Nuclear hourglass technique: an approach that detects electrically open nuclear pores in Xenopus laevis oocyte

Characterization of Single-Nucleus Electrical Properties by Microfluidic Constriction Channel

As key bioelectrical markers, equivalent capacitance (C_ne, i.e., capacitance per unit area) and resistance (R_ne, i.e., resistivity multiply thickness) of nuclear envelopes have emerged as promising electrical indicators, which cannot be effectively measured by conventional approaches. In this study, single nuclei were isolated from whole cells and trapped at the entrances of microfluidic constriction channels, and then corresponding impedance profiles were sampled and translated into single-nucleus C_ne and R_ne based on a home-developed equivalent electrical model. C_ne and R_ne of A549 nuclei were first quantified as 3.43 ± 1.81 μF/cm² and 2.03 ± 1.40 Ω·cm² (N_n = 35), which were shown not to be affected by variations of key parameters in nuclear isolation and measurement. The developed approach in this study was also used to measure a second type of nuclei, producing C_ne and R_ne of 3.75 ± 3.17 μF/cm² and 1.01 ± 0.70 Ω·cm² for SW620 (N_n = 17). This study may provide a new perspective in single-cell electrical characterization, enabling cell type classification and cell status evaluation based on bioelectrical markers of nuclei.

Nuclear pores enable sustained perinuclear calcium oscillations

Background

Calcium signalling relies on the flux of calcium ions across membranes yet how signals in different compartments are related remains unclear. In particular, similar calcium signals on both sides of the nuclear envelope have been reported and attributed to passive diffusion through nuclear pores. However, observed differing cytosolic and nucleosolic calcium signatures suggest that the signalling machinery in these compartments can act independently.

Results

We adapt the fire-diffuse-fire model to investigate the generation of perinuclear calcium oscillations. We demonstrate that autonomous spatio-temporal calcium patterns are still possible in the presence of nuclear and cytosolic coupling via nuclear pores. The presence or absence of this autonomy is dependent upon the strength of the coupling and the maximum firing rate of an individual calcium channel. In all cases, coupling through the nuclear pores enables robust signalling with respect to changes in the diffusion constant.

Conclusions

We show that contradictory interpretations of experimental data with respect to the autonomy of nuclear calcium oscillations can be reconciled within one model, with different observations being a consequence of varying nuclear pore permeabilities for calcium and refractory conditions of channels. Furthermore, our results provide an explanation for why calcium oscillations on both sides of the nuclear envelope may be beneficial for sustained perinuclear signaling.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-016-0289-9) contains supplementary material, which is available to authorized users.

Towards the Physics of Calcium Signalling in Plants

Calcium is an abundant element with a wide variety of important roles within cells. Calcium ions are inter- and intra-cellular messengers that are involved in numerous signalling pathways. Fluctuating compartment-specific calcium ion concentrations can lead to localised and even plant-wide oscillations that can regulate downstream events. Understanding the mechanisms that give rise to these complex patterns that vary both in space and time can be challenging, even in cases for which individual components have been identified. Taking a systems biology approach, mathematical and computational techniques can be employed to produce models that recapitulate experimental observations and capture our current understanding of the system. Useful models make novel predictions that can be investigated and falsified experimentally. This review brings together recent work on the modelling of calcium signalling in plants, from the scale of ion channels through to plant-wide responses to external stimuli. Some in silico results that have informed later experiments are highlighted.

The human nuclear pore complex as revealed by cryo-electron tomography

Nuclear pore complexes (NPCs) are the sole passage through the nuclear envelope, connecting the cytoplasm to the nucleoplasm. These gigantic molecular machines, over 100 MDa in molecular weight, allow free diffusion of small molecules and ions while mediating selective energy-dependent nucleocytoplasmic transport of large macromolecules. Here, we applied cryo-electron tomography to human fibroblast cells, reconstructing their nuclear envelopes without applying any purification steps. From these reconstructions, we extracted subtomograms containing individual NPCs and utilized in silico subtomogram averaging procedures to determine the structure of the mammalian pore complex at a resolution of ∼6.6 nm. Beyond revealing the canonical features of the human NPC, our analysis identified inner lateral channels and fusing bridge-like structures, suggesting alternative routes of peripheral nuclear passage. Finally, we concluded from our structural analysis that the human NPC is structurally distinct from that of lower eukaryotes in terms of dimension and organization but resembles its amphibian (frog) counterpart.

