Conformation-specific antibodies reveal distinct actin structures in the nucleus and the cytoplasm

Nuclear actin filaments - a historical perspective

The view on nuclear filaments formed by non-skeletal β-actin has significantly changed over the decades. Initially, filamentous actin was observed in amphibian oocyte nuclei and only under specific cell stress conditions in mammalian cell nuclei. Improved labeling and imaging technologies have permitted insights into a transient but microscopically apparent filament network that is relevant for chromatin organization, biomechanics of the mammalian cell nucleus, gene expression, and DNA damage repair. Here, we will provide a historical perspective on the developing insight into nuclear actin filaments.

Bioinformatics Analysis of Actin Interactome: Characterization of the Nuclear and Cytoplasmic Actin-Binding Proteins

Actin is present in the cytoplasm and nucleus of every eukaryotic cell. In the cytoplasm, framework and motor functions of actin are associated with its ability to polymerize to form F-actin. In the nucleus, globular actin plays a significant functional role. For a globular protein, actin has a uniquely large number of proteins with which it interacts. Bioinformatics analysis of the actin interactome showed that only a part of actin-binding proteins are both cytoplasmic and nuclear. There are proteins that interact only with cytoplasmic, or only with nuclear actin. The first pool includes proteins associated with the formation, regulation, and functioning of the actin cytoskeleton predominate, while nuclear actin-binding proteins are involved in the majority of key nuclear processes, from regulation of transcription to DNA damage response. Bioinformatics analysis of the structure of actin-binding proteins showed that these are mainly intrinsically disordered proteins, many of which are part of membrane-less organelles. Interestingly, although the number of intrinsically disordered actin-binding proteins in the nucleus is greater than in the cytoplasm, the drivers for the formation of the membrane-less organelles in the cytoplasm are significantly (four times) greater than in the nucleus.

Effects of environmental stress factors on the actin cytoskeleton of fungi and plants: Ionizing radiation and ROS

Actin is an abundant and multifaceted protein in eukaryotic cells that has been detected in the cytoplasm as well as in the nucleus. In cooperation with numerous interacting accessory-proteins, monomeric actin (G-actin) polymerizes into microfilaments (F-actin) which constitute ubiquitous subcellular higher order structures. Considering the extensive spatial dimensions and multifunctionality of actin superarrays, the present study analyses the issue if and to what extent environmental stress factors, specifically ionizing radiation (IR) and reactive oxygen species (ROS), affect the cellular actin-entity. In that context, this review particularly surveys IR-response of fungi and plants. It examines in detail which actin-related cellular constituents and molecular pathways are influenced by IR and related ROS. This comprehensive survey concludes that the general integrity of the total cellular actin cytoskeleton is a requirement for IR-tolerance. Actin's functions in genome organization and nuclear events like chromatin remodeling, DNA-repair, and transcription play a key role. Beyond that, it is highly significant that the macromolecular cytoplasmic and cortical actin-frameworks are affected by IR as well. In response to IR, actin-filament bundling proteins (fimbrins) are required to stabilize cables or patches. In addition, the actin-associated factors mediating cellular polarity are essential for IR-survivability. Moreover, it is concluded that a cellular homeostasis system comprising ROS, ROS-scavengers, NADPH-oxidases, and the actin cytoskeleton plays an essential role here. Consequently, besides the actin-fraction which controls crucial genome-integrity, also the portion which facilitates orderly cellular transport and polarized growth has to be maintained in order to survive IR.

Dynamic coalescence of yeast Heat Shock Protein genes bypasses the requirement for actin

Nuclear actin has been implicated in dynamic chromatin rearrangements in diverse eukaryotes. In mammalian cells, it is required to reposition double-strand DNA breaks to enable homologous recombination repair and to enhance transcription by facilitating RNA Pol II recruitment to gene promoters. In the yeast Saccharomyces cerevisiae, nuclear actin modulates interphase chromosome dynamics and is required to reposition the induced INO1 gene to the nuclear periphery. Here, we have investigated the role of actin in driving intergenic interactions between Heat Shock Factor 1 (Hsf1)-regulated Heat Shock Protein (HSP) genes in budding yeast. These genes, dispersed on multiple chromosomes, dramatically reposition following exposure of cells to acute thermal stress, leading to their clustering within dynamic biomolecular condensates. Using an auxin-induced degradation strategy, we found that conditional depletion of nucleators of either linear or branched F-actin (Bni1/Bnr1 and Arp2, respectively) had little or no effect on heat shock-induced HSP gene coalescence or transcription. In addition, we found that pretreatment of cells with latrunculin A, an inhibitor of both filamentous and monomeric actin, failed to affect intergenic interactions between activated HSP genes and their heat shock-induced intragenic looping and folding. Moreover, latrunculin A pretreatment had little effect on HSP gene expression at either RNA or protein levels. In notable contrast, we confirmed that repositioning of activated INO1 to the nuclear periphery and its proper expression do require actin. Collectively, our work suggests that transcriptional activation and 3D genome restructuring of thermally induced, Hsf1-regulated genes can occur in the absence of actin.

Architectural control of mesenchymal stem cell phenotype through nuclear actin

There is growing appreciation that architectural components of the nucleus regulate gene accessibility by altering chromatin organization. While nuclear membrane connector proteins link the mechanosensitive actin cytoskeleton to the nucleoskeleton, actin’s contribution to the inner architecture of the nucleus remains enigmatic. Control of actin transport into the nucleus, plus the presence of proteins that control actin structure (the actin tool-box) within the nucleus, suggests that nuclear actin may support biomechanical regulation of gene expression. Cellular actin structure is mechanoresponsive: actin cables generated through forces experienced at the plasma membrane transmit force into the nucleus. We posit that dynamic actin remodeling in response to such biomechanical cues provides a novel level of structural control over the epigenetic landscape. We here propose to bring awareness to the fact that mechanical forces can promote actin transfer into the nucleus and control structural arrangements as illustrated in mesenchymal stem cells, thereby modulating lineage commitment.

