Many mechanisms, one entrance: membrane protein translocation into the nucleus

Nuclear translocation of the membrane oxoeicosanoid/androgen receptor, OXER1: Possible mechanisms involved

OXER1, the receptor for the arachidonic acid metabolite 5-οxo-eicosatetraenoic acid (5-oxo-ETE), has been reported to also bind and mediate the membrane-initiated actions of androgens. Indeed, androgens antagonize the 5-oxo-ETE effects through OXER1, affecting a number of signaling pathways and inhibiting cancer cell proliferation and migration. OXER1, being a GPCR, was classically described to be localized in the plasma membrane. However, for numerous GPCRs, there is now strong evidence that they can be also found in other cellular compartments, including the nucleus. The aim of the present work was to investigate OXER1's possible localization in the nucleus and identify the mechanism(s) involved. For this purpose, we verified OXER1's nuclear presence by immunofluorescence and western blot, in whole cells and nuclei of two different prostate cancer cell lines (DU-145 and LNCaP) and in CHO cells transfected with a GFP labelled OXER1, both in untreated and OXER1 ligands' treated cells. Mutated, OXER1-tGFP expressing, CHO cells were used to verify that OXER1 agonist (5-oxo-ETE) binding is necessary for OXER1 nuclear translocation. NLS sequences were in silico identified, and a specific inhibitor, as well as, specific importins' siRNAs were also utilized to explore the mechanism involved. Moreover, we examined the role of palmitoylation in OXER1 nuclear translocation by in silico identifying possible palmitoylation sites and using a palmitoylation inhibitor. Our results clearly show that OXER1 can be localized in the nucleus, in an agonist-dependent manner, that is inhibited by androgens. We also provide evidence for two possible mechanisms for its nuclear trafficking, that involve receptor palmitoylation and importin-mediated cytoplasmic-nuclear transport. In our knowledge, it is the first time that a membrane androgen receptor is identified into the nucleus, suggesting an alternative, more direct, mode of action, involving nuclear mechanisms. Therefore, our findings provide new insights on androgen-mediated actions and androgen-lipid interactions, and reveal new possible therapeutic targets, not only for cancer, but also for other pathological conditions in which OXER1 may have an important role.

Phosphorylation of ELYS promotes its interaction with VAPB at decondensing chromosomes during mitosis

ELYS is a nucleoporin that localizes to the nuclear side of the nuclear pore complex (NPC) in interphase cells. In mitosis, it serves as an assembly platform that interacts with chromatin and then with nucleoporin subcomplexes to initiate post-mitotic NPC assembly. Here we identify ELYS as a major binding partner of the membrane protein VAPB during mitosis. In mitosis, ELYS becomes phosphorylated at many sites, including a predicted FFAT (two phenylalanines in an acidic tract) motif, which mediates interaction with the MSP (major sperm protein)-domain of VAPB. Binding assays using recombinant proteins or cell lysates and co-immunoprecipitation experiments show that VAPB binds the FFAT motif of ELYS in a phosphorylation-dependent manner. In anaphase, the two proteins co-localize to the non-core region of the newly forming nuclear envelope. Depletion of VAPB results in prolonged mitosis, slow progression from meta- to anaphase and in chromosome segregation defects. Together, our results suggest a role of VAPB in mitosis upon recruitment to or release from ELYS at the non-core region of the chromatin in a phosphorylation-dependent manner.

Therapeutic interventions on human breast cancer xenografts promote systemic dissemination of oncogenes

Metastatic dissemination following successful treatment of the primary tumour remains a common cause of death. There is mounting evidence that therapeutic interventions themselves may promote development of metastatic disease. We earlier reported that cell-free chromatin particles (cfChPs) released from dying cancer cells are potentially oncogenic. Based on this observation we hypothesized that therapeutic interventions may lead to the release of cfChPs from therapy induced dying cancer cells which could be carried via the blood stream to distant organs to transform healthy cells into new cancers that would masquerade as metastasis. To test this hypothesis, we generated xenografts of MDA-MB-231 human breast cancer cells in severe combined immune-deficient mice, and using immuno-fluorescence and FISH analysis looked for cfChPs in their brain cells. We detected multiple human DNA signals representing cfChPs in nuclei of brain cells of mice which co-localized with eight human onco-proteins. No intact MDA-MB-231 cells were detected. The number of co-localizing human DNA and human c-Myc signals increased dramatically following treatment with chemotherapy, localized radiotherapy or surgery, which could be prevented by concurrent treatment with three different cfChPs deactivating agents. These results suggest that therapeutic interventions lead to the release cfChPs from therapy induced dying cancer cells carrying oncogenes and are transported via the blood stream to brain cells to potentially transform them to generate new cancers that would appear as metastases. cfChPs induced metastatic spread of cancer is preventable by concurrent treatment with agents that deactivate cfChPs.

Working a second job: Cell adhesion proteins that moonlight in the nucleus

Cells are adept at sensing changes in their environment, transmitting signals internally to coordinate responses to external stimuli, and thereby influencing adaptive changes in cell states and behavior. Often, this response involves modulation of gene expression in the nucleus, which is seen largely as a physically separated process from the rest of the cell. Mechanosensing, whereby a cell senses physical stimuli, and integrates and converts these inputs into downstream responses including signaling cascades and gene regulatory changes, involves the participation of several macromolecular structures. Of note, the extracellular matrix (ECM) and its constituent macromolecules comprise an essential part of the cellular microenvironment, allowing cells to interact with each other, and providing both structural and biochemical stimuli sensed by adhesion transmembrane receptors. This highway of information between the ECM, cell adhesion proteins, and the cytoskeleton regulates cellular behavior, the disruption of which results in disease. Emerging evidence suggests a more direct role for some of these adhesion proteins in chromatin structure and gene regulation, RNA maturation and other non-canonical functions. While many of these discoveries were previously limited to observations of cytoplasmic-nuclear transport, recent advances in microscopy, and biochemical, proteomic and genomic technologies have begun to significantly enhance our understanding of the impact of nuclear localization of these proteins. This review will briefly cover known cell adhesion proteins that migrate to the nucleus, and their downstream functions. We will outline recent advances in this very exciting yet still emerging field, with impact ranging from basic biology to disease states like cancer.