Single-molecule transport across an individual biomimetic nuclear pore complex

Nuclear pore complexes regulate the selective exchange of RNA and proteins across the nuclear envelope in eukaryotic cells. Biomimetic strategies offer new opportunities to investigate this remarkable transport phenomenon. Here, we show selective transport of proteins across individual biomimetic nuclear pore complexes at the single-molecule level. Each biomimetic complex is constructed by covalently tethering either Nup98 or Nup153 (phenylalanine-glycine (FG) nucleoporins) to a solid-state nanopore. Individual translocation events are monitored using ionic current measurements with sub-millisecond temporal resolution. Transport receptors (Impβ) proceed with a dwell time of ∼2.5 ms for both Nup98- and Nup153-coated pores, whereas the passage of non-specific proteins is strongly inhibited with different degrees of selectivity. For pores up to ∼25 nm in diameter, Nups form a dense and low-conducting barrier, whereas they adopt a more open structure in larger pores. Our biomimetic nuclear pore complex provides a quantitative platform for studying nucleocytoplasmic transport phenomena at the single-molecule level in vitro.

Permeating the nuclear pore complex

The extensive and multifaceted traffic between nucleus and cytoplasm is handled by a single type of macromolecular assembly called the nuclear pore complex (NPC). While being readily accessible to ions and metabolites, the NPC imposes stringent selectivity on the passage of proteins and RNA, tightly regulating their traffic between the two major cellular compartments. Here we discuss how shuttling carriers, which mediate the transport of macromolecules through NPCs, cross its permeability barrier. We also discuss the co-existence of receptor-mediated macromolecular transport with the passive diffusion of small molecules in the context of the various models suggested for the permeability barrier of the NPC. Finally, we speculate on how nuclear transport receptors negotiate the dependence of their NPC-permeating abilities on hydrophobic interactions with the necessity of avoiding these promiscuous interactions in the cytoplasm and nucleus.

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.

Using oocyte nuclei for studies on chromatin structure and gene expression

The giant nucleus of amphibian oocytes is generally referred to as the germinal vesicle (GV). Its size allows relatively easy manual isolation from the rest of the oocyte and also presents a large target in situ for microinjection of macromolecules including plasmid DNA, RNA species, antibodies and other proteins and even whole organelles, including somatic cell nuclei. Thus the use of GVs is excellent for two major types of study: the function of endogenous nuclear processes such as gene transcription, RNA processing and intra-nuclear dynamics; and the use of the nuclear components to effect processes such as chromatin assembly, expression of foreign genes and nucleocytoplasmic transport of injected biomolecules. This article outlines some basic techniques appropriate for GV studies, particularly the preparation of oocytes for microinjection and the isolation of germinal vesicles into an oil phase. As an aid to the targeting of the GV within the nucleus, descriptions are given of the use of oocytes from albino animals.

Towards reconciling structure and function in the nuclear pore complex

The spatial separation between the cytoplasm and the cell nucleus necessitates the continuous exchange of macromolecular cargo across the double-membraned nuclear envelope. Being the only passageway in and out of the nucleus, the nuclear pore complex (NPC) has the principal function of regulating the high throughput of nucleocytoplasmic transport in a highly selective manner so as to maintain cellular order and function. Here, we present a retrospective review of the evidence that has led to the current understanding of both NPC structure and function. Looking towards the future, we contemplate on how various outstanding effects and nanoscopic characteristics ought to be addressed, with the goal of reconciling structure and function into a single unified picture of the NPC.

Biology and biophysics of the nuclear pore complex and its components

Nucleocytoplasmic exchange of proteins and ribonucleoprotein particles occurs via nuclear pore complexes (NPCs) that reside in the double membrane of the nuclear envelope (NE). Significant progress has been made during the past few years in obtaining better structural resolution of the three-dimensional architecture of NPC with the help of cryo-electron tomography and atomic structures of domains from nuclear pore proteins (nucleoporins). Biophysical and imaging approaches have helped elucidate how nucleoporins act as a selective barrier in nucleocytoplasmic transport. Nucleoporins act not only in trafficking of macromolecules but also in proper microtubule attachment to kinetochores, in the regulation of gene expression and signaling events associated with, for example, innate and adaptive immunity, development and neurodegenerative disorders. Recent research has also been focused on the dynamic processes of NPC assembly and disassembly that occur with each cell cycle. Here we review emerging results aimed at understanding the molecular arrangement of the NPC and how it is achieved, defining the roles of individual nucleoporins both at the NPC and at other sites within the cell, and finally deciphering how the NPC serves as both a barrier and a conduit of active transport.