Nuclear Cytoskeleton in Virus Infection

The nuclear lamina is the main component of the nuclear cytoskeleton that maintains the integrity of the nucleus. However, it represents a natural barrier for viruses replicating in the cell nucleus. The lamina blocks viruses from being trafficked to the nucleus for replication, but it also impedes the nuclear egress of the progeny of viral particles. Thus, viruses have evolved mechanisms to overcome this obstacle. Large viruses induce the assembly of multiprotein complexes that are anchored to the inner nuclear membrane. Important components of these complexes are the viral and cellular kinases phosphorylating the lamina and promoting its disaggregation, therefore allowing virus egress. Small viruses also use cellular kinases to induce lamina phosphorylation and the subsequent disruption in order to facilitate the import of viral particles during the early stages of infection or during their nuclear egress. Another component of the nuclear cytoskeleton, nuclear actin, is exploited by viruses for the intranuclear movement of their particles from the replication sites to the nuclear periphery. This study focuses on exploitation of the nuclear cytoskeleton by viruses, although this is just the beginning for many viruses, and promises to reveal the mechanisms and dynamic of physiological and pathological processes in the nucleus.

New Insights into Cellular Functions of Nuclear Actin

Simple Summary

It is well known that actin forms a cytoplasmic network of microfilaments, the part of the cytoskeleton, in the cytoplasm of eukaryotic cells. The presence of nuclear actin was elusive for a very long time. Now, there is a very strong evidence that actin plays many important roles in the nucleus. Here, we discuss the recently discovered functions of the nuclear actin pool. Actin does not have nuclear localization signal (NLS), so its import to the nucleus is facilitated by the NLS-containing proteins. Nuclear actin plays a role in the maintenance of the nuclear structure and the nuclear envelope breakdown. It is also involved in chromatin remodeling, and chromatin and nucleosome movement necessary for DNA recombination, repair, and the initiation of transcription. It also binds RNA polymerases, promoting transcription. Because of the multifaceted role of nuclear actin, the future challenge will be to further define its functions in various cellular processes and diseases.

Abstract

Actin is one of the most abundant proteins in eukaryotic cells. There are different pools of nuclear actin often undetectable by conventional staining and commercial antibodies used to identify cytoplasmic actin. With the development of more sophisticated imaging and analytical techniques, it became clear that nuclear actin plays a crucial role in shaping the chromatin, genomic, and epigenetic landscape, transcriptional regulation, and DNA repair. This multifaceted role of nuclear actin is not only important for the function of the individual cell but also for the establishment of cell fate, and tissue and organ differentiation during development. Moreover, the changes in the nuclear, chromatin, and genomic architecture are preamble to various diseases. Here, we discuss some of the newly described functions of nuclear actin.

Keratin 17 regulates nuclear morphology and chromatin organization

Keratin 17 (KRT17; K17), a non-lamin intermediate filament protein, was recently found to occur in the nucleus. We report here on K17-dependent differences in nuclear morphology, chromatin organization, and cell proliferation. Human tumor keratinocyte cell lines lacking K17 exhibit flatter nuclei relative to normal. Re-expression of wild-type K17, but not a mutant form lacking an intact nuclear localization signal (NLS), rescues nuclear morphology in KRT17-null cells. Analyses of primary cultures of skin keratinocytes from a mouse strain expressing K17 with a mutated NLS corroborated these findings. Proteomics screens identified K17-interacting nuclear proteins with known roles in gene expression, chromatin organization and RNA processing. Key histone modifications and LAP2β (an isoform encoded by TMPO) localization within the nucleus are altered in the absence of K17, correlating with decreased cell proliferation and suppression of GLI1 target genes. Nuclear K17 thus impacts nuclear morphology with an associated impact on chromatin organization, gene expression, and proliferation in epithelial cells.This article has an associated First Person interview with the first author of the paper.

Nuclear actin in cancer biology

The presence of actin in the nucleus has been a matter of debate for many years. In recent years many important roles of actin in the nucleus (transcriptional regulation, chromatin remodeling, DNA repair, cell division, maintenance of nuclear architecture) have been identified, and the precise control of nuclear actin levels has been demonstrated. The vital importance of the actin driven processes in the cell make it highly likely that dysregulation of nuclear actin dynamics and structure can be linked to tumor induction and -progression. In this chapter I summarize our current knowledge about nuclear actin in the cancer context.

Nuclear myosins - roles for molecular transporters and anchors

The myosin family of molecular motors are well-characterised cytoskeletal proteins. However, myosins are also present in the nucleus, where they have been shown to have roles in transcription, DNA repair and viral infections. Despite their involvement in these fundamental cellular processes, our understanding of these functions and their regulation remains limited. Recently, research on nuclear myosins has been gathering pace, and this Review will evaluate the current state of the field. Here, we will focus on the variation in structure of nuclear myosins, their nuclear import and their roles within transcription, DNA damage, chromatin organisation and viral infections. We will also consider both the biochemical and biophysical properties and restraints that are placed on these multifunctional motors, and how they link to their cytoplasmic counterparts. By highlighting these properties and processes, we show just how integral nuclear myosins are for cellular survival.

