Regulation of YAP Promotor Accessibility in Endothelial Mechanotransduction

BACKGROUND

Endothelial cells are constantly exposed to mechanical forces in the form of fluid shear stress, extracellular stiffness, and cyclic strain. The mechanoresponsive activity of YAP (Yes-associated protein) and its role in vascular development are well described; however, whether changes to transcription or epigenetic regulation of YAP are involved in these processes remains unanswered. Furthermore, how mechanical forces are transduced to the nucleus to drive transcriptional reprogramming in endothelial cells is poorly understood. The YAP target gene, AmotL2 (angiomotin-like 2), is a junctional mechanotransducer that connects cell-cell junctions to the nuclear membrane via the actin cytoskeleton.

METHODS

We applied mechanical manipulations including shear flow, stretching, and substrate stiffness to endothelial cells to investigate the role of mechanical forces in modulating YAP transcription. Using in vitro and in vivo endothelial depletion of AmotL2, we assess nuclear morphology, chromatin organization (using transposase-accessible chromatin sequencing), and whole-mount immunofluorescent staining of the aorta to determine the regulation and functionality of YAP. Finally, we use genetic and chemical inhibition to uncouple the nuclear-cytoskeletal connection to investigate the role of this pathway on YAP transcription.

RESULTS

Our results reveal that mechanical forces sensed at cell-cell junctions by the YAP target gene AmotL2 are directly involved in changes in global chromatin accessibility and activity of the histone methyltransferase EZH2, leading to modulation of YAP promotor activity. Functionally, shear stress-induced proliferation of endothelial cells in vivo was reliant on AmotL2 and YAP/TAZ (transcriptional coactivator with PDZ-binding motif) expression. Mechanistically, uncoupling of the nuclear-cytoskeletal connection from junctions and focal adhesions led to altered nuclear morphology, chromatin accessibility, and YAP promotor activity.

CONCLUSIONS

Our findings reveal a role for AmotL2 and nuclear-cytoskeletal force transmission in modulating the epigenetic and transcriptional regulation of YAP to maintain a mechano-enforced positive feedback loop of vascular homeostasis. These findings may offer an explanation as to the proinflammatory phenotype that leads to aneurysm formation observed in AmotL2 endothelial deletion models.

Membranes that make fat: roles of membrane lipids as acyl donors for triglyceride synthesis and organelle function

Triglycerides constitute an inert storage form for fatty acids deposited in lipid droplets and are mobilized to provide metabolic energy or membrane building blocks. The biosynthesis of triglycerides is highly conserved within eukaryotes and normally involves the sequential esterification of activated fatty acids with a glycerol backbone. Some eukaryotes, however, can also use cellular membrane lipids as direct fatty acid donors for triglyceride synthesis. The biological significance of a pathway that generates triglycerides at the expense of organelle membranes has remained elusive. Here we review current knowledge on how cells use membrane lipids as fatty acid donors for triglyceride synthesis and discuss the hypothesis that a primary function of this pathway is to regulate membrane lipid remodeling and organelle function.

Plant nuclear envelope as a hub connecting genome organization with regulation of gene expression

Eukaryotic cells organize their genome within the nucleus with a double-layered membrane structure termed the nuclear envelope (NE) as the physical barrier. The NE not only shields the nuclear genome but also spatially separates transcription from translation. Proteins of the NE including nucleoskeleton proteins, inner nuclear membrane proteins, and nuclear pore complexes have been implicated in interacting with underlying genome and chromatin regulators to establish a higher-order chromatin architecture. Here, I summarize recent advances in the knowledge of NE proteins that are involved in chromatin organization, gene regulation, and coordination of transcription and mRNA export. These studies support an emerging view of plant NE as a central hub that contributes to chromatin organization and gene expression in response to various cellular and environmental cues.

Identification and characterization of BmNPV Bm5 protein required for the formation of nuclear vesicle structures

BmNPV infection induces nuclear vesicle-like structures and its Bm5 protein mediates the intranuclear lipid accumulation, which is thought to participate in the formation of nuclear vesicles. However, the relationship between viral-induced nuclear vesicles and Bm5 protein is still unclear. Here, our results indicated that BmNPV Bm5 protein participated in the baculovirus infection-induced nuclear vesicle-like structures' invagination thereby influencing the production of occlusion-derived virion (ODV) and occlusion body (OB). The process of nuclear vesicle-like structures' formation was dispensable for the transport or recruitment of ODV major envelope proteins, such as P74 and Bm14. Furthermore, baculovirus-induced nuclear F-actin might provide a direct mechanical force to mediate the scission of large vesicle-like structures from the nuclear membrane. Collectively, these findings illustrated a BmNPV Bm5 protein-induced nuclear membrane invagination pathway and revealed the function of nuclear vesicle-like structures in ODV production.

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.

Tau-FG-nucleoporin98 interaction and impaired nucleocytoplasmic transport in Alzheimer's disease

An emerging pathophysiology associated with the neurodegenerative Alzheimer's disease (AD) is the impairment of nucleocytoplasmic transport (NCT). The impairment can originate from damage to the nuclear pore complex (NPC) or other factors involved in NCT. The phenylalanine-glycine nucleoporins (FG-Nups) form a crucial component of the NPC, which is central to NCT. Recent discoveries have highlighted that the neuropathological protein tau is involved in direct interactions with the FG-Nups and impairment of the NCT process. Targeting such interactions may lead to the identification of novel interaction inhibitors and offer new therapeutic alternatives for the treatment of AD. This review highlights recent findings associated with impaired NCT in AD and the interaction between tau and the FG-Nups.

Components and Mechanisms of Nuclear Mechanotransduction

The cell nucleus is best known as the container of the genome. Its envelope provides a barrier for passive macromolecule diffusion, which enhances the control of gene expression. As its largest and stiffest organelle, the nucleus also defines the minimal space requirements of a cell. Internal or external pressures that deform a cell to its physical limits cause a corresponding nuclear deformation. Evidence is consolidating that the nucleus, in addition to its genetic functions, serves as a physical sensing device for critical cell body deformation. Nuclear mechanotransduction allows cells to adapt their acute behaviors, mechanical stability, paracrine signaling, and fate to their physical surroundings. This review summarizes the basic chemical and mechanical properties of nuclear components, and how these properties are thought to be utilized for mechanosensing.