GPCRs in Intracellular Compartments: New Targets for Drug Discovery

The architecture of eukaryotic cells is defined by extensive membrane-delimited compartments, which entails separate metabolic processes that would otherwise interfere with each other, leading to functional differences between cells. G protein-coupled receptors (GPCRs) are the largest class of cell surface receptors, and their signal transduction is traditionally viewed as a chain of events initiated from the plasma membrane. Furthermore, their intracellular trafficking, internalization, and recycling were considered only to regulate receptor desensitization and cell surface expression. On the contrary, accumulating data strongly suggest that GPCRs also signal from intracellular compartments. GPCRs localize in the membranes of endosomes, nucleus, Golgi and endoplasmic reticulum apparatuses, mitochondria, and cell division compartments. Importantly, from these sites they have shown to orchestrate multiple signals that regulate different cell pathways. In this review, we summarize the current knowledge of this fascinating phenomenon, explaining how GPCRs reach the intracellular sites, are stimulated by the endogenous ligands, and their potential physiological/pathophysiological roles. Finally, we illustrate several mechanisms involved in the modulation of the compartmentalized GPCR signaling by drugs and endogenous ligands. Understanding how GPCR signaling compartmentalization is regulated will provide a unique opportunity to develop novel pharmaceutical approaches to target GPCRs and potentially lead the way towards new therapeutic approaches.

Nucleoplasmic signals promote directed transmembrane protein import simultaneously via multiple channels of nuclear pores

Roughly 10% of eukaryotic transmembrane proteins are found on the nuclear membrane, yet how such proteins target and translocate to the nucleus remains in dispute. Most models propose transport through the nuclear pore complexes, but a central outstanding question is whether transit occurs through their central or peripheral channels. Using live-cell high-speed super-resolution single-molecule microscopy we could distinguish protein translocation through the central and peripheral channels, finding that most inner nuclear membrane proteins use only the peripheral channels, but some apparently extend intrinsically disordered domains containing nuclear localization signals into the central channel for directed nuclear transport. These nucleoplasmic signals are critical for central channel transport as their mutation blocks use of the central channels; however, the mutated proteins can still complete their translocation using only the peripheral channels, albeit at a reduced rate. Such proteins can still translocate using only the peripheral channels when central channel is blocked, but blocking the peripheral channels blocks translocation through both channels. This suggests that peripheral channel transport is the default mechanism that was adapted in evolution to include aspects of receptor-mediated central channel transport for directed trafficking of certain membrane proteins.

Flexible and Extended Linker Domains Support Efficient Targeting of Heh2 to the Inner Nuclear Membrane

The nuclear pore complex (NPC) is embedded in the nuclear envelope and forms the main gateway to the nuclear interior including the inner nuclear membrane (INM). Two INM proteins in yeast are selectively imported. Their sorting signals consist of a nuclear localization signal, separated from the transmembrane domain by a long intrinsically disordered (ID) linker. We used computational models to predict the dynamic conformations of ID linkers and analyzed the INM targeting efficiency of proteins with linker regions with altered Stokes radii and decreased flexibilities. We find that flexibility, Stokes radius, and the frequency at which the linkers are at an extended end-to-end distance larger than 25 nm are good predictors for the targeting of the proteins. The data are consistent with a transport mechanism in which INM targeting of Heh2 is dependent on an ID linker that facilitates the crossing of the approximately 25-nm thick NPC scaffold.

Delivery systems exploiting natural cell transport processes of macromolecules for intracellular targeting of Auger electron emitters

The presence of Auger electrons (AE) among the decay products of a number of radionuclides makes these radionuclides an attractive means for treating cancer because these short-range electrons can cause significant damage in the immediate vicinity of the decomposition site. Moreover, the extreme locality of the effect provides a potential for selective eradication of cancer cells with minimal damage to adjacent normal cells provided that the delivery of the AE emitter to the most vulnerable parts of the cell can be achieved. Few cellular compartments have been regarded as the desired target site for AE emitters, with the cell nucleus generally recognized as the preferred site for AE decay due to the extreme sensitivity of nuclear DNA to direct damage by radiation of high linear energy transfer. Thus, the advantages of AE emitters for cancer therapy are most likely to be realized by their selective delivery into the nucleus of the malignant cells. To achieve this goal, delivery systems must combine a challenging complex of properties that not only provide cancer cell preferential recognition but also cell entry followed by transport into the cell nucleus. A promising strategy for achieving this is the recruitment of natural cell transport processes of macromolecules, involved in each of the aforementioned steps. To date, a number of constructs exploiting intracellular transport systems have been proposed for AE emitter delivery to the nucleus of a targeted cell. An example of such a multifunctional vehicle that provides smart step-by-step delivery is the so-called modular nanotransporter, which accomplishes selective recognition, binding, internalization, and endosomal escape followed by nuclear import of the delivered radionuclide. The current review will focus on delivery systems utilizing various intracellular transport pathways and their combinations in order to provide efficient targeting of AE to the cancer cell nucleus.