D-Aspartate Upregulates DAAM1 Protein Levels in the Rat Testis and Induces Its Localization in Spermatogonia Nucleus

Cell differentiation during spermatogenesis requires a proper actin dynamic, regulated by several proteins, including formins. Disheveled-Associated-Activator of Morphogenesis1 (DAAM1) belongs to the formins and promotes actin polymerization. Our results showed that oral D-Aspartate (D-Asp) administration, an excitatory amino acid, increased DAAM1 protein levels in germ cells cytoplasm of rat testis. Interestingly, after the treatment, DAAM1 also localized in rat spermatogonia (SPG) and mouse GC-1 cells nuclei. We provided bioinformatic evidence that DAAM1 sequence has two predicted NLS, supporting its nuclear localization. The data also suggested a role of D-Asp in promoting DAAM1 shuttling to the nuclear compartment of those proliferative cells. In addition, the proliferative action induced by D-Asp is confirmed by the increased levels of PCNA, a protein expressed in the nucleus of cells in the S phase and p-H3, a histone crucial for chromatin condensation during mitosis and meiosis. In conclusion, we demonstrated, for the first time, an increased DAAM1 protein levels following D-Asp treatment in rat testis and also its localization in the nucleus of rat SPG and in mouse GC-1 cells. Our results suggest an assumed role for this formin as a regulator of actin dynamics in both cytoplasm and nuclei of the germ cells.

O-GlcNAcylation and phosphorylation of β-actin Ser199 in diabetic nephropathy

The function of actin is regulated by various posttranslational modifications. We have previously shown that in the kidneys of nonobese type 2 diabetes model Goto-Kakizaki rats, increased O-GlcNAcylation of β-actin protein is observed. It has also been reported that both O-GlcNAcylation and phosphorylation occur on Ser199 of β-actin. However, their roles are not known. To elucidate their roles in diabetic nephropathy, we examined the rat kidney for changes in O-GlcNAcylation of Ser199 (gS199)-actin and in the phosphorylation of Ser199 (pS199)-actin. Both gS199- and pS199-actin molecules had an apparent molecular weight of 40 kDa and were localized as nonfilamentous actin in both the cytoplasm and nucleus. Compared with the normal kidney, the immunostaining intensity of gS199-actin increased in podocytes of the glomeruli and in proximal tubules of the diabetic kidney, whereas that of pS199-actin did not change in podocytes but decreased in proximal tubules. We confirmed that the same results could be observed in the glomeruli of the human diabetic kidney. In podocytes of glomeruli cultured in the presence of the O-GlcNAcase inhibitor Thiamet G, increased O-GlcNAcylation was accompanied by a concomitant decrease in the amount of filamentous actin and in morphological changes. Our present results demonstrate that dysregulation of O-GlcNAcylation and phosphorylation of Ser199 occurred in diabetes, which may contribute partially to the causes of the morphological changes in the glomeruli and tubules. gS199- and pS199-actin will thus be useful for the pathological evaluation of diabetic nephropathy.

Nuclear Actin and Actin-Binding Proteins in DNA Repair

Nuclear actin has been implicated in a variety of DNA-related processes including chromatin remodeling, transcription, replication, and DNA repair. However, the mechanistic understanding of actin in these processes has been limited, largely due to a lack of research tools that address the roles of nuclear actin specifically, that is, distinct from its cytoplasmic functions. Recent findings support a model for homology-directed DNA double-strand break (DSB) repair in which a complex of ARP2 and ARP3 (actin-binding proteins 2 and 3) binds at the break and works with actin to promote DSB clustering and homology-directed repair. Further, it has been reported that relocalization of heterochromatic DSBs to the nuclear periphery in Drosophila is ARP2/3 dependent and actin-myosin driven. Here we provide an overview of the role of nuclear actin and actin-binding proteins in DNA repair, critically evaluating the experimental tools used and potential indirect effects.

Multiple Pools of Nuclear Actin

While nuclear actin was reported ~50 years ago, it's in vivo prevalence and structure remain largely unknown. Here, we use Drosophila oogenesis, that is, follicle development, to characterize nuclear actin. We find that three different reagents-DNase I, anti-actin C4, and anti-actin AC15-recognize distinct pools of nuclear actin. DNase I labels monomeric or G-actin, and, during follicle development, G-actin is present in the nucleus of every cell. Some G-actin is recognized by the C4 antibody. In particular, C4 nuclear actin colocalizes with DNase I to the nucleolus in anterior escort cells, follicle stem cells, some mitotic follicle cells, and a subset of nurse cells during early oogenesis. C4 also labels polymeric nuclear actin in the nucleoplasm of the germline stem cells, early cystoblasts, and oocytes. The AC15 antibody labels a completely distinct pool of nuclear actin from that of DNase I and C4. Specifically, AC15 nuclear actin localizes to the chromatin in the nurse and follicle cells during mid-to-late oogenesis. Within the oocyte, AC15 nuclear actin progresses from localizing to puncta surrounding the DNA, to forming a filamentous cage around the chromosomes. Together these findings reveal that nuclear actin is highly prevalent in vivo, and multiple pools of nuclear actin exist and can be recognized using different reagents. Additionally, our localization studies suggest that nuclear actin may regulate stemness, nucleolar structure and function, transcription, and nuclear structure. Such findings call for further studies to explore the prevalence, diversity, and functions of nuclear actin across tissues and organisms. Anat Rec, 301:2014-2036, 2018. © 2018 Wiley Periodicals, Inc.