Torsin and NEP1R1-CTDNEP1 phosphatase affect interphase nuclear pore complex insertion by lipid-dependent and lipid-independent mechanisms

The interphase nuclear envelope (NE) is extensively remodeled during nuclear pore complex (NPC) insertion. How this remodeling occurs and why it requires Torsin ATPases, which also regulate lipid metabolism, remains poorly understood. Here, we show that Drosophila Torsin (dTorsin) affects lipid metabolism via the NEP1R1‐CTDNEP1 phosphatase and the Lipin phosphatidic acid (PA) phosphatase. This includes that Torsins remove NEP1R1‐CTDNEP1 from the NE in fly and mouse cells, leading to subsequent Lipin exclusion from the nucleus. NEP1R1‐CTDNEP1 downregulation also restores nuclear pore membrane fusion in post‐mitotic dTorsin^KO fat body cells. However, dTorsin‐associated nuclear pore defects do not correlate with lipidomic abnormalities and are not resolved by silencing of Lipin. Further testing confirmed that membrane fusion continues in cells with hyperactivated Lipin. It also led to the surprising finding that excessive PA metabolism inhibits recruitment of the inner ring complex Nup35 subunit, resulting in elongated channel‐like structures in place of mature nuclear pores. We conclude that the NEP1R1‐CTDNEP1 phosphatase affects interphase NPC biogenesis by lipid‐dependent and lipid‐independent mechanisms, explaining some of the pleiotropic effects of Torsins.

Response characteristics and optimization of electroporation: simulation based on finite element method

Electroporation has been widely used in biology, medicine, and the food industry as a means to transport various molecules through the cell membrane. The phenomenon of electroporation is the result of cell membrane damage caused by the application of an electric field. In order to understand more precisely how cells function, we established a dielectric model of a spherical cell and analyzed its characteristics by the finite element method. The effects of altering different electrical parameters were determined. The results showed that the electric field strength was positively related to the transmembrane voltage (TMV) and pore density. There was a minimum electric field strength necessary to induce a critical TMV for the formation of pores. Pulse width also had to be long enough to charge the cell membrane, compared with the normal membrane charging time constant of about 1 μs. When the pulse width was shorter than the charging time constant, it was necessary to increase pulse frequency to create a high enough TMV. The rise-time of the electric pulse also affected electroporation: a fast rise-time pulse not only allowed penetration of the plasma membrane but also the organelle membrane. With slow rise-time pulse, the organelle was shielded from electroporation. This study defines the response characteristics of electrical parameters on the electric load cell and establishes the specificity of parameters for different purposes.

The Alkylating Agent Methyl Methanesulfonate Triggers Lipid Alterations at the Inner Nuclear Membrane That Are Independent from Its DNA-Damaging Ability

In order to tackle the study of DNA repair pathways, the physical and chemical agents creating DNA damage, the genotoxins, are frequently employed. Despite their utility, their effects are rarely restricted to DNA, and therefore simultaneously harm other cell biomolecules. Methyl methanesulfonate (MMS) is an alkylating agent that acts on DNA by preferentially methylating guanine and adenine bases. It is broadly used both in basic genome stability research and as a model for mechanistic studies to understand how alkylating agents work, such as those used in chemotherapy. Nevertheless, MMS exerts additional actions, such as oxidation and acetylation of proteins. In this work, we introduce the important notion that MMS also triggers a lipid stress that stems from and affects the inner nuclear membrane. The inner nuclear membrane plays an essential role in virtually all genome stability maintenance pathways. Thus, we want to raise awareness that the relative contribution of lipid and genotoxic stresses when using MMS may be difficult to dissect and will matter in the conclusions drawn from those studies.

A Pro-Tumorigenic Effect of Heparanase 2 (Hpa2) in Thyroid Carcinoma Involves Its Localization to the Nuclear Membrane

Activity of the endo-beta-glucuronidase heparanase, capable of cleaving heparan sulfate (HS), is most often elevated in many types of tumors, associating with increased tumor metastasis and decreased patients' survival. Heparanase is therefore considered to be a valid drug target, and heparanase inhibitors are being evaluated clinically in cancer patients. Heparanase 2 (Hpa2) is a close homolog of heparanase that gained very little attention, likely because it lacks HS-degrading activity typical of heparanase. The role of Hpa2 in cancer was not examined in detail. In head and neck cancer, high levels of Hpa2 are associated with decreased tumor cell dissemination to regional lymph nodes and prolonged patients' survival, suggesting that Hpa2 functions to attenuate tumor growth. Here, we examined the role of Hpa2 in normal thyroid tissue and in benign thyroid tumor, non-metastatic, and metastatic papillary thyroid carcinoma (PTC) utilizing immunostaining in correlation with clinicopathological parameters. Interestingly, we found that Hpa2 staining intensity does not significantly change in the transition from normal thyroid gland to benign, non-metastatic, or metastatic thyroid carcinoma. Remarkably, we observed that in some biopsies, Hpa2 is accumulating on the membrane (envelop) of the nucleus and termed this cellular localization NM (nuclear membrane). Notably, NM localization of Hpa2 occurred primarily in metastatic PTC and was associated with an increased number of positive (metastatic) lymph nodes collected at surgery. These results describe for the first time unrecognized localization of Hpa2 to the nuclear membrane, implying that in PTC, Hpa2 functions to promote tumor metastasis.

Characterizing the degeneration of nuclear membrane and mitochondria of adipose-derived mesenchymal stem cells from patients with type II diabetes

Regenerative therapeutic approaches involving the transplantation of stem cells differentiated into insulin‐producing cells are being studied in patients with rapidly progressing severe diabetes. Adipose‐derived mesenchymal stem cells have been reported to have varied cellular characteristics depending on the biological environment of the location from which they were harvested. However, the characteristics of mesenchymal stem cells in type II diabetes have not been clarified. In this study, we observed the organelles of mesenchymal stem cells from patients with type II diabetes under a transmission electron microscope to determine the structure of stem cells in type II diabetes. Transmission electron microscopic observation of mesenchymal stem cells from healthy volunteers (N‐ADSC) and those from patients with type II diabetes (T2DM‐ADSC) revealed enlarged nuclei and degenerated mitochondrial cristae in T2DM‐ADSCs. Moreover, T2DM‐ADSCs were shown to exhibit a lower expression of Emerin, a constituent protein of the nuclear membrane, and a decreased level of mitochondrial enzyme activity. In this study, we successfully demonstrated the altered structure of nuclear membrane and the decreased mitochondrial enzyme activity in adipose‐derived mesenchymal cells from patients with type II diabetes. These findings have contributed to the understanding of type II diabetes‐associated changes in mesenchymal stem cells used for regenerative therapy.

New kid on the block: lipid droplets in the nucleus

The regulation of lipid homeostasis is essential for normal cell physiology, and its disruption can lead to disease. Lipid droplets (LDs) are ubiquitous organelles dedicated to storing nonpolar lipids that are used for metabolic energy production or membrane biogenesis. LDs normally emerge from, and associate with, the endoplasmic reticulum and interact with other cytoplasmic organelles to deliver the stored lipids. Recently, LDs were found to reside also at the inner side of the nuclear envelope and inside the nucleus in yeast and mammalian cells. This unexpected finding raises fundamental questions about the nature of the inner nuclear membrane, its connection with the endoplasmic reticulum and the pathways of LD formation. In this viewpoint, we will highlight recent developments relating to these questions and discuss possible roles of LDs in nuclear physiology.