Pathogenic mutations in genes encoding nuclear envelope proteins and defective nucleocytoplasmic connections

Mutations in genes encoding nuclear lamins and associated nuclear envelope proteins have been linked to a broad range of inherited diseases affecting different tissues and organs. These diseases are often referred to as laminopathies. Scientists have yet to elucidate exactly how pathogenic mutations leading to alteration of a nuclear envelope protein cause disease. Our relatively recent research has shown that pathogenic mutations in genes encoding nuclear envelope proteins lead to defective nucleocytoplasmic connections that disrupt proper functioning of the linker of nucleoskeleton and cytoskeleton complex in the establishment of cell polarity. These defects may explain, at least in part, pathogenic mechanisms underlying laminopathies.

Impact statement

Mutations in genes encoding nuclear lamins and associated nuclear envelope proteins have been linked to several diseases affecting different tissues and organs. The pathogenic mechanisms underlying these diseases, often called laminopathies, remain poorly understood. Increased knowledge of the functions of different nuclear envelope proteins and the interactions between them is crucial to elucidate these disease mechanisms. Our research has shown that pathogenic mutations in genes encoding nuclear envelope proteins lead to defective nucleocytoplasmic connections that disrupt proper functioning of the linker of nucleoskeleton and cytoskeleton (LINC) complex in the establishment of cell polarity. These defects may contribute to the pathogenesis of laminopathies and provide novel targets for therapeutics.

Patrolling the nucleus: inner nuclear membrane-associated degradation

Protein quality control and transport are important for the integrity of organelles such as the endoplasmic reticulum, but it is largely unknown how protein homeostasis is regulated at the nuclear envelope (NE) despite the connection between NE protein function and human disease. Elucidating mechanisms that regulate the NE proteome is key to understanding nuclear processes such as gene expression, DNA replication and repair as NE components, particularly proteins at the inner nuclear membrane (INM), are involved in the maintenance of nuclear structure, nuclear positioning and chromosome organization. Nuclear pore complexes control the entry and exit of proteins in and out of the nucleus, restricting movement across the nuclear membrane based on protein size, or the size of the extraluminal-facing domain of a transmembrane protein, providing one level of INM proteome regulation. Research in budding yeast has identified a protein quality control system that targets mislocalized and misfolded proteins at the INM. Here, we review what is known about INM-associated degradation, including recent evidence suggesting that it not only targets mislocalized or misfolded proteins, but also contributes to homeostasis of resident INM proteins.

Systematic analysis of NLMP suggests nuclear localization of RTK/MET kinases resemble cancer cell clearance

BACKGROUND

Some membrane proteins can translocate into the nucleus, defined as nuclear localized membrane proteins (NLMPs), including receptor tyrosine kinases (RTKs). We previously showed that nuclear MET (nMET), a member of RTKs, mediates cancer stem-like cells self-renewal to promote cancer recurrence. However, it is unknown that nMET or mMET, which is the ancestor in the evolution of cancer cell survival and clearance. Here, we aim to study the NLMP functions in cell death, differentiation and survival.

METHOD

We applied the systematic reanalysis of functional NLMP and clinical investigations of nMET from databases. In addition, we used soft agar assay, immunoblotting, flow cytometry, and immunofluorescence confocal microscopy for examinations of nMET functions including stem-like cell formation, cell signaling, cell cycle regulation, and co-localization with regulators of cell signaling. ShRNA, antibody of recognizing surface membrane MET based treatment were used to downregulate endogenous nMET to uncover its function.

RESULTS

We predicted and demonstrated that nMET and nEGFR are most likely not ancestors. nMET overexpression induces both cell death and survival with drug resistance and stem cell-like characters. Moreover, the paradoxical function of nMET in both cell death and cell survival is explained by the fact that nMET induces stem cell-like cell growth, DNA damage repair, to evade the drug sensitization for survival of single cells while non-stem cell-like nMET expressing single cells may undergo clearance by cell death through cell cycle arrest induced by p21.

CONCLUSION

Taken together, our data suggest a link between nuclear RTK and cancer cell evolutionary clearance via cell death, and drug resistance for survival through stemness selection. Targeting evolved nuclear RTKs in cancer stem cells would be a novel avenue for precision cancer therapy.

Targeting of LRRC59 to the Endoplasmic Reticulum and the Inner Nuclear Membrane

LRRC59 (leucine-rich repeat-containing protein 59) is a tail-anchored protein with a single transmembrane domain close to its C-terminal end that localizes to the endoplasmic reticulum (ER) and the nuclear envelope. Here, we investigate the mechanisms of membrane integration of LRRC59 and its targeting to the inner nuclear membrane (INM). Using purified microsomes, we show that LRRC59 can be post-translationally inserted into ER-derived membranes. The TRC-pathway, a major route for post-translational membrane insertion, is not required for LRRC59. Like emerin, another tail-anchored protein, LRRC59 reaches the INM, as demonstrated by rapamycin-dependent dimerization assays. Using different approaches to inhibit importin α/β-dependent nuclear import of soluble proteins, we show that the classic nuclear transport machinery does not play a major role in INM-targeting of LRRC59. Instead, the size of the cytoplasmic domain of LRRC59 is an important feature, suggesting that targeting is governed by passive diffusion.