Nuclear Actin: From Discovery to Function

While actin was discovered in the nucleus over 50 years ago, research lagged for decades due to strong skepticism. The revitalization of research into nuclear actin occurred after it was found that cellular stresses induce the nuclear localization and alter the structure of actin. These studies provided the first hints that actin has a nuclear function. Subsequently, it was established that the nuclear import and export of actin is highly regulated. While the structures of nuclear actin remain unclear, it can function as monomers, polymers, and even rods. Furthermore, even within a given structure, distinct pools of nuclear actin that can be differentially labeled have been identified. Numerous mechanistic studies have uncovered an array of functions for nuclear actin. It regulates the activity of RNA polymerases, as well as specific transcription factors. Actin also modulates the activity of several chromatin remodeling complexes and histone deacetylases, to ultimately impinge on transcriptional programing and DNA damage repair. Further, nuclear actin mediates chromatin movement and organization. It has roles in meiosis and mitosis, and these functions may be functionally conserved from ancient bacterial actin homologs. The structure and integrity of the nuclear envelope and sub-nuclear compartments are also regulated by nuclear actin. Furthermore, nuclear actin contributes to human diseases like cancer, neurodegeneration, and myopathies. Here, we explore the early discovery of actin in the nucleus and discuss the forms and functions of nuclear actin in both normal and disease contexts. Anat Rec, 301:1999-2013, 2018. © 2018 Wiley Periodicals, Inc.

Chromosome Dynamics in Response to DNA Damage

Recent advances in both the technologies used to measure chromatin movement and the biophysical analysis used to model them have yielded a fuller understanding of chromatin dynamics and the polymer structure that underlies it. Changes in nucleosome packing, checkpoint kinase activation, the cell cycle, chromosomal tethers, and external forces acting on nuclei in response to external and internal stimuli can alter the basal mobility of DNA in interphase nuclei of yeast or mammalian cells. Although chromatin movement is assumed to be necessary for many DNA-based processes, including gene activation by distal enhancer–promoter interaction or sequence-based homology searches during double-strand break repair, experimental evidence supporting an essential role in these activities is sparse. Nonetheless, high-resolution tracking of chromatin dynamics has led to instructive models of the higher-order folding and flexibility of the chromatin polymer. Key regulators of chromatin motion in physiological conditions or after damage induction are reviewed here.

Nuclear Actin Polymerized by mDia2 Confines Centromere Movement during CENP-A Loading

Centromeres are specialized chromosomal regions epigenetically defined by the histone H3 variant centromere protein A (CENP-A). CENP-A needs to be replenished in every cell cycle, but how new CENP-A is stably incorporated into centromeric chromatin remains unclear. We have discovered that a cytoskeletal protein, diaphanous formin mDia2, is essential for the stable incorporation of new CENP-A proteins into centromeric nucleosomes. Here we report that mDia2-mediated formation of dynamic and short nuclear actin filaments in G1 nucleus is required to maintain CENP-A levels at the centromere. Importantly, mDia2 and nuclear actin are required for constrained centromere movement during CENP-A loading, and depleting nuclear actin or MgcRacGAP, which lies upstream of mDia2, extends centromeric association of the CENP-A loading chaperone Holliday junction recognition protein (HJURP). Our findings thus suggest that nuclear actin polymerized by mDia2 contributes to the physical confinement of G1 centromeres so that HJURP-mediated CENP-A loading reactions can be productive, and centromere's epigenetic identity can be stably maintained.

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Formin mDia2 is required for nuclear actin polymerization at G1 centromeres

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Nuclear actin polymerization is required to maintain centromeric CENP-A levels

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mDia2 and nuclear actin restrict centromere movement during CENP-A loading

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Nuclear actin and MgcRacGAP are required for timely turnover of centromeric HJURP

The Nature of Actin-Family Proteins in Chromatin-Modifying Complexes

Actin is not only one of the most abundant proteins in eukaryotic cells, but also one of the most versatile. In addition to its familiar involvement in enabling contraction and establishing cellular motility and scaffolding in the cytosol, actin has well-documented roles in a variety of processes within the confines of the nucleus, such as transcriptional regulation and DNA repair. Interestingly, monomeric actin as well as actin-related proteins (Arps) are found as stoichiometric subunits of a variety of chromatin remodeling complexes and histone acetyltransferases, raising the question of precisely what roles they serve in these contexts. Actin and Arps are present in unique combinations in chromatin modifiers, helping to establish structural integrity of the complex and enabling a wide range of functions, such as recruiting the complex to nucleosomes to facilitate chromatin remodeling and promoting ATPase activity of the catalytic subunit. Actin and Arps are also thought to help modulate chromatin dynamics and maintain higher-order chromatin structure. Moreover, the presence of actin and Arps in several chromatin modifiers is necessary for promoting genomic integrity and an effective DNA damage response. In this review, we discuss the involvement of actin and Arps in these nuclear complexes that control chromatin remodeling and histone modifications, while also considering avenues for future study to further shed light on their functional importance.

Nuclear F-actin and myosins drive relocalization of heterochromatic breaks

Heterochromatin mainly comprises repeated DNA sequences that are prone to ectopic recombination. In Drosophila cells, 'safe' repair of heterochromatic double-strand breaks by homologous recombination relies on the relocalization of repair sites to the nuclear periphery before strand invasion. The mechanisms responsible for this movement were unknown. Here we show that relocalization occurs by directed motion along nuclear actin filaments assembled at repair sites by the Arp2/3 complex. Relocalization requires nuclear myosins associated with the heterochromatin repair complex Smc5/6 and the myosin activator Unc45, which is recruited to repair sites by Smc5/6. ARP2/3, actin nucleation and myosins also relocalize heterochromatic double-strand breaks in mouse cells. Defects in this pathway result in impaired heterochromatin repair and chromosome rearrangements. These findings identify de novo nuclear actin filaments and myosins as effectors of chromatin dynamics for heterochromatin repair and stability in multicellular eukaryotes.

