Passive transport of macromolecules through Xenopus laevis nuclear envelope

Intracellular calcium levels can regulate Importin-dependent nuclear import

We previously showed that increased intracellular calcium can modulate Importin (Imp)β1-dependent nuclear import of SRY-related chromatin remodeling proteins. Here we extend this work to show for the first time that high intracellular calcium inhibits Impα/β1- or Impβ1-dependent nuclear protein import generally. The basis of this relates to the mislocalisation of the transport factors Impβ1 and Ran, which show significantly higher nuclear localization in contrast to various other factors, and RCC1, which shows altered subnuclear localisation. The results here establish for the first time that intracellular calcium modulates conventional nuclear import through direct effects on the nuclear transport machinery.

Nucleoporin Gene Fusions and Hematopoietic Malignancies

Nuclear pore complexes (NPCs) are the sole gateways between the nucleus and the cytoplasm of eukaryotic cells and they mediate all macromolecular trafficking between these cellular compartments. Nucleocytoplasmic transport is highly selective and precisely regulated and as such an important aspect of normal cellular function. Defects in this process or in its machinery have been linked to various human diseases, including cancer. Nucleoporins, which are about 30 proteins that built up NPCs, are critical players in nucleocytoplasmic transport and have also been shown to be key players in numerous other cellular processes, such as cell cycle control and gene expression regulation. This review will focus on the three nucleoporins Nup98, Nup214, and Nup358. Common to them is their significance in nucleocytoplasmic transport, their multiple other functions, and being targets for chromosomal translocations that lead to haematopoietic malignancies, in particular acute myeloid leukaemia. The underlying molecular mechanisms of nucleoporin-associated leukaemias are only poorly understood but share some characteristics and are distinguished by their poor prognosis and therapy outcome.

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.

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 role of nuclear envelope calcium in modifying nuclear pore complex structureThis paper is one of a selection of papers published in this Special Issue, entitled The Nucleus: A Cell Within A Cell

Some of the most important trafficking processes in cells involve transport across the nuclear envelope. Whether it is the import of transcription factors or the export of RNA, the only known portal across the double lipid bilayer that forms the nuclear envelope are the macromolecular pores known as nuclear pore complexes (NPCs). Understanding how signals influence the conformation of the NPC is important for testing models of, and perhaps modifying, transport across the nuclear envelope. Here we summarize high-resolution atomic force microscopy studies of NPC structure following manipulation of nuclear envelope calcium stores of nuclei from Xenopus laevis oocytes. The results show that the release of calcium from these stores through the specific activation of inositol 1,4,5-trisphosphate receptors leads to changes in NPC structure observable from both sides of the nuclear envelope. The diameter of the NPC is also sensitive to these calcium stores and increases upon calcium release. Western blot analysis reveals the presence of ryanodine receptors in the nuclear envelope of X. laevis oocytes, although in low abundance. Activation of these calcium channels also leads to the displacement of the central mass and changes in NPC diameter. This change in structure may involve a displacement of the cytoplasmic and nuclear rings of the NPC towards each other, leading to the apparent emergence of the central mass from both sides of the NPC. The changes in conformation and diameter of the NPC may alter cargo access and binding to phenylalanine-glycine repeats lining the pore, thus altering transport.

Hot Spot Formation in the Nuclear Envelope of Oocytes in Response to Steroids

A Glucocorticoid-sensitive cell rapidly responds to hormone stimulation with bidirectional exchange of specific macromolecules between cytosol and nucleus. Glucocorticoid-initiated macromolecules (GIMs) must overcome the nuclear envelope (NE) to enter or leave the nucleus. GIM translocation occurs through nuclear pore complexes (NPCs) that span the NE. We investigated the question whether transport of GIMs through NPCs occurs random or involves selected groups of NPCs (hot spots). Glucocorticoid receptors were expressed in Xenopus laevis oocytes and GIM transport was activated by triamcinolone acetonide, a potent synthetic glucocorticoid analogon. Glucocorticoid receptors associated with the NE and the chromatin were identified using western blot analysis and, at single molecule level, atomic force microscopy. Fluorescence-labeled dextran was used to describe passive NE permeability. We observed that after hormone injection (i) small GIMs, most likely GRs, localize within seconds on both sides of the NE. (ii) large GIMs, most likely ribonucleoproteins, localize within minutes on NPCs at the nucleoplasmic side (iii) both small and large GIMs accumulate on selected NPC clusters (iv) NE permeability transiently decreases when GIMs attach to NPCs. We conclude that GIM transport across the nuclear barrier does not randomly take place but is carried out by a selected population of NPCs.

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.