Intracellular GPCRs Play Key Roles in Synaptic Plasticity

The trillions of synaptic connections within the human brain are shaped by experience and neuronal activity, both of which underlie synaptic plasticity and ultimately learning and memory. G protein-coupled receptors (GPCRs) play key roles in synaptic plasticity by strengthening or weakening synapses and/or shaping dendritic spines. While most studies of synaptic plasticity have focused on cell surface receptors and their downstream signaling partners, emerging data point to a critical new role for the very same receptors to signal from inside the cell. Intracellular receptors have been localized to the nucleus, endoplasmic reticulum, lysosome, and mitochondria. From these intracellular positions, such receptors may couple to different signaling systems, display unique desensitization patterns, and/or show distinct patterns of subcellular distribution. Intracellular GPCRs can be activated at the cell surface, endocytosed, and transported to an intracellular site or simply activated in situ by de novo ligand synthesis, diffusion of permeable ligands, or active transport of non-permeable ligands. Current findings reinforce the notion that intracellular GPCRs play a dynamic role in synaptic plasticity and learning and memory. As new intracellular GPCR roles are defined, the need to selectively tailor agonists and/or antagonists to both intracellular and cell surface receptors may lead to the development of more effective therapeutic tools.

GPCR signalling from within the cell

Traditionally, signal transduction from GPCRs is thought to emanate from the cell surface where receptor interactions with external stimuli can be transformed into a broad range of cellular responses. However, emergent data show that numerous GPCRs are also associated with various intracellular membranes where they may couple to different signalling systems, display unique desensitization patterns and/or exhibit distinct patterns of subcellular distribution. Although many GPCRs can be activated at the cell surface and subsequently endocytosed and transported to a unique intracellular site, other intracellular GPCRs can be activated in situ either via de novo ligand synthesis, diffusion of permeable ligands or active transport of nonpermeable ligands. Current findings reinforce the notion that intracellular GPCRs play a dynamic role in various biological functions including learning and memory, contractility and angiogenesis. As new intracellular GPCR roles are defined, the need to selectively tailor agonists and/or antagonists to both intracellular and cell surface receptors may lead to the development of more effective therapeutic tools.

LINKED ARTICLES

This article is part of a themed section on Molecular Pharmacology of GPCRs. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.21/issuetoc.

Determination of Membrane Protein Distribution on the Nuclear Envelope by Single-Point Single-Molecule FRAP

Nuclear envelope transmembrane proteins (NETs) are synthesized on the endoplasmic reticulum and then transported from the outer nuclear membrane (ONM) to the inner nuclear membrane (INM) in eukaryotic cells. The abnormal distribution of NETs has been associated with many human diseases. However, quantitative determination of the spatial distribution and translocation dynamics of NETs on the ONM and INM is still very limited in currently existing approaches. Here we demonstrate a single-point single-molecule fluorescence recovery after photobleaching (FRAP) microscopy technique that enables quick determination of distribution and translocation rates for NETs in vivo. © 2017 by John Wiley & Sons, Inc.

Sequences within the C Terminus of the Metabotropic Glutamate Receptor 5 (mGluR5) Are Responsible for Inner Nuclear Membrane Localization

Traditionally, G-protein-coupled receptors (GPCR) are thought to be located on the cell surface where they transmit extracellular signals to the cytoplasm. However, recent studies indicate that some GPCRs are also localized to various subcellular compartments such as the nucleus where they appear required for various biological functions. For example, the metabotropic glutamate receptor 5 (mGluR5) is concentrated at the inner nuclear membrane (INM) where it mediates Ca²⁺ changes in the nucleoplasm by coupling with G_q/11 Here, we identified a region within the C-terminal domain (amino acids 852-876) that is necessary and sufficient for INM localization of the receptor. Because these sequences do not correspond to known nuclear localization signal motifs, they represent a new motif for INM trafficking. mGluR5 is also trafficked to the plasma membrane where it undergoes re-cycling/degradation in a separate receptor pool, one that does not interact with the nuclear mGluR5 pool. Finally, our data suggest that once at the INM, mGluR5 is stably retained via interactions with chromatin. Thus, mGluR5 is perfectly positioned to regulate nucleoplasmic Ca²⁺in situ.

A ternary complex comprising transportin1, Rab8 and the ciliary targeting signal directs proteins to ciliary membranes

The sensory functions of cilia are dependent on the enrichment of cilium-resident proteins. Although it is known that ciliary targeting signals (CTSs) specifically target ciliary proteins to cilia, it is still unclear how CTSs facilitate the entry and retention of cilium-resident proteins at the molecular level. We found that non-ciliary membrane reporters can passively diffuse into cilia through the lateral transport pathway, and the translocation of membrane reporters through the ciliary diffusion barrier is facilitated by importin binding motifs and domains. Screening known CTSs of ciliary membrane residents uncovered that fibrocystin, photoreceptor retinol dehydrogenase, rhodopsin and retinitis pigmentosa 2 interact with transportin1 (TNPO1) through previously identified CTSs. We further discovered that a new ternary complex, comprising TNPO1, Rab8 and a CTS, can assemble or disassemble under the guanine nucleotide exchange activity of Rab8. Our study suggests a new mechanism in which the TNPO1-Rab8-CTS complex mediates selective entry into and retention of cargos within cilia.

Single-point single-molecule FRAP distinguishes inner and outer nuclear membrane protein distribution

The normal distribution of nuclear envelope transmembrane proteins (NETs) is disrupted in several human diseases. NETs are synthesized on the endoplasmic reticulum and then transported from the outer nuclear membrane (ONM) to the inner nuclear membrane (INM). Quantitative determination of the distribution of NETs on the ONM and INM is limited in available approaches, which moreover provide no information about translocation rates in the two membranes. Here we demonstrate a single-point single-molecule FRAP microscopy technique that enables determination of distribution and translocation rates for NETs in vivo.