AIM1 is an actin-binding protein that suppresses cell migration and micrometastatic dissemination

A defining hallmark of primary and metastatic cancers is the migration and invasion of malignant cells. These invasive properties involve altered dynamics of the cytoskeleton and one of its major structural components β-actin. Here we identify AIM1 (absent in melanoma 1) as an actin-binding protein that suppresses pro-invasive properties in benign prostate epithelium. Depletion of AIM1 in prostate epithelial cells increases cytoskeletal remodeling, intracellular traction forces, cell migration and invasion, and anchorage-independent growth. In addition, decreased AIM1 expression results in increased metastatic dissemination in vivo. AIM1 strongly associates with the actin cytoskeleton in prostate epithelial cells in normal tissues, but not in prostate cancers. In addition to a mislocalization of AIM1 from the actin cytoskeleton in invasive cancers, advanced prostate cancers often harbor AIM1 deletion and reduced expression. These findings implicate AIM1 as a key suppressor of invasive phenotypes that becomes dysregulated in primary and metastatic prostate cancer.

Invasion of malignant cells involves changes in cytoskeleton dynamics. Here the authors identify absent in melanoma 1 as an actin binding protein and show that it regulates cytoskeletal remodeling and cell migration in prostate epithelial cells, acting as a metastatic suppressor in cancer cells.

Keratins Are Going Nuclear

Previously thought to reside exclusively in the cytoplasm, the cytoskeletal protein keratin 17 (K17) has been recently identified inside the nucleus of tumor epithelial cells with a direct impact on cell proliferation and gene expression. We comment on fundamental questions raised by this new finding and the associated significance.

The Nucleoskeleton

SUMMARYThe nucleoskeleton is an important structural feature of the metazoan nucleus and is involved in the regulation of genome expression and maintenance. The nuclear lamins are intermediate filament proteins that form a peripheral nucleoskeleton in concert with other lamin-associated proteins. Several other proteins normally found in the cytoskeleton have also been identified in the nucleus, but, as will be discussed here, their roles in forming a nucleoskeleton have not been elucidated. Nevertheless, mutations in lamins and lamin-associated proteins cause a spectrum of diseases, making them interesting targets for future research.

A Role for Nuclear F-Actin Induction in Human Cytomegalovirus Nuclear Egress

Herpesviruses, which include important pathogens, remodel the host cell nucleus to facilitate infection. This remodeling includes the formation of structures called replication compartments (RCs) in which herpesviruses replicate their DNA. During infection with the betaherpesvirus, human cytomegalovirus (HCMV), viral DNA synthesis occurs at the periphery of RCs within the nuclear interior, after which assembled capsids must reach the inner nuclear membrane (INM) for translocation to the cytoplasm (nuclear egress). The processes that facilitate movement of HCMV capsids to the INM during nuclear egress are unknown. Although an actin-based mechanism of alphaherpesvirus capsid trafficking to the INM has been proposed, it is controversial. Here, using a fluorescently-tagged, nucleus-localized actin-binding peptide, we show that HCMV, but not herpes simplex virus 1, strongly induced nuclear actin filaments (F-actin) in human fibroblasts. Based on studies using UV inactivation and inhibitors, this induction depended on viral gene expression. Interestingly, by 24 h postinfection, nuclear F-actin formed thicker structures that appeared by super-resolution microscopy to be bundles of filaments. Later in infection, nuclear F-actin primarily localized along the RC periphery and between the RC periphery and the nuclear rim. Importantly, a drug that depolymerized nuclear F-actin caused defects in production of infectious virus, capsid accumulation in the cytoplasm, and capsid localization near the nuclear rim, without decreasing capsid accumulation in the nucleus. Thus, our results suggest that for at least one herpesvirus, nuclear F-actin promotes capsid movement to the nuclear periphery and nuclear egress. We discuss our results in terms of competing models for these processes.

The mechanisms underlying herpesvirus nuclear egress have not been fully determined. In particular, how newly assembled capsids move to the inner nuclear membrane for envelopment is uncertain and controversial. In this study, we show that HCMV, an important human pathogen, induces actin filaments in the nuclei of infected cells and that an inhibitor of nuclear F-actin impairs nuclear egress and capsid localization toward the nuclear periphery. Herpesviruses are widespread pathogens that cause or contribute to an array of human diseases. A better understanding of how herpesvirus capsids traffic in the nucleus may uncover novel targets for antiviral intervention and elucidate aspects of the nuclear cytoskeleton, about which little is known.

Fascin regulates nuclear actin during Drosophila oogenesis

Study of Drosophila oogenesis reveals that the nuclear localization of actin is controlled by both development and Fascin. Fascin regulates both endogenous nuclear actin and ectopic nuclear actin rod formation by controlling Cofilin.

Drosophila oogenesis provides a developmental system with which to study nuclear actin. During Stages 5–9, nuclear actin levels are high in the oocyte and exhibit variation within the nurse cells. Cofilin and Profilin, which regulate the nuclear import and export of actin, also localize to the nuclei. Expression of GFP-tagged Actin results in nuclear actin rod formation. These findings indicate that nuclear actin must be tightly regulated during oogenesis. One factor mediating this regulation is Fascin. Overexpression of Fascin enhances nuclear GFP-Actin rod formation, and Fascin colocalizes with the rods. Loss of Fascin reduces, whereas overexpression of Fascin increases, the frequency of nurse cells with high levels of nuclear actin, but neither alters the overall nuclear level of actin within the ovary. These data suggest that Fascin regulates the ability of specific cells to accumulate nuclear actin. Evidence indicates that Fascin positively regulates nuclear actin through Cofilin. Loss of Fascin results in decreased nuclear Cofilin. In addition, Fascin and Cofilin genetically interact, as double heterozygotes exhibit a reduction in the number of nurse cells with high nuclear actin levels. These findings are likely applicable beyond Drosophila follicle development, as the localization and functions of Fascin and the mechanisms regulating nuclear actin are widely conserved.