Role of Nucleoporins and Transport Receptors in Cell Differentiation

Bidirectional molecular movements between the nucleus and cytoplasm take place through nuclear pore complexes (NPCs) embedded in the nuclear membrane. These macromolecular structures are composed of several nucleoporins, which form seven different subcomplexes based on their biochemical affinity. These nucleoporins are integral components of the complex, not only allowing passive transport but also interacting with importin, exportin, and other molecules that are required for transport of protein in various cellular processes. Transport of different proteins is carried out either dependently or independently on transport receptors. As well as facilitating nucleocytoplasmic transport, nucleoporins also play an important role in cell differentiation, possibly by their direct gene interaction. This review will cover the general role of nucleoporins (whether its dependent or independent) and nucleocytoplasmic transport receptors in cell differentiation.

CRISPR-Cas9/phosphoproteomics identifies multiple noncanonical targets of myosin light chain kinase

Prior studies have implicated myosin light chain kinase (MLCK) in the regulation of aquaporin-2 (AQP2) in the renal collecting duct. To discover signaling targets of MLCK, we used CRISPR-Cas9 to delete the MLCK gene (Mylk) to obtain MLCK-null mpkCCD cells and carried out comprehensive phosphoproteomics using stable isotope labeling with amino acids in cell culture for quantification. Immunocytochemistry and electron microscopy demonstrated a defect in the processing of AQP2-containing early endosomes to late endosomes. The phosphoproteomics experiments revealed that, of the 1,743 phosphopeptides quantified over multiple replicates, 107 were changed in abundance by MLCK deletion (29 decreased and 78 increased). One of the decreased phosphopeptides corresponded to the canonical target site in myosin regulatory light chain. Network analysis indicated that targeted phosphoproteins clustered into distinct structural/functional groups: actomyosin, signaling, nuclear envelope, gene transcription, mRNA processing, energy metabolism, intermediate filaments, adherens junctions, and tight junctions. There was significant overlap between the derived MLCK signaling network and a previously determined PKA signaling network. The presence of multiple proteins in the actomyosin category prompted experiments showing that MLCK deletion inhibits the normal effect of vasopressin to depolymerize F-actin, providing a potential explanation for the AQP2 trafficking defect. Changes in phosphorylation of multiple proteins in the nuclear envelope prompted measurement of nuclear size, showing a significant increase in average nuclear volume. We conclude that MLCK is part of a multicomponent signaling pathway in both the cytoplasm and nucleus that includes much more than simple regulation of conventional nonmuscle myosins through myosin regulatory light chain phosphorylation.

Analysis of RNA-Seq datasets reveals enrichment of tissue-specific splice variants for nuclear envelope proteins

Laminopathies yield tissue-specific pathologies, yet arise from mutation of ubiquitously-expressed genes. A little investigated hypothesis to explain this is that the mutated proteins or their partners have tissue-specific splice variants. To test this, we analyzed RNA-Seq datasets, finding novel isoforms or isoform tissue-specificity for: Lap2, linked to cardiomyopathy; Nesprin 2, linked to Emery-Dreifuss muscular dystrophy and Lmo7, that regulates the Emery-Dreifuss muscular dystrophy linked emerin gene. Interestingly, the muscle-specific Lmo7 exon is rich in serine phosphorylation motifs, suggesting regulatory function. Muscle-specific splice variants in non-nuclear envelope proteins linked to other muscular dystrophies were also found. Nucleoporins tissue-specific variants were found for Nup54, Nup133, Nup153 and Nup358/RanBP2. RT-PCR confirmed novel Lmo7 and RanBP2 variants and specific knockdown of the Lmo7 variantreduced myogenic index. Nuclear envelope proteins were enriched for tissue-specific splice variants compared to the rest of the genome, suggesting that splice variants contribute to its tissue-specific functions.

Compartmentalized Synthesis of Triacylglycerol at the Inner Nuclear Membrane Regulates Nuclear Organization

Cells dynamically adjust organelle organization in response to growth and environmental cues. This requires regulation of synthesis of phospholipids, the building blocks of organelle membranes, or remodeling of their fatty-acyl (FA) composition. FAs are also the main components of triacyglycerols (TGs), which enable energy storage in lipid droplets. How cells coordinate FA metabolism with organelle biogenesis during cell growth remains unclear. Here, we show that Lro1, an acyltransferase that generates TGs from phospholipid-derived FAs in yeast, relocates from the endoplasmic reticulum to a subdomain of the inner nuclear membrane. Lro1 nuclear targeting is regulated by cell cycle and nutrient starvation signals and is inhibited when the nucleus expands. Lro1 is active at this nuclear subdomain, and its compartmentalization is critical for nuclear integrity. These data suggest that Lro1 nuclear targeting provides a site of TG synthesis, which is coupled with nuclear membrane remodeling.

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Nutrients regulate the phospholipid:diacylglycerol acyltransferase Lro1

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Lro1 targets the INM subdomain bordering the nucleolus

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Lro1 is active at the INM and can sustain cell survival during starvation

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Compartmentalized synthesis of nuclear TG is important for nuclear envelope integrity

Predominant localization of phosphatidylserine at the cytoplasmic leaflet of the ER, and its TMEM16K-dependent redistribution

TMEM16K, a membrane protein carrying 10 transmembrane regions, has phospholipid scramblase activity. TMEM16K is localized to intracellular membranes, but whether it actually scrambles phospholipids inside cells has not been demonstrated, due to technical difficulties in studying intracellular lipid distributions. Here, we developed a freeze-fracture electron microscopy method that enabled us to determine the phosphatidylserine (PtdSer) distribution in the individual leaflets of cellular membranes. Using this method, we found that the endoplasmic reticulum (ER) of mammalian cells harbored abundant PtdSer in its cytoplasmic leaflet and much less in the luminal leaflet, whereas the outer and inner nuclear membranes (NMs) had equivalent amounts of PtdSer in both leaflets. The ER and NMs of budding yeast also harbored PtdSer in their cytoplasmic leaflet, but asymmetrical distribution in the ER was not observed. Treating mouse embryonic fibroblasts with the Ca2+ ionophore A23187 compromised the cytoplasmic leaflet-dominant PtdSer asymmetry in the ER and increased PtdSer in the NMs, especially in the nucleoplasmic leaflet of the inner NM. This Ca2+-induced PtdSer redistribution was not observed in TMEM16K-null fibroblasts, but was recovered in these cells by reexpressing TMEM16K. These results indicate that, similar to the plasma membrane, PtdSer in the ER of mammalian cells is predominantly localized to the cytoplasmic leaflet, and that TMEM16K directly or indirectly mediates Ca2+-dependent phospholipid scrambling in the ER.

