity may help our understanding of other important issues such as cell suicide, programmed cell death or apoptosis.

Photodynamic effects on the nuclear envelope of human skin fibroblasts

Photodynamic effects on the nuclear envelope of human skin fibroblasts were investigated by confocal laser fluorescence microscopy and transmembrane resting potential measurements. The results show staining of the nuclear envelope after short incubation times with Photosan III, Photofrin II and haematoporphyrin derivative (HPD) enriched with monomers. Maximum staining was found at the centre of the nuclear envelope. The sequence of fluorescence intensity was HPD enriched with monomers > Photofrin II > Photosan III. After lethal treatment with Photosan III and tetrasulphonated aluminium chloride phthalocyanine, the nuclear transmembrane potential of the nuclear membrane decreased from -20 mV to about -10 mV with reference to the plasma membrane potential.

Electrical-ionic control of gene expression

1. Changes in turgor, in cell volume, in membrane potential, in intracellular ionic activities and, more recently, in spontaneous electrical activity have been reported to be causally linked to the expression of specific genes. 2. As a result, it has become clear that changes in membrane properties and/or in the intracellular "ionic environment" can play an important role in generating cell type specific physiological responses which indirectly--or maybe directly--affect gene expression. 3. Possible targets of the ionic "environment" are: the selective transport across biological membranes; the activity of certain (regulatory) enzymes; the conformation of some (regulatory) proteins; of chromatin; of the cytoskeleton; of the nuclear matrix; the association of the cytoskeleton with plasmamembrane proteins or RNA; the association chromatin-nuclear matrix; protein-DNA and protein-protein interactions etc. All these sites may be instrumental to "fine or coarse" tuning of gene expression. 4. The exact mechanisms by which changes in intracellular ionic environment are transduced, directly or indirectly, into alterations of the activity of trans-acting factors have not yet been fully uncovered. Changes in the degree of phosphorylation of regulatory proteins and/or of trans-acting factors may provoke fine tuning effects on cell type specific gene expression activity. 5. The intranuclear ionic environment is difficult to measure in an exact way. It can be influenced in a number of ways. The location of a gene, as determined by the position of the nucleus in the cytoplasm and by the association of chromatin to the nuclear matrix may be especially important in cells which can generate some type of intracellular gradient or in excitable cells. 6. In some somatic cell types--germinal vesicles may behave differently--the intranuclear inorganic ionic "environment" has been reported to be distinct from the cytoplasmic one. This challenges the widespread assumption that the nuclear envelope is always freely permeable to small molecules and inorganic ions. 7. It can be expected that the fast progress in the cloning of "electrically" controlled genes, in the identification of trans-acting factors, in their mode of interaction with genes and in the precise localization of genes within the nucleus may soon lead to substantial progress in this domain.

A large conductance ion channel in the nuclear envelope of a higher plant cell

To detect and characterize ion channel activity in the nuclear envelope of a higher plant cell, we performed patch clamp experiments on nuclei isolated from coconut endosperm cells and on giant liposomes containing nuclear envelope fragments prepared from the same cells. An ion channel exhibiting a number of conductance substates, with a maximum of ca. 1,000 pS, was observed. Above an applied potential of +/- 100 mV, the behavior of the channel was similar in isolated nuclei and liposomes, indicating that both patch clamp modes were detecting the same channel. That such a channel has now been identified in members of both the animal and plant kingdoms reinforces the notion that the nuclear pores are not always open to ions.