Diaphanous formin mDia2 regulates CENP-A levels at centromeres

The diaphanous formin mDia2, a protein involved in cytoskeletal control, is required for new CENP-A loading at centromeres during the cell cycle to maintain epigenetic markers.

Centromeres of higher eukaryotes are epigenetically defined by centromere protein A (CENP-A), a centromere-specific histone H3 variant. The incorporation of new CENP-A into centromeres to maintain the epigenetic marker after genome replication in S phase occurs in G1 phase; however, how new CENP-A is loaded and stabilized remains poorly understood. Here, we identify the formin mDia2 as essential for stable replenishment of new CENP-A at centromeres. Quantitative imaging, pulse-chase analysis, and high-resolution ratiometric live-cell studies demonstrate that mDia2 and its nuclear localization are required to maintain CENP-A levels at centromeres. Depletion of mDia2 results in a prolonged centromere association of holiday junction recognition protein (HJURP), the chaperone required for CENP-A loading. A constitutively active form of mDia2 rescues the defect in new CENP-A loading caused by depletion of male germ cell Rac GTPase-activating protein (MgcRacGAP), a component of the small GTPase pathway essential for CENP-A maintenance. Thus, the formin mDia2 functions downstream of the MgcRacGAP-dependent pathway in regulating assembly of new CENP-A containing nucleosomes at centromeres.

Actin, actin-binding proteins, and actin-related proteins in the nucleus

Extensive research in the past decade has significantly broadened our view about the role actin plays in the life of the cell and added novel aspects to actin research. One of these new aspects is the discovery of the existence of nuclear actin which became evident only recently. Nuclear activities including transcriptional activation in the case of all three RNA polymerases, editing and nuclear export of mRNAs, and chromatin remodeling all depend on actin. It also became clear that there is a fine-tuned equilibrium between cytoplasmic and nuclear actin pools and that this balance is ensured by an export-import system dedicated to actin. After over half a century of research on conventional actin and its organizing partners in the cytoplasm, it was also an unexpected finding that the nucleus contains more than 30 actin-binding proteins and new classes of actin-related proteins which are not able to form filaments but had evolved nuclear-specific functions. The actin-binding and actin-related proteins in the nucleus have been linked to RNA transcription and processing, nuclear transport, and chromatin remodeling. In this paper, we attempt to provide an overview of the wide range of information that is now available about actin, actin-binding, and actin-related proteins in the nucleus.

Evidence for a dual role of actin in regulating chromosome organization and dynamics in yeast

Eukaryotic chromosomes undergo movements that are involved in the regulation of functional processes such as DNA repair. To better understand the origin of these movements, we used fluorescence microscopy, image analysis and chromosome conformation capture to quantify the actin contribution to chromosome movements and interactions in budding yeast. We show that both the cytoskeletal and nuclear actin drive local chromosome movements, independently of Csm4, a putative LINC protein. Inhibition of actin polymerization reduces subtelomere dynamics, resulting in more confined territories and enrichment in subtelomeric contacts. Artificial tethering of actin to nuclear pores increased both nuclear pore complex (NPC) and subtelomere motion. Chromosome loci that were positioned away from telomeres exhibited reduced motion in the presence of an actin polymerization inhibitor but were unaffected by the lack of Csm4. We further show that actin was required for locus mobility that was induced by targeting the chromatin-remodeling protein Ino80. Correlated with this, DNA repair by homologous recombination was less efficient. Overall, interphase chromosome dynamics are modulated by the additive effects of cytoskeletal actin through forces mediated by the nuclear envelope and nuclear actin, probably through the function of actin in chromatin-remodeling complexes.

Emerging roles of sumoylation in the regulation of actin, microtubules, intermediate filaments, and septins

Sumoylation is a powerful regulatory system that controls many of the critical processes in the cell, including DNA repair, transcriptional regulation, nuclear transport, and DNA replication. Recently, new functions for SUMO have begun to emerge. SUMO is covalently attached to components of each of the four major cytoskeletal networks, including microtubule-associated proteins, septins, and intermediate filaments, in addition to nuclear actin and actin-regulatory proteins. However, knowledge of the mechanisms by which this signal transduction system controls the cytoskeleton is still in its infancy. One story that is beginning to unfold is that SUMO may regulate the microtubule motor protein dynein by modification of its adaptor Lis1. In other instances, cytoskeletal elements can both bind to SUMO non-covalently and also be conjugated by it. The molecular mechanisms for many of these new functions are not yet clear, but are under active investigation. One emerging model links the function of MAP sumoylation to protein degradation through SUMO-targeted ubiquitin ligases, also known as STUbL enzymes. Other possible functions for cytoskeletal sumoylation are also discussed.

The potential roles of actin in the nucleus

Over the past few decades, actin’s presence in the nucleus has been demonstrated. Actin is a key protein necessary for different nuclear processes. Although actin is well known for its functional role in dynamic behavior of the cytoskeleton, emerging studies are now highlighting new roles for actin. At the present time there is no doubt about the presence of actin in the nucleus. A number of studies have uncovered the functional involvement of actin in nuclear processes. Actin as one of the nuclear components has its own structured and functional rules, such as nuclear matrix association, chromatin remodeling, transcription by RNA polymerases I, II, III and mRNA processing. In this historical review, we attempt to provide an overview of our current understanding of the functions of actin in the nucleus.