Roles of the Interhexamer Contact Site for Hexagonal Lattice Formation of the Herpes Simplex Virus 1 Nuclear Egress Complex in Viral Primary Envelopment and Replication

During the nuclear export of nascent nucleocapsids of herpes simplex virus 1 (HSV-1), the nucleocapsids acquire a primary envelope by budding through the inner nuclear membrane into the perinuclear space between the inner and outer nuclear membranes. This unique budding process, termed primary envelopment, is initiated by the nuclear egress complex (NEC), composed of the HSV-1 UL31 and UL34 proteins. Earlier biochemical approaches have shown that the NEC has an intrinsic ability to vesiculate membranes through the formation of a hexagonal lattice structure. The significance of intrahexamer interactions of the NEC in the primary envelopment of HSV-1-infected cells has been reported. In contrast, the contribution of lattice formation of the NEC hexamer to primary envelopment in HSV-1-infected cells remains to be elucidated. Therefore, we constructed and characterized a recombinant HSV-1 strain carrying an amino acid substitution in a UL31 residue that is an interhexamer contact site for the lattice formation of the NEC hexamer. This mutation was reported to destabilize the interhexamer interactions of the HSV-1 NEC. Here, we demonstrate that the mutation causes the aberrant accumulation of nucleocapsids in the nucleus and reduces viral replication in Vero and HeLa cells. Thus, the ability of HSV-1 to form the hexagonal lattice structure of the NEC was linked to an increase in primary envelopment and viral replication. Our results suggest that the lattice formation of the NEC hexamer has an important role in HSV-1 replication by regulating primary envelopment.IMPORTANCE The scaffolding proteins of several envelope viruses required for virion assembly form high-order lattice structures. However, information on the significance of their lattice formation in infected cells is limited. Herpesviruses acquire envelopes twice during their viral replication. The first envelop acquisition (primary envelopment) is one of the steps in the vesicle-mediated nucleocytoplasmic transport of nascent nucleocapsids, which is unique in biology. HSV-1 NEC, thought to be conserved in all members of the Herpesviridae family, is critical for primary envelopment and was shown to form a hexagonal lattice structure. Here, we investigated the significance of the interhexamer contact site for hexagonal lattice formation of the NEC in HSV-1-infected cells and present evidence suggesting that the lattice formation of the NEC hexamer has an important role in HSV-1 replication by regulating primary envelopment. Our results provide insights into the mechanisms of the envelopment of herpesviruses and other envelope viruses.

Postmitotic annulate lamellae assembly contributes to nuclear envelope reconstitution in daughter cells

In higher eukaryotic cells, the nuclear envelope (NE) is composed of double nuclear membranes studded with nuclear pore complexes (NPCs) and undergoes dynamic disassembly and reassembly during the cell cycle. However, how the NE and NPC reassemble remains largely unclear. Here, using HeLa, HEK293, and Drosophila cells, along with immunofluorescence microscopy and transmission EM methods, we found that postmitotic annulate lamellae (AL) assembly contributes to NE and NPC assembly. We observed that the AL are parallel membrane-pair stacks and possess regularly spaced AL pore complexes (ALPCs) that are morphologically similar to the NPCs. We found that the AL assemble in the cytoplasm during mitotic exit simultaneously with NE re-formation in daughter cells. Then, the assembled AL either bound the decondensing chromatin to directly transform into the NE or bound and fused with the outer nuclear membrane to join the assembling NE. The AL did not colocalize with sheet and tubular endoplasmic reticulum (ER) marker proteins on the ER or the lamin B receptor-localized membrane in the cytoplasm, suggesting that postmitotic AL assembly occurs independently of the chromatin and ER. Collectively, our results indicate that postmitotic AL assembly is a common cellular event and an intermediate step in NE and NPC assembly and in NE expansion in higher eukaryotic cells.

Abnormal Microtubule Dynamics Impair the Nuclear-Cytoplasmic Transport in Dementia

Disrupted nuclear-cytoplasmic transport (NCT) is a common pathophysiological event in several neurodegenerative disorders. However, the correlation between the mutations in the pathogenic microtubule-associated protein tau and NCT and neuronal dysfunction is not yet clearly understood. A recent study revealed that tau is mislocalized to the neuronal cell body and, thus, deforms the nuclear membrane in the frontotemporal dementia (FTD). This causes a defect in NCT, leading to neurodegeneration. The microtubule depolymerization could rescue the NCT defects as well as neurodegeneration. Therefore, agents that can modulate the microtubule functions or NCT can constitute a potential therapeutic method for the treatment of neurodegenerative disorders.

Microtubules Deform the Nuclear Membrane and Disrupt Nucleocytoplasmic Transport in Tau-Mediated Frontotemporal Dementia

The neuronal microtubule-associated protein tau, MAPT, is central to the pathogenesis of many dementias. Autosomal-dominant mutations in MAPT cause inherited frontotemporal dementia (FTD), but the underlying pathogenic mechanisms are unclear. Using human stem cell models of FTD due to MAPT mutations, we find that tau becomes hyperphosphorylated and mislocalizes to cell bodies and dendrites in cortical neurons, recapitulating a key early event in FTD. Mislocalized tau in the cell body leads to abnormal microtubule movements in FTD-MAPT neurons that grossly deform the nuclear membrane. This results in defective nucleocytoplasmic transport, which is corrected by microtubule depolymerization. Neurons in the post-mortem human FTD-MAPT cortex have a high incidence of nuclear invaginations, indicating that tau-mediated nuclear membrane dysfunction is an important pathogenic process in FTD. Defects in nucleocytoplasmic transport in FTD point to important commonalities in the pathogenic mechanisms of tau-mediated dementias and ALS-FTD due to TDP-43 and C9orf72 mutations.

Nuclear Lipid Microdomains Regulate Daunorubicin Resistance in Hepatoma Cells

Daunorubicin is an anticancer drug, and cholesterol is involved in cancer progression, but their relationship has not been defined. In this study, we developed a novel experimental model that utilizes daunorubicin, cholesterol, and daunorubicin plus cholesterol in the same cells (H35) to search for the role of nuclear lipid microdomains, rich in cholesterol and sphingomyelin, in drug resistance. We find that the daunorubicin induces perturbation of nuclear lipid microdomains, localized in the inner nuclear membrane, where active chromatin is anchored. As changes of sphingomyelin species in nuclear lipid microdomains depend on neutral sphingomyelinase activity, we extended our studies to investigate whether the enzyme is modulated by daunorubicin. Indeed the drug stimulated the sphingomyelinase activity that induced reduction of saturated long chain fatty acid sphingomyelin species in nuclear lipid microdomains. Incubation of untreated-drug cells with high levels of cholesterol resulted in the inhibition of sphingomyelinase activity with increased saturated fatty acid sphingomyelin species. In daunodubicin-treated cells, incubation with cholesterol reversed the action of the drug by acting via neutral sphingomyelinase. In conclusion, we suggest that cholesterol and sphingomyelin-forming nuclear lipid microdomains are involved in the drug resistance.