A Pathway Separate from the Central Channel through the Nuclear Pore Complex for Inorganic Ions and Small Macromolecules

Nuclear pore complexes (NPCs) are supramolecular nanomachines that mediate the exchange of macromolecules and inorganic ions between the nucleus and the cytosol. Although there is no doubt that large cargo is transported through the centrally located channel, the route of ions and small molecules remains debatable. We thus tested the hypothesis that there are two separate pathways by imaging NPCs using atomic force microscopy, NPC electrical conductivity measurements, and macromolecule permeability assays. Our data indicate a spatial separation between the active transport of macromolecules through the central channel and the passive transport of ions and small macromolecules through the pore periphery.

Passive and facilitated transport in nuclear pore complexes is largely uncoupled

Nuclear pore complexes provide the sole gateway for the exchange of material between nucleus and cytoplasm of interphase eukaryotic cells. They support two modes of transport: passive diffusion of ions, metabolites, and intermediate-sized macromolecules and facilitated, receptor-mediated translocation of proteins, RNA, and ribonucleoprotein complexes. It is generally assumed that both modes of transport occur through a single diffusion channel located within the central pore of the nuclear pore complex. To test this hypothesis, we studied the mutual effects between transporting molecules utilizing either the same or different modes of translocation. We find that the two modes of transport do not interfere with each other, but molecules utilizing a particular mode of transport do hinder motion of others utilizing the same pathway. We therefore conclude that the two modes of transport are largely segregated.

Ethanol alters access to the cell nucleus

Ethanol is the most frequently used drug among humans. We tested the hypothesis whether ethanol, at clinically relevant concentrations modifies, signaling across the nuclear envelope (NE). In cell nuclei isolated from Xenopus oocytes, we measured NE electrical resistance and NE macromolecule permeability 1 to 20 h after addition of ethanol (0.05 to 0.2%). Furthermore, with atomic force microscopy, nuclear pores of the NE were imaged after exposure to ethanol. We found that NE permeability decreased within hours of ethanol exposure. In parallel, nuclei swell and nuclear pores form clusters in the NE. Force measurements on individual nuclear pores indicate that pores found in clusters are stiffer than those found randomly distributed in the NE. Application of a transcription blocker (actinomycin D) or RNase treatment of isolated nuclei in vitro after ethanol exposure prevents the permeability changes. In conclusion, ethanol, at commonly used concentrations, changes NE structure by transcriptional processes in the cell nucleus. Within hours, the NE becomes less permeable for diffusible ions and macromolecules. This could explain altered signaling to and communication with the cell nucleus in the pathophysiology of alcohol abuse.

Current concepts in nuclear pore electrophysiology

Over 4 decades ago, microelectrode studies of in situ nuclei showed that, under certain conditions, the nuclear envelope (NE) behaves as a barrier opposing the nucleocytoplasmic flow of physiological ions. As the nuclear pore complexes (NPCs) of the NE are the only pathways for direct nucleocytoplasmic flow, those experiments implied that the NPCs are capable of restricting ion flow. These early studies validated electrophysiology as a useful approach to quantify some of the mechanisms by which NPCs mediate gene activity and expression. Since electron microscopy (EM) and other non-electrophysiological investigations, showed that the NPC lumen is a nanochannel, the opinion prevailed that the NPC could not oppose the flow of ions and, therefore, that electrophysiological observations resulted from technical artifacts. Consequently, the initial enthusiasm with nuclear electrophysiology faded out in less than a decade. In 1990, nuclear electrophysiology was revisited with patch-clamp, the most powerful electrophysiological technique to date. Patch-clamp has consistently demonstrated that the NE has intrinsic ion channel activity. Direct demonstrations of the NPC on-off ion channel gating behavior were published for artificial conditions in 1995 and for intact living nuclei in 2002. This on-off switching/gating behavior can be interpreted in terms of a metastable energy barrier. In the hope of advancing nuclear electrophysiology, and to complement the other papers contained in this special issue of the journal, here I review some of the main technical, experimental, and theoretical issues of the field, with special focus on NPCs.