Involvement of the SATB1/F-actin complex in chromatin reorganization during active cell death

Over the past years, confirmations on the presence of actin and/or its polymerized form, F-actin, in the cell nucleus are progressively accumulating. Nevertheless, the function and localization of F-actin in the nucleus is still not fully characterized. Thus, the aim of the present study was to evaluate the association between F-actin and sequence-binding protein 1 (SATB1) and their involvement in chromatin remodeling associated with active cell death. Both SATB1 and F-actin were colocalized in the transcriptional active regions of the cell nucleus and a functional interaction was observed between SATB1 and higher-organized nuclear F-actin structures at the border between condensed and decondensed chromatin. These results extend the knowledge on the role of SATB1 and nuclear F-actin in three-dimensional chromatin organization and their functions during active cell death. Additionally, this study opens the discussion on the involvement of the SATB1/F-actin functional complex in active cell death; further studies are required to fully elucidate these issues.

Thermal memory in self-assembled collagen fibril networks

Collagen fibrils form extracellular networks that regulate cell functions and provide mechanical strength to tissues. Collagen fibrillogenesis is an entropy-driven process promoted by warming and reversed by cooling. Here, we investigate the influence of noncovalent interactions mediated by the collagen triple helix on fibril stability. We measure the kinetics of cold-induced disassembly of fibrils formed from purified collagen I using turbimetry, probe the fibril morphology by atomic force microscopy, and measure the network connectivity by confocal microscopy and rheometry. We demonstrate that collagen fibrils disassemble by subunit release from their sides as well as their ends, with complex kinetics involving an initial fast release followed by a slow release. Surprisingly, the fibrils are gradually stabilized over time, leading to thermal memory. This dynamic stabilization may reflect structural plasticity of the collagen fibrils arising from their complex structure. In addition, we propose that the polymeric nature of collagen monomers may lead to slow kinetics of subunit desorption from the fibril surface. Dynamic stabilization of fibrils may be relevant in the initial stages of collagen assembly during embryogenesis, fibrosis, and wound healing. Moreover, our results are relevant for tissue repair and drug delivery applications, where it is crucial to control fibril stability.

Mechanisms of nuclear actin in chromatin-remodeling complexes

The mystery of nuclear actin has puzzled biologists for decades largely due to the lack of defined experimental systems. However, the development of actin-containing chromatin-modifying complexes as a defined genetic and biochemical system in the past decade has provided an unprecedented opportunity to dissect the mechanism of actin in the nucleus. Although the established functions of actin mostly rely on its dynamic polymerization, the novel finding of the mechanism of action of actin in the INO80 chromatin-remodeling complex suggests a conceptually distinct mode of actin that functions as a monomer. In this review we highlight the new paradigm and discuss how actin interaction with chromatin suggests a fundamental divergence between conventional cytoplasmic actin and nuclear actin. Furthermore, we provide how this framework could be applied to investigations of nuclear actin in other actin-containing chromatin-modifying complexes.

Mechanics of the nucleus

The nucleus is the distinguishing feature of eukaryotic cells. Until recently, it was often considered simply as a unique compartment containing the genetic information of the cell and associated machinery, without much attention to its structure and mechanical properties. This article provides compelling examples that illustrate how specific nuclear structures are associated with important cellular functions, and how defects in nuclear mechanics can cause a multitude of human diseases. During differentiation, embryonic stem cells modify their nuclear envelope composition and chromatin structure, resulting in stiffer nuclei that reflect decreased transcriptional plasticity. In contrast, neutrophils have evolved characteristic lobulated nuclei that increase their physical plasticity, enabling passage through narrow tissue spaces in their response to inflammation. Research on diverse cell types further demonstrates how induced nuclear deformations during cellular compression or stretch can modulate cellular function. Pathological examples of disturbed nuclear mechanics include the many diseases caused by mutations in the nuclear envelope proteins lamin A/C and associated proteins, as well as cancer cells that are often characterized by abnormal nuclear morphology. In this article, we will focus on determining the functional relationship between nuclear mechanics and cellular (dys-)function, describing the molecular changes associated with physiological and pathological examples, the resulting defects in nuclear mechanics, and the effects on cellular function. New insights into the close relationship between nuclear mechanics and cellular organization and function will yield a better understanding of normal biology and will offer new clues into therapeutic approaches to the various diseases associated with defective nuclear mechanics.

Nck1 deficiency accelerates unloading-induced bone loss

Mechanical stress is an important signal to determine the levels of bone mass. Unloading-induced osteoporosis is a critical issue in bed-ridden patients and astronauts. Many molecules have been suggested to be involved in sensing mechanical stress in bone, though the mechanisms involved in this phenomenon are not fully understood. Nck1 is an adaptor protein known to mediate signaling from plasma membrane-activated receptors to cytosolic effectors regulating actin cytoskeleton remodeling. Nck1 has also been implicated in cellular responses to endoplasmic reticulum stress. In vitro, in case of cell stress the actin cytoskeleton is disrupted and in such cases Nck1 has been reported to enter the nucleus of the cells to mediate the nuclear actin polymerization. However, the role of Nck1 in vivo during the bone response to mechanical stimuli is unknown. The purpose of this study is to examine the role of Nck1 in unloading-induced bone loss in vivo. Sciatic and femoral nerve resection was conducted. Neurectomy-based unloading enhanced Nck1 gene expression in bone about twofold. Using the Nck1 deficient mice and control Nck1+/+, effects of neurectomy-based unloading on bone structure were examined. Unloading reduced bone volume in wild type mice by 30% whereas the levels in bone loss were exacerbated to 50% in Nck1 deficient mice due to neurectomy after 4 weeks. These data demonstrate that Nck1 gene deficiency accelerates the mechanical unloading-induced bone loss suggesting Nck1 to be a crucial molecule in mechanical stress mediated regulation in bone metabolism.