Molecular and cellular basis for the unique functioning of Nrf1, an indispensable transcription factor for maintaining cell homoeostasis and organ integrity

The consensuscis-regulatory AP-1 (activator protein-1)-like AREs (antioxidant-response elements) and/or EpREs (electrophile-response elements) allow for differential recruitment of Nrf1 [NF-E2 (nuclear factor-erythroid 2)-related factor 1], Nrf2 and Nrf3, together with each of their heterodimeric partners (e.g. sMaf, c-Jun, JunD or c-Fos), to regulate different sets of cognate genes. Among them, NF-E2 p45 and Nrf3 are subject to tissue-specific expression in haemopoietic and placental cell lineages respectively. By contrast, Nrf1 and Nrf2 are two important transcription factors expressed ubiquitously in various vertebrate tissues and hence may elicit putative combinational or competitive functions. Nevertheless, they have de facto distinct biological activities because knockout of their genes in mice leads to distinguishable phenotypes. Of note, Nrf2 is dispensable during development and growth, albeit it is accepted as a master regulator of antioxidant, detoxification and cytoprotective genes against cellular stress. Relative to the water-soluble Nrf2, less attention has hitherto been drawn to the membrane-bound Nrf1, even though it has been shown to be indispensable for embryonic development and organ integrity. The biological discrepancy between Nrf1 and Nrf2 is determined by differences in both their primary structures and topovectorial subcellular locations, in which they are subjected to distinct post-translational processing so as to mediate differential expression of ARE-driven cytoprotective genes. In the present review, we focus on the molecular and cellular basis for Nrf1 and its isoforms, which together exert its essential functions for maintaining cellular homoeostasis, normal organ development and growth during life processes. Conversely, dysfunction of Nrf1 results in spontaneous development of non-alcoholic steatohepatitis, hepatoma, diabetes and neurodegenerative diseases in animal models.

High-Resolution Scanning Electron Microscopy and Immuno-Gold Labeling of the Nuclear Lamina and Nuclear Pore Complex

Scanning electron microscopy (SEM) is a technique used to image surfaces. Field emission SEMs (feSEMs) can resolve structures that are ~0.5-1.5 nm apart. FeSEM, therefore is a useful technique for imaging molecular structures that exist at surfaces such as membranes. The nuclear envelope consists of four membrane surfaces, all of which may be accessible for imaging. Imaging of the cytoplasmic face of the outer membrane gives information about ribosomes and cytoskeletal attachments, as well as details of the cytoplasmic peripheral components of the nuclear pore complex, and is the most easily accessed surface. The nucleoplasmic face of the inner membrane is easily accessible in some cells, such as amphibian oocytes, giving valuable details about the organization of the nuclear lamina and how it interacts with the nuclear pore complexes. The luminal faces of both membranes are difficult to access, but may be exposed by various fracturing techniques. Protocols are presented here for the preparation, labeling, and feSEM imaging of Xenopus laevis oocyte nuclear envelopes.

Probing Protein Distribution Along the Nuclear Envelope In Vivo by Using Single-Point FRAP

Determining the locations of nuclear envelope transmembrane proteins and their concentrations across the outer and inner nuclear membranes has been a challenging and time-consuming process. Typically, this required the week-long process of fixing and immunogold staining of cells prior to analysis by electron microscopy. Here, we describe a method, single-point fluorescence recovery after photobleaching (spFRAP), which is able to quickly determine the localization and distribution of nuclear membrane proteins along the double nuclear envelope membranes with a precision of 10-15 nm in a matter of 10-20 min the day after transfection.

Emery-Dreifuss muscular dystrophy mutations impair TRC40-mediated targeting of emerin to the inner nuclear membrane

Emerin is a tail-anchored protein that is found predominantly at the inner nuclear membrane (INM), where it associates with components of the nuclear lamina. Mutations in the emerin gene cause Emery-Dreifuss muscular dystrophy (EDMD), an X-linked recessive disease. Here, we report that the TRC40/GET pathway for post-translational insertion of tail-anchored proteins into membranes is involved in emerin-trafficking. Using proximity ligation assays, we show that emerin interacts with TRC40 in situ. Emerin expressed in bacteria or in a cell-free lysate was inserted into microsomal membranes in an ATP- and TRC40-dependent manner. Dominant-negative fragments of the TRC40-receptor proteins WRB and CAML (also known as CAMLG) inhibited membrane insertion. A rapamycin-based dimerization assay revealed correct transport of wild-type emerin to the INM, whereas TRC40-binding, membrane integration and INM-targeting of emerin mutant proteins that occur in EDMD was disturbed. Our results suggest that the mode of membrane integration contributes to correct targeting of emerin to the INM.