The nuclear barrier is structurally and functionally highly responsive to glucocorticoids

Nuclear pore complexes mediate and control transport between the cytosol and the nucleus. They form a highly selective and, thus, tight nuclear barrier between these compartments. The nuclear barrier provides the cell with the opportunity to control access to its DNA, a defining feature of eukaryotes. The tightness of the nuclear barrier is therefore physiologically pivotal and any remarkable change in its structure and permeability can prove pathophysiological, e.g. as a result of viral attack. However, there is accumulating evidence that nuclear barrier structure and permeability are highly responsive to hydrophobic cargos of crucial physiological and therapeutic relevance, glucocorticoids (steroid hormones). The present review highlights the glucocorticoid-induced effects on the nuclear barrier structure and permeability concluding that they are physiologically essential to mediate glucocorticoid action.

Glucocorticoids remodel nuclear envelope structure and permeability

The present study describes glucocorticoid induced remodelling of nuclear envelope (NE) structure and permeability. A glucocorticoid analogue, triamcinolone acetonide (TA), is injected into Xenopus laevis oocytes that express an exogeneous glucocorticoid receptor (GR). Electrical, fluorescence and nano-imaging techniques are applied to study the permeability and the structure of the NE at 5 and 60 minutes after injection of TA. A remarkable dilation of nuclear pore complexes (NPCs), a rearrangement of NPC distribution and a significant increase of NE permeability for ions and fluorescent 20 kDa dextran are observed within 5 minutes of TA exposure. At regular distances on local NE patches, NPCs seem to adjoin forming clusters each consisting of several hundred NPCs. Interestingly, at the same time of exposure, hydrophobicity of NPC central channels and NPC-free NE surface increases. The changes in permeability and structure are transient as the NE permeability returns to its initial state within 60 minutes. In conclusion, the NE is a barrier of high plasticity sensitive to hydrophobic molecules. Remodelling of NE structure and permeability is a prerequisite for mediating physiological actions of glucocorticoids.

Transient permeability leak of nuclear envelope induced by aldosterone

The mineralocorticoid hormone aldosterone controls fluid and electrolyte transport in target cells of the kidney and the cardiovascular system. Classic genomic aldosterone action involves the activation of cytosolic mineralocorticoid receptors and translocation into the cell nucleus where specific transcription processes are initiated. A key barrier of the intracellular signalling pathway is the nuclear envelope, which physically separates the nucleoplasm from the cytoplasm. It was shown recently that aldosterone changes ion conductivity of the nuclear envelope mediated by nuclear pore complexes. The latter are supramolecular nanomachines responsible for import and export of inorganic ions and macromolecules. The aim of the present study was to test whether aldosterone changes the macromolecule permeability of the nuclear envelope. Aldosterone-responsive Xenopus laevis oocytes were used as a model system. We isolated the cell nuclei at defined times after hormone injection. By means of confocal fluorescence microscopy and fluorescence-labelled dextrans we evaluated passive macromolecule import and export in isolated nuclei. 10 minutes after aldosterone injection nuclear envelope permeability of 10 kD dextran was found sharply increased. At the same time cell nuclei were found swollen by about 28%. Changes in nuclear volume and nuclear envelope permeability lasted 5 to 15 minutes and could be inhibited by the mineralocorticoid receptor blocker spironolactone. We conclude that aldosterone transiently changes the barrier function of the nuclear envelope. This short-lasting permeability change signals the start of a sustained transcription process that follows in response to steroids.

Permeability of the Nuclear Envelope at Isolated Xenopus Oocyte Nuclei Studied by Scanning Electrochemical Microscopy

In interphase eukaryotic cells, molecular transport between the cytoplasm and the nucleus is mediated by the nuclear pore complex (NPC), which perforates the double-membraned nuclear envelope (NE). Local permeability of the NE at large intact nuclei (approximately 400 microm in diameter) isolated from Xenopus laevis oocytes was studied by scanning electrochemical microscopy (SECM). Steady-state tip current versus tip-nucleus distance curves (approach curves) were measured with 10- and 2-microm-diameter Pt disk microelectrodes at the nuclei in isotonic buffer solutions containing redox-active molecules. The approach curves in the normalized form are independent of the tip diameter, indicating diffusion-limited membrane transport of the redox molecules. SECM chronoamperometry demonstrated that a decrease in the steady-state tip current at short tip-nucleus distances is due to smaller diffusion coefficients and concentrations of the redox molecules in the nucleus than those in the buffer solution. The experimental approach curves fit very well with theoretical ones for freely permeable membranes, yielding the NE permeability to the molecules that is at least 2 orders of magnitude larger than permeability of bilayer lipid membranes and cell membranes. This result indicates that passive transport of the redox molecules across the NE is facilitated by open NPC pores. The flux of the redox molecules sustainable by a single NPC channel (>9.8 x 10(6) molecules per NPC per second) and the diameter of the channel pore (>15 nm) were estimated from the SECM data by assuming the NE as an array of nanometer-sized NPC pores. The effects of the redox molecules on the nucleus and the NPC function were examined by studying signal-mediated nuclear import of rhodamine-labeled bovine serum albumin with and without nuclear localization signals by fluorescence microscopy.