A nuclear membrane-derived structure associated with Atg8 is involved in the sequestration of selective cargo, the Cvt complex, during autophagosome formation in yeast

Macroautophagy (hereafter autophagy) is a conserved intracellular degradation mechanism required for cell survival. A double-membrane structure, the phagophore, is generated to sequester cytosolic cargos destined for degradation in the vacuole. The mechanism involved in the biogenesis of the phagophore is still an open question. We focused on 4 autophagy-related (Atg) proteins (Atg2, Atg9, Atg14, and Atg18), which are involved in the formation of the phagophore in order to gain a more complete understanding of the membrane dynamics that occur during formation of the autophagosome. The corresponding mutants, while defective in autophagy, nonetheless generate the membrane-bound form of Atg8, allowing us to use this protein as a marker for the nascent autophagosome precursor membrane. Using electron microscopy (EM), we discovered in these atg mutants a novel single-membrane structure (~120 to 150 nm in size). Electron tomography revealed that this structure originates from a part of the nuclear membrane, and we have named it the alphasome. Our data suggest that the alphasome is associated with Atg8, and sequesters selective cargo, the Cvt complex, during autophagy. Abbreviations: 3D: three-dimensional; AB: autophagic body; AP: autophagosome; Atg: autophagy-related; Cvt: cytoplasm-to-vacuole targeting; EM: electron microscopy; IEM: immunoelectron microscopy; L: lipid droplet; N: nucleus; NM: nuclear membrane; PAS: phagophore assembly site; PE: phosphatidylethanolamine; prApe1: precursor aminopeptidase I; rER: rough endoplasmic reticulum; TEM: transmission electron microscopy; V: vacuole; VLP: virus-like particle.

LEM domain-containing protein 3 antagonizes TGFβ-SMAD2/3 signaling in a stiffness-dependent manner in both the nucleus and cytosol

Transforming growth factor-β (TGFβ) signaling through SMAD2/3 is an important driver of pathological fibrosis in multiple organ systems. TGFβ signaling and extracellular matrix (ECM) stiffness form an unvirtuous pathological circuit in which matrix stiffness drives activation of latent TGFβ, and TGFβ signaling then drives cellular stress and ECM synthesis. Moreover, ECM stiffness also appears to sensitize cells to exogenously activated TGFβ through unknown mechanisms. Here, using human fibroblasts, we explored the effect of ECM stiffness on a putative inner nuclear membrane protein, LEM domain-containing protein 3 (LEMD3), which is physically connected to the cell's actin cytoskeleton and inhibits TGFβ signaling. We showed that LEMD3-SMAD2/3 interactions are inversely correlated with ECM stiffness and TGFβ-driven luciferase activity and that LEMD3 expression is correlated with the mechanical response of the TGFβ-driven luciferase reporter. We found that actin polymerization but not cellular stress or LEMD3-nuclear-cytoplasmic couplings were necessary for LEMD3-SMAD2/3 interactions. Intriguingly, LEMD3 and SMAD2/3 frequently interacted in the cytosol, and we discovered LEMD3 was proteolytically cleaved into protein fragments. We confirmed that a consensus C-terminal LEMD3 fragment binds SMAD2/3 in a stiffness-dependent manner throughout the cell and is sufficient for antagonizing SMAD2/3 signaling. Using human lung biopsies, we observed that these nuclear and cytosolic interactions are also present in tissue and found that fibrotic tissues exhibit locally diminished and cytoplasmically shifted LEMD3-SMAD2/3 interactions, as noted in vitro Our work reveals novel LEMD3 biology and stiffness-dependent regulation of TGFβ by LEMD3, providing a novel target to antagonize pathological TGFβ signaling.

Lem2 is retained at the nuclear envelope through its interaction with Bqt4 in fission yeast

Inner nuclear membrane (INM) proteins are thought to play important roles in modulating nuclear organization and function through their interactions with chromatin. However, these INM proteins share redundant functions in metazoans that pose difficulties for functional studies. The fission yeast Schizosaccharomyces pombe exhibits a relatively small number of INM proteins, and molecular genetic tools are available to separate their redundant functions. In S. pombe, it has been reported that among potentially redundant INM proteins, Lem2 displays a unique genetic interaction with another INM protein, Bqt4, which is involved in anchoring telomeres to the nuclear envelope. Double mutations in the lem2 and bqt4 genes confer synthetic lethality during vegetative growth. Here, we show that Lem2 is retained at the nuclear envelope through its interaction with Bqt4, as the loss of Bqt4 results in the exclusive accumulation of Lem2 to the spindle pole body (SPB). An N-terminal nucleoplasmic region of Lem2 bears affinity to both Bqt4 and the SPB in a competitive manner. In contrast, the synthetic lethality of the lem2 bqt4 double mutant is suppressed by the C-terminal region of Lem2. These results indicate that the N-terminal and C-terminal domains of Lem2 show independent functions with respect to Bqt4.

Nuclear membrane-localised NOX4D generates pro-survival ROS in FLT3-ITD-expressing AML

Internal tandem duplication of the juxtamembrane domain of FMS-like tyrosine kinase 3 (FLT3-ITD) is the most prevalent genetic aberration present in 20-30% of acute myeloid leukaemia (AML) cases and is associated with a poor prognosis. FLT3-ITD expressing cells express elevated levels of NADPH oxidase 4 (NOX4)-generated pro-survival hydrogen peroxide (H₂O₂) contributing to increased levels of DNA oxidation and double strand breaks. NOX4 is constitutively active and has been found to have various isoforms expressed at multiple locations within a cell. The purpose of this study was to investigate the expression, localisation and regulation of NOX4 28 kDa splice variant, NOX4D. NOX4D has previously been shown to localise to the nucleus and nucleolus in various cell types and is implicated in the generation of reactive oxygen species (ROS) and DNA damage. Here, we demonstrate that FLT3-ITD expressing-AML patient samples as well as -cell lines express the NOX4D isoform resulting in elevated H₂O₂ levels compared to FLT3-WT expressing cells, as quantified by flow cytometry. Cell fractionation indicated that NOX4D is nuclear membrane-localised in FLT3-ITD expressing cells. Treatment of MV4-11 cells with receptor trafficking inhibitors, tunicamycin and brefeldin A, resulted in deglycosylation of NOX4 and NOX4D. Inhibition of the FLT3 receptor revealed that the FLT3-ITD oncogene is responsible for the production of NOX4D-generated H₂O₂ in AML. We found that inhibition of the PI3K/AKT and STAT5 pathways resulted in down-regulation of NOX4D-generated pro-survival ROS. Taken together these findings indicate that nuclear membrane-localised NOX4D-generated pro-survival H₂O₂ may be contributing to genetic instability in FLT3-ITD expressing AML.