Active Nuclear Import of Membrane Proteins Revisited

It is poorly understood how membrane proteins destined for the inner nuclear membrane pass the crowded environment of the Nuclear Pore Complex (NPC). For the Saccharomyces cerevisiae proteins Src1/Heh1 and Heh2, a transport mechanism was proposed where the transmembrane domains diffuse through the membrane while the extralumenal domains encoding a nuclear localization signal (NLS) and intrinsically disordered linker (L) are accompanied by transport factors and travel through the NPC. Here, we validate the proposed mechanism and explore and discuss alternative interpretations of the data. First, to disprove an interpretation where the membrane proteins become membrane embedded only after nuclear import, we present biochemical and localization data to support that the previously used, as well as newly designed reporter proteins are membrane-embedded irrespective of the presence of the sorting signals, the specific transmembrane domain (multipass or tail anchored), independent of GET, and also under conditions that the proteins are trapped in the NPC. Second, using the recently established size limit for passive diffusion of membrane proteins in yeast, and using an improved assay, we confirm active import of polytopic membrane protein with extralumenal soluble domains larger than those that can pass by diffusion on similar timescales. This reinforces that NLS-L dependent active transport is distinct from passive diffusion. Thirdly, we revisit the proposed route through the center of the NPC and conclude that the previously used trapping assay is, unfortunately, poorly suited to address the route through the NPC, and the route thus remains unresolved. Apart from the uncertainty about the route through the NPC, the data confirm active, transport factor dependent, nuclear transport of membrane-embedded mono- and polytopic membrane proteins in baker’s yeast.

Distinctive Properties of the Nuclear Localization Signals of Inner Nuclear Membrane Proteins Heh1 and Heh2

Targeting of ER-synthesized membrane proteins to the inner nuclear membrane (INM) has long been explained by the diffusion-retention model. However, several INM proteins contain non-classical nuclear localization signal (NLS) sequences, which, in a few instances, have been shown to promote importin α/β- and Ran-dependent translocation to the INM. Here, using structural and biochemical methods, we show that yeast INM proteins Heh2 and Src1/Heh1 contain bipartite import sequences that associate intimately with the minor NLS-binding pocket of yeast importin α and unlike classical NLSs efficiently displace the IBB domain in the absence of importin β. In vivo, the intimate interactions at the minor NLS-binding pocket make the h2NLS highly efficient at recruiting importin α at the ER and drive INM localization of endogenous Heh2. Thus, h1/h2NLSs delineate a novel class of super-potent, IBB-like membrane protein NLSs, distinct from classical NLSs found in soluble cargos and of general interest in biology.

Nuclear envelope protein Lem2 is required for mouse development and regulates MAP and AKT kinases

The nuclear lamina, along with associated nuclear membrane proteins, is a nexus for regulating signaling in the nucleus. Numerous human diseases arise from mutations in lamina proteins, and experimental models for these disorders have revealed aberrant regulation of various signaling pathways. Previously, we reported that the inner nuclear membrane protein Lem2, which is expressed at high levels in muscle, promotes the differentiation of cultured myoblasts by attenuating ERK signaling. Here, we have analyzed mice harboring a disrupted allele for the Lem2 gene (Lemd2). No gross phenotypic defects were seen in heterozygotes, although muscle regeneration induced by cardiotoxin was delayed. By contrast, homozygous Lemd2 knockout mice died by E11.5. Although many normal morphogenetic hallmarks were observed in E10.5 knockout embryos, most tissues were substantially reduced in size. This was accompanied by activation of multiple MAP kinases (ERK1/2, JNK, p38) and AKT. Knockdown of Lem2 expression in C2C12 myoblasts also led to activation of MAP kinases and AKT. These findings indicate that Lemd2 plays an essential role in mouse embryonic development and that it is involved in regulating several signaling pathways. Since increased MAP kinase and AKT/mTORC signaling is found in other animal models for diseases linked to nuclear lamina proteins, LEMD2 should be considered to be another candidate gene for human disease.

Size-dependent leak of soluble and membrane proteins through the yeast nuclear pore complex

The permeability of the baker's yeast nuclear pore complex for multidomain proteins of different sizes, both soluble and transmembrane, was measured. The permeability for soluble proteins correlated with models of the disordered phase of wild-type and mutant NPCs generated using a one bead per amino acid molecular dynamics model.

Nuclear pore complexes (NPCs) allow selective import and export while forming a barrier for untargeted proteins. Using fluorescence microscopy, we measured in vivo the permeability of the Saccharomyces cerevisiae NPC for multidomain proteins of different sizes and found that soluble proteins of 150 kDa and membrane proteins with an extralumenal domain of 90 kDa were still partly localized in the nucleus on a time scale of hours. The NPCs thus form only a weak barrier for the majority of yeast proteins, given their monomeric size. Using FGΔ-mutant strains, we showed that specific combinations of Nups, especially with Nup100, but not the total mass of FG-nups per pore, were important for forming the barrier. Models of the disordered phase of wild-type and mutant NPCs were generated using a one bead per amino acid molecular dynamics model. The permeability measurements correlated with the density predictions from coarse-grained molecular dynamics simulations in the center of the NPC. The combined in vivo and computational approach provides a framework for elucidating the structural and functional properties of the permeability barrier of nuclear pore complexes.

An assay for clogging the ciliary pore complex distinguishes mechanisms of cytosolic and membrane protein entry

As a cellular organelle, the cilium contains a unique protein composition. Entry of both membrane and cytosolic components is tightly regulated by gating mechanisms at the cilium base; however, the mechanistic details of ciliary gating are largely unknown. We previously proposed that entry of cytosolic components is regulated by mechanisms similar to those of nuclear transport and is dependent on nucleoporins (NUPs), which comprise a ciliary pore complex (CPC). To investigate ciliary gating mechanisms, we developed a system to clog the pore by inhibiting NUP function via forced dimerization. We targeted NUP62, a component of the central channel of the nuclear pore complex (NPC), for forced dimerization by tagging it with the homodimerizing Fv domain. As proof of principle, we show that forced dimerization of NUP62-Fv attenuated (1) active transport of BSA into the nuclear compartment and (2) the kinesin-2 motor KIF17 into the ciliary compartment. Using the pore-clogging technique, we find that forced dimerization of NUP62 attenuated the gated entry of cytosolic proteins but did not affect entry of membrane proteins or diffusional entry of small cytosolic proteins. We propose a model in which active transport of cytosolic proteins into both nuclear and ciliary compartments requires functional NUPs of the central pore, whereas lateral entry of membrane proteins utilizes a different mechanism that is likely specific to each organelle's limiting membrane.