New aspects of nuclear calcium signalling

Nuclear calcium signalling has been a controversial battlefield for many years and the question of how permeable the nuclear pore complexes (NPCs) are to Ca2+ has been the subject of a particularly hot dispute. Recent data from isolated nuclei suggest that the NPCs are open even after depletion of the Ca2+ store in the nuclear envelope. Other research has suggested that a new Ca2+ -releasing messenger, nicotinic acid adenine dinucleotide phosphate (NAADP), can liberate Ca2+ only from acidic organelles, probably lysosomes, rather than from the traditional Ca2+ store in the endoplasmic reticulum (ER). Recent work indicates that NAADP can release Ca2+ from the nuclear envelope (NE), which has a thapsigargin-sensitive, ER-type Ca2+ store. NAADP acts in a manner similar to inositol (1,4,5)-trisphosphate [Ins(1,4,5)P3] or cyclic ADP-ribose (cADPR): all three messengers are equally able to reduce the Ca2+ concentration inside the NE and this is associated with a transient rise in the nucleoplasmic Ca2+ concentration. The NE contains ryanodine receptors (RyRs) and Ins(1,4,5)P3 receptors [Ins(1,4,5)P3Rs], and these can be activated separately and independently: the RyRs by either NAADP or cADPR, and the Ins(1,4,5)P3Rs by Ins(1,4,5)P3.

Nuclear envelope barrier leak induced by dexamethasone

Nuclear pore complexes (NPCs) are multiprotein channels that span the nuclear envelope. They strongly limit the efficiency of gene transfection by restriction of nuclear delivery of exogenously applied therapeutic macromolecules. NPC dilation could significantly increase this efficiency. Recently, it was shown in oocytes of Xenopus laevis that NPCs dilate from about 82 to 110 nm within min after injection of the glucocorticoid analog dexamethasone (dex). In the present paper we analyzed by means of atomic force microscopy the structural details of NPC dilation and correlated them with functional changes in nuclear envelope permeability. 5-11 min after Dex injection NPC dilation was found at its maximum (approximately 140 nm). In addition, a yet unknown configuration, so-called giant pore, up to 300 nm in diameter, was visualized. Giant pore formation was paralleled by an increase in nuclear envelope permeability tested by electrophysiology and confocal fluorescence microscopy. Even large macromolecules lacking any nuclear localization signal (77 kDa FITC-dextran, molecule diameter up to 36 nm) could gain access to the nucleus. We conclude that dex transiently opens unspecific pathways for large macromolecules. Dex treatment could be potentially useful for improving the efficiency of nuclear gene transfection.

Passive transport of macromolecules through Xenopus laevis nuclear envelope

Although nuclear pore complexes (NPC) are considered to be key structures in gene expression, little is known about their regulatory control. In order to explore the regulatory mechanism of passive transport of small macromolecules we examined the influence of different factors on the diffusional pathway of NPCs in isolated Xenopus laevis oocyte nuclei. Diffusion of fluorescence-labeled 10-kD dextran was measured across the nuclear envelope with confocal fluorescence microscopy. Surprisingly, the filling state of the perinuclear Ca(2+) store had no influence on passive transport of 10-kD dextran. Furthermore, nuclear envelope permeability was independent of cytoplasmic pH (pH range 8.3-6.3). In contrast, nuclear swelling, induced by omission of the endogenous cytosolic macromolecules, clearly increased nuclear permeability. An antibody against the glycoprotein gp62, located at the central channel entrance, reduced macromolecule diffusion. In addition, nuclei from transcriptionally active, early developmental stages (stage II) were less permeable compared to transcriptionally inactive, late-developmental-stage (stage VI) nuclei. In stage II nuclei, atomic force microscopy disclosed NPC central channels with plugs that most likely were ribonucleoproteins exiting the nucleus. In conclusion, the difference between macromolecule permeability and previous measurements of electrical resistance strongly indicates separate routes for macromolecules and ions across the nuclear envelope.