Samp1, a RanGTP binding transmembrane protein in the inner nuclear membrane

Samp1 is a transmembrane protein of the inner nuclear membrane (INM), which interacts with the nuclear lamina and the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex in interphase and during mitosis, it localizes to the mitotic spindle. Samp1 was recently found to coprecipitate a protein complex containing Ran, a GTPase with fundamental regulatory functions both in interphase and in mitosis. To investigate the interaction between Samp1 and Ran in further detail, we have designed and expressed recombinant fusion proteins of the Chaetomium thermophilum homolog of Samp1 (Ct.Samp1) and human Ran. Pulldown experiments show that Samp1 binds directly to Ran and that Samp1 binds better to RanGTP compared to RanGDP. Samp1 also preferred RanGTP over RanGDP in living tsBN2 cells. We also show that the Ran binding domain is located between amino acids 75–135 in the nucleoplasmically exposed N-terminal tail of Samp1. This domain is unique for Samp1, without homology in any other proteins in fungi or metazoa. Samp1 is the first known transmembrane protein that binds to Ran and could provide a unique local binding site for RanGTP in the INM. Samp1 overexpression resulted in increased Ran concentrations in the nuclear periphery supporting this idea.

Torsin ATPases: Harnessing Dynamic Instability for Function

Torsins are essential, disease-relevant AAA+ (ATPases associated with various cellular activities) proteins residing in the endoplasmic reticulum and perinuclear space, where they are implicated in a variety of cellular functions. Recently, new structural and functional details about Torsins have emerged that will have a profound influence on unraveling the precise mechanistic details of their yet-unknown mode of action in the cell. While Torsins are phylogenetically related to Clp/HSP100 proteins, they exhibit comparatively weak ATPase activities, which are tightly controlled by virtue of an active site complementation through accessory cofactors. This control mechanism is offset by a TorsinA mutation implicated in the severe movement disorder DYT1 dystonia, suggesting a critical role for the functional Torsin-cofactor interplay in vivo. Notably, TorsinA lacks aromatic pore loops that are both conserved and critical for the processive unfolding activity of Clp/HSP100 proteins. Based on these distinctive yet defining features, we discuss how the apparent dynamic nature of the Torsin-cofactor system can inform emerging models and hypotheses for Torsin complex formation and function. Specifically, we propose that the dynamic assembly and disassembly of the Torsin/cofactor system is a critical property that is required for Torsins' functional roles in nuclear trafficking and nuclear pore complex assembly or homeostasis that merit further exploration. Insights obtained from these future studies will be a valuable addition to our understanding of disease etiology of DYT1 dystonia.

Identification of candidates for interacting partners of the tail domain of DcNMCP1, a major component of the Daucus carota nuclear lamina-like structure

NMCP/CRWN (NUCLEAR MATRIX CONSTITUENT PROTEIN/CROWDED NUCLEI) is a major component of a protein fibrous meshwork (lamina-like structure) on the plant inner nuclear membrane. NMCP/CRWN contributes to regulating nuclear shape and nuclear functions. An NMCP/CRWN protein in Daucus carota (DcNMCP1) is localized to the nuclear periphery in interphase cells, and surrounds chromosomes in cells in metaphase and anaphase. The N-terminal region and the C-terminal region of DcNMCP1 are both necessary for localizing DcNMCP1 to the nuclear periphery. Here candidate interacting partners of the amino acid position 975-1053 of DcNMCP1 (T975-1053), which is present in the C-terminal region and contains a conserved sequence that plays a role in localizing DcNMCP1 to the nuclear periphery, are screened for. Arabidopsis thaliana nuclear proteins were subjected to far-Western blotting with GST-fused T975-1053 as a probe, and signals were detected at the positions corresponding to ∼70, ∼40, and ∼18 kDa. These ∼70, ∼40, and ∼18 kDa nuclear proteins were identified by mass spectrometry, and subjected to a yeast 2-hybrid (Y2H) analysis with T975-1053 as bait. In this analysis, the ∼40 kDa protein ARP7, which is a nuclear actin-related protein possibly involved in regulating chromatin structures, was confirmed to interact with T975-1053. Independently of the far-Western blotting, a Y2H screen was performed using T975-1053 as bait. Targeted Y2H assays confirmed that 3 proteins identified in the screen, MYB3, SINAT1, and BIM1, interact with T975-1053. These proteins might have roles in NMCP/CRWN protein-mediated biologic processes.

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.

Acacia catechu ethanolic bark extract induces apoptosis in human oral squamous carcinoma cells

Oral cancer is in approximately 30% of all cancers in India. This study was conducted to evaluate the cytotoxic activity of ethanolic extract of Acacia catechu bark (ACB) against human squamous cell carcinoma cell line-25 (SCC-25). Cytotoxic effect of ACB extract was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium Bromide assay. A. catechu extract was treated SCC-25 cells with 25 and 50 μg/mL for 24 h. Apoptosis markers such as caspases-8 and 9, bcl-2, bax, and cytochrome c (Cyt-c) were done by RT-PCR. Morphological changes of ACB treated cells were evaluated using acridine orange/ethidium bromide (AO/EB) dual staining. Nuclear morphology and DNA fragmentation were evaluated using propidium iodide (PI) staining. Further, cell cycle analysis was performed using flow cytometry. A. catechu treatment caused cytotoxicity in SCC-25 cells with an IC₅₀ of 52.09 μg/mL. Apoptotic marker gene expressions were significantly increased on ACB treatment. Staining with AO/EB and PI shows membrane blebbing and nuclear membrane distortion, respectively, and it confirms the apoptosis induction in SCC-25 cells. These results suggest that ACB extract can be used as a modulating agent in oral squamous cell carcinoma.