Cellular mechanosensing: getting to the nucleus of it all

Cells respond to mechanical forces by activating specific genes and signaling pathways that allow the cells to adapt to their physical environment. Examples include muscle growth in response to exercise, bone remodeling based on their mechanical load, or endothelial cells aligning under fluid shear stress. While the involved downstream signaling pathways and mechanoresponsive genes are generally well characterized, many of the molecular mechanisms of the initiating 'mechanosensing' remain still elusive. In this review, we discuss recent findings and accumulating evidence suggesting that the cell nucleus plays a crucial role in cellular mechanotransduction, including processing incoming mechanoresponsive signals and even directly responding to mechanical forces. Consequently, mutations in the involved proteins or changes in nuclear envelope composition can directly impact mechanotransduction signaling and contribute to the development and progression of a variety of human diseases, including muscular dystrophy, cancer, and the focus of this review, dilated cardiomyopathy. Improved insights into the molecular mechanisms underlying nuclear mechanotransduction, brought in part by the emergence of new technologies to study intracellular mechanics at high spatial and temporal resolution, will not only result in a better understanding of cellular mechanosensing in normal cells but may also lead to the development of novel therapies in the many diseases linked to defects in nuclear envelope proteins.

A nuclear ubiquitin-proteasome pathway targets the inner nuclear membrane protein Asi2 for degradation

The nuclear envelope consists of inner and outer nuclear membranes. Whereas the outer membrane is an extension of the endoplasmic reticulum, the inner nuclear membrane (INM) represents a unique membranous environment containing specific proteins. The mechanisms of integral INM protein degradation are unknown. Here, we investigated the turnover of Asi2, an integral INM protein in Saccharomyces cerevisiae. We report that Asi2 is degraded by the proteasome independently of the vacuole and that it exhibited a half-life of ∼45 min. Asi2 exhibits enhanced stability in mutants lacking the E2 ubiquitin conjugating enzymes Ubc6 or Ubc7, or the E3 ubiquitin ligase Doa10. Consistent with these data, Asi2 is post-translationally modified by poly-ubiquitylation in a Ubc7- and Doa10-dependent manner. Importantly Asi2 degradation is significantly reduced in a sts1-2 mutant that fails to accumulate proteasomes in the nucleus, indicating that Asi2 is degraded in the nucleus. Our results reveal a molecular pathway that affects the stability of integral proteins of the inner nuclear membrane and indicate that Asi2 is subject to protein quality control in the nucleus.

Expression of DNAJB12 or DNAJB14 causes coordinate invasion of the nucleus by membranes associated with a novel nuclear pore structure

DNAJB12 and DNAJB14 are transmembrane proteins in the endoplasmic reticulum (ER) that serve as co-chaperones for Hsc70/Hsp70 heat shock proteins. We demonstrate that over-expression of DNAJB12 or DNAJB14 causes the formation of elaborate membranous structures within cell nuclei, which we designate DJANGOS for DNAJ-associated nuclear globular structures. DJANGOS contain DNAJB12, DNAJB14, Hsc70 and markers of the ER lumen and ER and nuclear membranes. Strikingly, they are evenly distributed underneath the nuclear envelope and are of uniform size in any one nucleus. DJANGOS are composed primarily of single-walled membrane tubes and sheets that connect to the nuclear envelope via a unique configuration of membranes, in which the nuclear pore complex appears anchored exclusively to the outer nuclear membrane, allowing both the inner and outer nuclear membranes to flow past the circumference of the nuclear pore complex into the nucleus. DJANGOS break down rapidly during cell division and reform synchronously in the daughter cell nuclei, demonstrating that they are dynamic structures that undergo coordinate formation and dissolution. Genetic studies showed that the chaperone activity of DNAJ/Hsc70 is required for the formation of DJANGOS. Further analysis of these structures will provide insight into nuclear pore formation and function, activities of molecular chaperones, and mechanisms that maintain membrane identity.

The cellular mastermind(?)-mechanotransduction and the nucleus

Cells respond to mechanical stimulation by activation of specific signaling pathways and genes that allow the cell to adapt to its dynamic physical environment. How cells sense the various mechanical inputs and translate them into biochemical signals remains an area of active investigation. Recent reports suggest that the cell nucleus may be directly implicated in this cellular mechanotransduction process. Taken together, these findings paint a picture of the nucleus as a central hub in cellular mechanotransduction-both structurally and biochemically-with important implications in physiology and disease.

Intracellular glycine receptor function facilitates glioma formation in vivo

The neuronal function of Cys-loop neurotransmitter receptors is established; however, their role in non-neuronal cells is poorly defined. As brain tumors accumulate the neurotransmitter glycine, we studied expression and function of glycine receptors (GlyR) in glioma cells. Human brain tumor biopsies selectively expressed GlyR subunits with nuclear import signal (NLS, α1 and α3). The mouse glioma cell line GL261 expressed GlyR α1, and knock-down of α1 protein expression impaired self-renewal capacity and tumorigenicity of GL261 glioma cells as evidenced by the neurosphere assay and GL261 cell inoculation in vivo, respectively. We furthermore show that the pronounced tumorigenic effect of GlyR α1 relies on a new intracellular signaling function that depends on the NLS region in the large cytosolic loop and impacts on GL261 glioma cell gene regulation. Stable expression of GlyR α1 and α3 loops rescued self-renewal capacity of GlyR α1 knock-down cells, which demonstrates their functional equivalence. The new intracellular signaling function identified here goes beyond the well-established role of GlyRs as neuronal ligand-gated ion channels and defines NLS-containing GlyRs as novel potential targets for brain tumor therapies.