Nuclear Envelope: Nanoarray Responsive to Aldosterone

Signalling between cytosol and nucleus is mediated by nuclear pores. These supramolecular complexes represent intelligent nanomachines regulated by a wide spectrum of factors. Among them, steroid hormones specifically interact with the pores and thus modify ion conductivity and macromolecule permeability of the nuclear envelope. In response to aldosterone the pores undergo dramatic changes in conformation, changes that depend on the nature of the transported cargo. Such changes can be imaged at the nanometer scale by using atomic force microscopy. Furthermore, steroid-induced macromolecule transport across the nuclear envelope causes osmotic water movements and nuclear swelling. Drugs that interact with intracellular steroid receptors (spironolactone) or with plasma membrane sodium channels (amiloride) inhibit swelling. Steroid hormone action is blocked when nuclear volume changes are prevented. This is shown in frog oocytes and human endothelial cells. In conclusion, nuclear pores serve as steroid-sensitive gates that determine nuclear activity.

The nuclear pore complex: nucleocytoplasmic transport and beyond

Over the past two years, it has become evident that there is an unexpected link between nuclear pore complex structure and dynamics, nucleocytoplasmic transport and chromosome segregation. In addition, a tomographic three-dimensional reconstruction of native nuclear pore complexes preserved in thick amorphous ice has unveiled a number of new structural features of this supramolecular machine. These data, together with some of the elementary physical principles that underlie nucleocytoplasmic transport, will be discussed in this review.

Cryo-electron tomography provides novel insights into nuclear pore architecture: implications for nucleocytoplasmic transport

To go beyond the current structural consensus model of the nuclear pore complex (NPC), we performed cryo-electron tomography of fully native NPCs from Xenopus oocyte nuclear envelopes (NEs). The cytoplasmic face of the NPC revealed distinct anchoring sites for the cytoplasmic filaments, whereas the nuclear face was topped with a massive distal ring positioned above the central pore with indications of the anchoring sites for the nuclear basket filaments and putative intranuclear filaments. The rather "spongy" central framework of the NPC was perforated by an elaborate channel and void system, and at the membrane pore interface it exhibited distinct "handles" protruding into the lumen of the NE. The most variable structural moiety of the NPC was a rather tenuous central plug partially obstructing the central pore. Its mobile character was documented by time-lapse atomic force microscopy. Taken together, the new insights we gained into NPC structure support the notion that the NPC acts as a constrained diffusion pore for molecules and particles without retention signal and as an affinity gate for signal-bearing cargoes.

Aldosterone signaling pathway across the nuclear envelope

We describe the route by which aldosterone-triggered macromolecules enter and exit the cell nucleus of Xenopus laevis oocyte. Oocytes were microinjected with 50 fmol aldosterone and then enucleated 2-30 min after injection. After isolation, nuclear envelope electrical resistance (NEER) was measured in the intact cell nuclei by using the nuclear hourglass technique. We observed three NEER stages: an early peak 2 min after injection, a sustained depression after 5-15 min, and a final late peak 20 min after injection. Because NEER reflects the passive electrical permeability of nuclear pores, we investigated with atomic force microscopy aldosterone-induced conformational changes of individual nuclear pore complexes (NPCs). At the early peak we observed small ( congruent with 100 kDa) molecules (flags) attached to the NPC surface. At the sustained depression NPCs were found free of flags. At the late peak large ( congruent with 800 kDa) molecules (plugs) were detected inside the central channels. Ribonuclease or actinomycin D treatment prevented the late NEER peak. Coinjection of aldosterone (50 fmol) and its competitive inhibitor spironolactone (500 fmol) eliminated the electrical changes as well as flag and plug formation. We conclude: (i) The genomic response of aldosterone can be electrically measured in intact oocyte nuclei. (ii) Flags represent aldosterone receptors on their way into the cell nucleus whereas plugs represent ribonucleoproteins carrying aldosterone-induced mRNA from the nucleoplasm into the cytoplasm. (iii) Because plugs can be mechanically harvested with the atomic force microscopy stylus, oocytes could serve as a bioassay system for identifying aldosterone-induced early genes.