Spatial distribution of lipid droplets during starvation: Implications for lipophagy

Survival during starvation depends largely on metabolic energy, which is stored in the form of neutral lipids in specialized organelles known as lipid droplets. The precursors for the synthesis of neutral lipids are also used for membrane biogenesis, which is required for cell growth and proliferation. Therefore cells must possess mechanisms to preferentially channel lipid precursors toward either membrane synthesis or lipid droplet storage, in response to nutrient status. How this partitioning is spatially regulated within the endoplasmic reticulum (ER) where lipid droplets co-localize, remains poorly understood. We have recently shown that at the onset of starvation lipid droplets concentrate at a perinuclear ER subdomain flanking the nucleus-vacuole junction (NVJ) and that this is crucial for maintaining proper nuclear shape and ER membrane organization. Here we show that disruption of the NVJ does not block the translocation and internalization of lipid droplets into the vacuole for their degradation, which takes place at later stages of starvation. We propose that alternative pathways of lipid droplet translocation from the ER to the vacuole may exist to enable stationary phase-induced lipophagy.

Induction of intranuclear membranes by overproduction of Opi1p and Scs2p, regulators for yeast phospholipid biosynthesis, suggests a mechanism for Opi1p nuclear translocation

In the yeast Saccharomyces cerevisiae, the expression of phospholipid biosynthetic genes is suppressed by the Opi1p negative regulator. Opi1p enters into the nucleoplasm from the nuclear membrane to suppress the gene expression under repressing conditions. The binding of Opi1p to the nuclear membrane requires an integral membrane protein, Scs2p and phosphatidic acid (PA). Although it is demonstrated that the association of Opi1p with membranes is affected by PA levels, how Opi1p dissociates from Scs2p is unknown. Here, we found that fluorescently labelled Opi1p accumulated on a perinuclear region in an Scs2p-dependent manner. Electron microscopic analyses indicated that the perinuclear region consists of intranuclear membranes, which may be formed by the invagination of the nuclear membrane due to the accumulation of Opi1p and Scs2p in a restricted area. As expected, localization of Opi1p and Scs2p in the intranuclear membranes was detected by immunoelectron microscopy. Biochemical analysis showed that Opi1p recovered in the membrane fraction was detergent insoluble while Scs2p was soluble, implying that Opi1p behaves differently from Scs2p in the fraction. We hypothesize that Opi1p dissociates from Scs2p after targeting to the nuclear membrane, making it possible to be released from the membrane quickly when PA levels decrease.

Minimization of extracellular space as a driving force in prokaryote association and the origin of eukaryotes

BACKGROUND

Internalization-based hypotheses of eukaryotic origin require close physical association of host and symbiont. Prior hypotheses of how these associations arose include chance, specific metabolic couplings between partners, and prey-predator/parasite interactions. Since these hypotheses were proposed, it has become apparent that mixed-species, close-association assemblages (biofilms) are widespread and predominant components of prokaryotic ecology. Which forces drove prokaryotes to evolve the ability to form these assemblages are uncertain. Bacteria and archaea have also been found to form membrane-lined interconnections (nanotubes) through which proteins and RNA pass. These observations, combined with the structure of the nuclear envelope and an energetic benefit of close association (see below), lead us to propose a novel hypothesis of the driving force underlying prokaryotic close association and the origin of eukaryotes.

RESULTS

Respiratory proton transport does not alter external pH when external volume is effectively infinite. Close physical association decreases external volume. For small external volumes, proton transport decreases external pH, resulting in each transported proton increasing proton motor force to a greater extent. We calculate here that in biofilms this effect could substantially decrease how many protons need to be transported to achieve a given proton motor force. Based as it is solely on geometry, this energetic benefit would occur for all prokaryotes using proton-based respiration.

CONCLUSIONS

This benefit may be a driving force in biofilm formation. Under this hypothesis a very wide range of prokaryotic species combinations could serve as eukaryotic progenitors. We use this observation and the discovery of prokaryotic nanotubes to propose that eukaryotes arose from physically distinct, functionally specialized (energy factory, protein factory, DNA repository/RNA factory), obligatorily symbiotic prokaryotes in which the protein factory and DNA repository/RNA factory cells were coupled by nanotubes and the protein factory ultimately internalized the other two. This hypothesis naturally explains many aspects of eukaryotic physiology, including the nuclear envelope being a folded single membrane repeatedly pierced by membrane-bound tubules (the nuclear pores), suggests that species analogous or homologous to eukaryotic progenitors are likely unculturable as monocultures, and makes a large number of testable predictions.

REVIEWERS

This article was reviewed by Purificación López-García and Toni Gabaldón.

The missing LINC: a mammalian KASH-domain protein coupling meiotic chromosomes to the cytoskeleton

Pairing of homologous chromosome is a unique event in meiosis that is essential for both haploidization of the genome and genetic recombination. Rapid chromosome movements during meiotic prophase are a key feature of the pairing process. This is usually telomere-led, and in metazoans is dependent upon microtubules and dynein. Chromosome movements culminate in the formation of a meiotic "bouquet" in which nuclear envelope-associated telomeres are clustered at the centrosomal pole of the nucleus. Bouquet formation is thought to facilitate homolog pairing. Recent studies reveal that coupling of telomeres to cytoplasmic dynein is mediated by SUN1 in the inner nuclear membrane (INM) and KASH5 a novel protein of the outer nuclear membrane (ONM). Together SUN1 and KASH5 assemble to form a transluminal LINC (linker of the nucleoskeleton and cytoskeleton) complex that spans both nuclear membranes. SUN1 forms attachment sites for telomeres at the INM while KASH5 functions as a dynein adaptor at the ONM. In mice deficient in KASH5, homologous chromosome pairing does not occur. The result is that meiosis is arrested at the leptotene/zygotene stage of meiotic prophase 1, and as a consequence both male and female mice are infertile. This study demonstrates an essential role for dynein directed telomere movement during meiotic prophase.

The dynamic nature of the nuclear envelope: lessons from closed mitosis

In eukaryotes, chromosomes are encased by a dynamic nuclear envelope. In contrast to metazoans, where the nuclear envelope disassembles during mitosis, many fungi including budding yeast undergo “closed mitosis,” where the nuclear envelope remains intact throughout the cell cycle. Consequently, during closed mitosis the nuclear envelope must expand to accommodate chromosome segregation to the two daughter cells. A recent study by Witkin et al. in budding yeast showed that if progression through mitosis is delayed, for example due to checkpoint activation, the nuclear envelope continues to expand despite the block to chromosome segregation. Moreover, this expansion occurs at a specific region of the nuclear envelope- adjacent to the nucleolus- forming an extension referred to as a “flare.” These observations raise questions regarding the regulation of nuclear envelope expansion both in budding yeast and in higher eukaryotes, the mechanisms confining mitotic nuclear envelope expansion to a particular region and the possible consequences of failing to regulate nuclear envelope expansion during the cell cycle.