Destination: inner nuclear membrane

The inner nuclear membrane (INM) of eukaryotic cells is enriched in proteins that are required for nuclear structure, chromosome organization, DNA repair, and transcriptional control. Mislocalization of INM proteins is observed in a wide spectrum of human diseases; however, the mechanism by which INM proteins reach their final destination is poorly understood. In this review we discuss how investigating INM composition, dissecting targeting pathways of conserved INM proteins in multiple systems and analyzing the nuclear transport of viruses and signaling complexes have broadened our knowledge of INM transport to include both nuclear pore complex-dependent and -independent pathways. The study of these INM targeting pathways is important to understanding nuclear organization and in both normal and diseased cells.

Entry into the nuclear pore complex is controlled by a cytoplasmic exclusion zone containing dynamic GLFG-repeat nucleoporin domains

Nuclear pore complexes (NPCs) mediate nucleocytoplasmic movement. The central channel contains proteins with phenylalanine-glycine (FG) repeats, or variations (GLFG, glycine-leucine-phenylalanine-glycine). These are 'intrinsically disordered' and often represent weak interaction sites that become ordered upon interaction. We investigated this possibility during nuclear transport. Using electron microscopy of S. cerevisiae, we show that NPC cytoplasmic filaments form a dome-shaped structure enclosing GLFG domains. GLFG domains extend out of this structure and are part of an 'exclusion zone' that might act as a partial barrier to entry of transport-inert proteins. The anchor domain of a GLFG nucleoporin locates exclusively to the central channel. By contrast, the localisation of the GLFG domains varied between NPCs and could be cytoplasmic, central or nucleoplasmic and could stretch up to 80 nm. These results suggest a dynamic exchange between ordered and disordered states. In contrast to diffusion through the NPC, transport cargoes passed through the exclusion zone and accumulated near the central plane. We also show that movement of cargo through the NPC is accompanied by relocation of GLFG domains, suggesting that binding, restructuring and movement of these domains could be part of the translocation mechanism.

Functional insights from studies on the structure of the nuclear pore and coat protein complexes

The nuclear envelope (NE) is a specific extension of the endoplasmic reticulum (ER) that wraps around the nucleus and enables the spatial separation of gene transcription and protein translation, one of the signature features of eukaryotes. Rather than being completely closed, the double lipid bilayer of the NE is perforated at sites where the inner and outer nuclear membranes fuse, resulting in circular openings lined with sharply bent membranes. These openings are filled with nuclear pore complexes (NPCs), enormous protein assemblies that facilitate nuclear transport. The scaffold components of the NPC surprisingly share interesting similarities with elements of coat protein complexes, which have general implications for function and evolution of these membrane-coating complexes. Here I discuss, from a structural perspective, what these findings might teach us.

Connecting the nucleus to the cytoskeleton by SUN-KASH bridges across the nuclear envelope

The nuclear-cytoskeleton connection influences many aspects of cellular architecture, including nuclear positioning, the stiffness of the global cytoskeleton, and mechanotransduction. Central to all of these processes is the assembly and function of conserved SUN-KASH bridges, or LINC complexes, that span the nuclear envelope. Recent studies provide details of the higher order assembly and targeting of SUN proteins to the inner nuclear membrane. Structural studies characterize SUN-KASH interactions that form the central link of the nuclear-envelope bridge. KASH proteins at the outer nuclear membrane link the nuclear envelope to the cytoskeleton where forces are generated to move nuclei. Significantly, SUN proteins were recently shown to contribute to the progression of laminopathies.

The way out: what we know and do not know about herpesvirus nuclear egress

Herpesvirus capsids are assembled in the nucleus of infected cells whereas final maturation occurs in the cytosol. To access the final maturation compartment, intranuclear capsids have to cross the nuclear envelope which represents a formidable barrier. They do so by budding at the inner nuclear membrane, thereby forming a primary enveloped particle residing in the perinuclear cleft. Formation of primary envelopes is driven by a heterodimeric complex of two conserved herpesviral proteins, designated in the herpes simplex virus nomenclature as pUL34, a tail-anchored transmembrane protein located in the nuclear envelope, and pUL31. This nuclear egress complex recruits viral and cellular kinases to soften the nuclear lamina and allowing access of capsids to the inner nuclear membrane. How capsids are recruited to the budding site and into the primary virus particle is still not completely understood, nor is the composition of the primary enveloped virion in the perinuclear cleft. Fusion of the primary envelope with the outer nuclear membrane then results in translocation of the capsid to the cytosol. This fusion event is clearly different from fusion during infectious entry of free virions into target cells in that it does not require the conserved essential core herpesvirus fusion machinery. Nuclear egress can thus be viewed as a vesicle (primary envelope)-mediated transport of cargo (capsids) through thenuclear envelope, a process which had been unique in cell biology. Only recently has a similar process been identified in Drosophila for nuclear egress of large ribonucleoprotein complexes. Thus, herpesviruses appear to subvert a hitherto cryptic cellular pathway for translocation of capsids from the nucleus to the cytosol.