Cell cycling determines integrin-mediated adhesion in osteoblastic ROS 17/2.8 cells exposed to space-related conditions

Six days of microgravity (Bion10 mission) induced dramatic shape changes in ROS 17/2.8 osteoblasts (7). During the Foton 11 and 12 space flights, we studied the kinetics (0-4 days) of ROS 17/2.8 morphology and adhesion, the relationships between adhesion and cell cycle progression after 4 days in space, and osteoblastic growth and activity after 6 days in space. Quantitative analysis of high-resolution adhesion [focal adhesion area imaged by total interference reflection fluorescent microscopy (TIRFM)] and integrin-dependent adhesion (imaged on confocal microscope by vinculin and phosphotyrosine staining) as well as cell cycle phase classification [Ki-67 staining, S-G2, mitotic cells and G1 (postmitotic cells)] were performed using programs validated in parabolic flight and clinostat. We observed disorganization of the cytoskeleton associated with disassembling of vinculin spots and phosphorylated proteins within focal contacts with no major change in TIRFM adhesion after 2 and 4 days of microgravity. Postmitotic cells, alone, accounted for the differences observed in the whole population. They are characterized by immature peripheral contacts with complete loss of central spots and decreased spreading. Osteocalcin, P1CP and alkaline phosphatase, and proliferation were similar in flight cells and 1 g centrifuge and ground controls after 6 days. In conclusion, microgravity substantially affected osteoblastic integrin-mediated cell adhesion. ROS17/2.8 cells responded differently, whether or not they were cycling by reorganizing adhesion plaque topography or morphology. In ROS 17/2.8, this reorganization did not impair osteoblastic phenotype.

Evidence for Ca2+- and ATP-sensitive peripheral channels in nuclear pore complexes

In eukaryotic cells the nuclear envelope (NE) serves as a functional barrier between cytosol and nucleoplasm perforated by nuclear pore complexes (NPCs). Both active and passive transport of ions and macromolecules are thought to be mediated by the centrally located large NPC channel. However, 3-dimensional imaging of NPCs based on electron microscopy indicates the existence of additional small channels of unknown function located in the NPC periphery. By means of the recently developed nuclear hourglass technique that measures NE electrical conductance, we evaluated passive electrically driven transport through NPCs. In isolated Xenopus laevis oocyte nuclei, we varied ambient Ca2+ and ATP in the cytosolic solution and/or chelated Ca2+ in the perinuclear stores in order to assess the role of Ca2+ in regulating passive ion transport. We noticed that NE electrical conductance is large under conditions where macromolecule permeability is known to be low. In addition, atomic force microscopy applied to native NPCs detects multiple small pores in the NPC periphery consistent with channel openings. Peripheral pores were detectable only in the presence of ATP. We conclude that NPC transport of ions and macromolecules occurs through different routes. We present a model in which NE ion flux does not occur through the central NPC channel but rather through Ca2+- and ATP-activated peripheral channels of individual NPCs.

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.

Yeast nucleoporins involved in passive nuclear envelope permeability

The vertebrate nuclear pore complex (NPC) harbors an approximately 10-nm diameter diffusion channel that is large enough to admit 50-kD polypeptides. We have analyzed the permeability properties of the Saccharomyces cerevisiae nuclear envelope (NE) using import (NLS) and export (NES) signal-containing green fluorescent protein (GFP) reporters. Compared with wild-type, passive export rates of a classical karyopherin/importin (Kap) Kap60p/Kap95p-targeted NLS-GFP reporter (cNLS-GFP) were significantly faster in nup188-Delta and nup170-Delta cells. Similar results were obtained using two other NLS-GFP reporters, containing either the Kap104p-targeted Nab2p NLS (rgNLS) or the Kap121p-targeted Pho4p NLS (pNLS). Elevated levels of Hsp70 stimulated cNLS-GFP import, but had no effect on the import of rgNLS-GFP. Thus, the role of Hsp70 in NLS-directed import may be NLS- or targeting pathway-specific. Equilibrium sieving limits for the diffusion channel were assessed in vivo using NES-GFP reporters of 36-126 kD and were found to be greater than wild-type in nup188-Delta and nup170-Delta cells. We propose that Nup170p and Nup188p are involved in establishing the functional resting diameter of the NPC's central transport channel.