Nuclear mechanics: lamin webs and pathological blebs

Anomalies in the three-dimensional shape of the nucleus are associated with a number of genetic diseases. These shape distortions include lobulated structures, with localized bulges referred to as nuclear blebs. Blebbing can result from mutations in genes encoding lamin intermediate filaments that form the lamin cortex, a thin meshwork lining the nuclear envelope. However, the biophysical origins of nuclear blebs remain a mystery. A recent study by Funkhouser et al. provides a theoretical model in which the lamin cortex is modeled as a thin, inhomogeneous elastic shell. This model shows that partial segregation of different lamin sub-networks-each with distinct mechanical properties-can lead to shell morphologies similar to blebbed nuclei in living cells.

Lamin B receptor recognizes specific modifications of histone H4 in heterochromatin formation

Inner nuclear membrane proteins provide a structural framework for chromatin, modulating transcription beneath the nuclear envelope. Lamin B receptor (LBR) is a classical inner nuclear membrane protein that associates with heterochromatin, and its mutations are known to cause Pelger-Huët anomaly in humans. However, the mechanisms by which LBR organizes heterochromatin remain to be elucidated. Here, we show that LBR represses transcription by binding to chromatin regions that are marked by specific histone modifications. The tudor domain (residues 1-62) of LBR primarily recognizes histone H4 lysine 20 dimethylation and is essential for chromatin compaction, whereas the whole nucleoplasmic region (residues 1-211) is required for transcriptional repression. We propose a model in which the nucleoplasmic domain of LBR tethers epigenetically marked chromatin to the nuclear envelope and transcriptional repressors are loaded onto the chromatin through their interaction with LBR.

Determining nuclear shape: the role of farnesylated nuclear membrane proteins

Changes in nuclear morphology are observed in diverse developmental processes as well as in pathological conditions. Modification of nuclear membrane and nuclear lamina protein levels results in altered nuclear shapes, as it has been demonstrated in experimental systems ranging from yeast to human cells. The important role of nuclear membrane components in regulating nuclear morphology is additionally highlighted by the abnormally shaped nuclei observed in diseases where nuclear lamina proteins are mutated. Even though the effect of nuclear envelope components on nuclear shape has been thoroughly described, not much is known about the molecular mechanisms that govern these events. In addition to the known role of intermediate filament formation by lamins, here we discuss several mechanisms that might alone or in combination participate in the regulation of nuclear shape observed upon modification of the levels of nuclear membrane and lamina proteins. Based on recent work with the two farnesylated nuclear membrane Drosophila proteins, kugelkern and lamin Dm0, we propose that the direct interaction of farnesylated nuclear membrane proteins with the phospholipid bilayer leads to nuclear envelope deformation. In addition to this mechanism, we suggest that the interaction of nuclear membrane and lamina proteins with cytoskeletal elements and chromatin, and modifications in lipid biosynthesis might also be involved in the formation of abnormally shaped nuclei.

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.

Cell cycle dynamics of the nuclear envelope

The nuclear envelope (NE) consists of an inner and an outer membrane, nuclear pore complexes, and the underlying nuclear lamina, a filamentous scaffold structure formed by lamins. The inner membrane is linked to the lamina and chromatin by its integral membrane proteins, such as lamin B receptor (LBR), emerin, and various isoforms of lamina-associated polypeptides (LAP) 1 and 2, which bind lamins and/or chromatin. During mitosis, the NE is disassembled upon phosphorylation of its core components, and the NE is torn apart by a dynein-driven microtubule-dependent mechanism. Nuclear reassembly after sister chromatid separation requires a timely coordinated and dephosphorylation-dependent association of lamin-binding proteins and lamins with chromosomal proteins and targeting of membranes to specific sites on chromosomes. Various chromatin-binding domains in lamina proteins, such as the LEM domain, present in all LAP2 isoforms and in emerin, as well as unique regions in lamina proteins and in specific LAP2 isoforms have been implicated in defined steps of NE reformation. Furthermore, novel mechanisms of membrane fusion involving Ran GTPase are just beginning to emerge.

Human sperm telomere-binding complex involves histone H2B and secures telomere membrane attachment

Telomeres are unique chromatin domains located at the ends of eukaryotic chromosomes. Telomere functions in somatic cells involve complexes between telomere proteins and TTAGGG DNA repeats. During the differentiation of germ-line cells, telomeres undergo significant reorganization most likely required for additional specific functions in meiosis and fertilization. A telomere-binding protein complex from human sperm (hSTBP) has been isolated by detergent treatment and was partially purified. hSTBP specifically binds double-stranded telomeric DNA and does not contain known somatic telomere proteins TRF1, TRF2, and Ku. Surprisingly, the essential component of this complex has been identified as a specific variant of histone H2B. Indirect immunofluorescence shows punctate localization of H2B in sperm nuclei, which in part coincides with telomeric DNA localization established by fluorescent in situ hybridization. Anti-H2B antibodies block interactions of hSTBP with telomere DNA, and spH2B forms specific complex with this DNA in vitro, indicating that this protein plays a role in telomere DNA recognition. We propose that hSTBP participates in the membrane attachment of telomeres that may be important for ordered chromosome withdrawal after fertilization.

Ultrastructure and immuno-cytochemistry of BHK-21 cells infected with a modified Bucyrus strain of equine arteritis virus

Morphogenesis of a modified Bucyrus strain of equine arteritis virus (EAV) in BHK-21 cells was studied. Bacillary tubules were first detected in the cytoplasm 8 h after infection, and mature virions 79 to 122 nm in diameter, 101 nm on average, were mostly observed in the cisternae of the rough endoplasmic reticulum (RER) at 12 h or later. They had isometrical cores and morphological subunits in the outer layer. Budding occurred from the RER and the outer nuclear membrane, but not from the cell surface. Structural linkage was detected between the tubule and the virus core. Aberrant strands were occasionally demonstrated within the nucleus 12 h after infection, and immunofluorescence and immunogold labeling revealed viral antigen also in the nucleus.

Sites of replication of chromosomal DNA in a eukaryotic cell

In mouse cells (line P815), newly synthesized DNA labeled for 20-30 sec during exponential growth is found by electron microscope autoradiography at sites throughout the cell nucleus. These sites are relatively more concentrated in the peripheral region of the nucleus (averaged over a random population of S-phase cells), probably reflecting a higher local concentration of DNA in this region. Newly synthesized DNA is not preferentially associated with purified nuclear envelopes, but is found in a fraction of the chromosomal deoxynucleoprotein whose buoyant density in CsCl after formaldehyde treatment is about 1% lower than that of the deoxynucleoprotein peak. Kinetics experiments suggest that this material is a precursor of mature deoxynucleoprotein; it may represent regions of deoxynucleoprotein containing replicating DNA and the additional proteins involved in DNA replication. Other complexes of newly replicated DNA that are found in the interphase after phenol extraction of nuclei are formed during the extraction procedure, probably due to the partially single-stranded nature of replicating DNA, and do not appear to exist in vivo.