To be or not to be assembled: progressing into nuclear actin filaments

The paradigm states that cytoplasmic actin operates as filaments and nuclear actin is mainly monomeric, acting as a scaffold in transcription complexes. However, why should a powerful function of actin, namely polymerization, not be used in the nucleus? Recent progress in the field forces us to rethink this issue, as many actin filament assembly proteins have been linked to nuclear functions and new experimental approaches have provided the first direct visualizations of polymerized nuclear actin.

What we talk about when we talk about nuclear actin

In the cytoplasm, actin filaments form crosslinked networks that enable eukaryotic cells to transport cargo, change shape, and move. Actin is also present in the nucleus but, in this compartment, its functions are more cryptic and controversial. If we distill the substantial literature on nuclear actin down to its essentials, we find four, recurring, and more-or-less independent, claims: (1) crosslinked networks of conventional actin filaments span the nucleus and help maintain its structure and organize its contents; (2) assembly or contraction of filaments regulates specific nuclear events; (3) actin monomers moonlight as subunits of chromatin remodeling complexes, independent of their ability to form filaments; and (4) modified actin monomers or oligomers, structurally distinct from canonical, cytoskeletal filaments, mediate nuclear events by unknown mechanisms. We discuss the evidence underlying these claims and as well as their strengths and weaknesses. Next, we describe our recent work, in which we built probes specific for nuclear actin and used them to describe the form and distribution of actin in somatic cell nuclei. Finally, we discuss how different forms of nuclear actin may play different roles in different cell types and physiological contexts.

Nuanced but significant: how ethanol perturbs avian cranial neural crest cell actin cytoskeleton, migration and proliferation

Children with fetal alcohol syndrome (FAS) display striking craniofacial abnormalities. These features are proposed to result from perturbations in the morphology and function of cranial neural crest cells (cNCCs), which contribute significantly to the craniofacial complex. While certain pathways by which this may occur have been suggested, precise teratogenic mechanisms remain intensely investigated, as does the question of the teratogenic dose. The present study focused on examining how avian cNCC actin cytoskeleton, migratory distance, and proliferation are affected ex vivo by exposure to ethanol concentrations that simulate maternal intoxication. Chick cNCCs were cultured in 0.2% and 0.4% v/v ethanol. Distances migrated by both ethanol-treated and control cells at 24 and 48 h were recorded. Following phalloidin immunocytochemistry, treated and control cNCCs were compared morphologically and quantitatively. Apoptosis and proliferation in control versus treated cNCCs were also studied. Chick cNCCs cultured in ethanol lost their spindle-like shapes and their ordered cytoskeleton. There was a significant stage-dependent effect on cNCC migration at 24 h (p = 0.035), which was greatest at stage 10 (HH). Ethanol treatment for 48 h revealed a significant main effect for ethanol, chiefly at the 0.4% level. There was also an interaction effect between ethanol dose and stage of development (stage 9 HH). Actin microfilament disruption was quantitatively increased by ethanol at the doses studied while cNCC proliferation was increased but not significantly. Ethanol had no effect on cNCC apoptosis. At ethanol levels likely to induce human FAS, avian cNCCs exhibit various subtle, potentially significant changes in morphology, migration, and proliferation, with possible consequences for fated structures.

Visualization of actin filaments and monomers in somatic cell nuclei

Fluorescent nuclear actin reporters are used to determine the distribution of nuclear actin in live somatic cells and evaluate its potential functions. They reveal distinct monomeric and filamentous actin populations in nuclei of live somatic cells and implicate nuclear actin in mRNA processing and organization of the nucleoplasm.

In addition to its long-studied presence in the cytoplasm, actin is also found in the nuclei of eukaryotic cells. The function and form (monomer, filament, or noncanonical oligomer) of nuclear actin are hotly debated, and its localization and dynamics are largely unknown. To determine the distribution of nuclear actin in live somatic cells and evaluate its potential functions, we constructed and validated fluorescent nuclear actin probes. Monomeric actin probes concentrate in nuclear speckles, suggesting an interaction of monomers with RNA-processing factors. Filamentous actin probes recognize discrete structures with submicron lengths that are excluded from chromatin-rich regions. In time-lapse movies, these actin filament structures exhibit one of two types of mobility: 1) diffusive, with an average diffusion coefficient of 0.06–0.08 μm²/s, or (2) subdiffusive, with a mobility coefficient of 0.015 μm²/s. Individual filament trajectories exhibit features of particles moving within a viscoelastic mesh. The small size of nuclear actin filaments is inconsistent with a role in micron-scale intranuclear transport, and their localization suggests that they do not participate directly in chromatin-based processes. Our results instead suggest that actin filaments form part of a large, viscoelastic structure in the nucleoplasm and may act as scaffolds that help organize nuclear contents.

Co-transcriptional nuclear actin dynamics

Actin is a key player for nuclear structure and function regulating both chromosome organization and gene activity. In the cell nucleus actin interacts with many different proteins. Among these proteins several studies have identified classical nuclear factors involved in chromatin structure and function, transcription and RNA processing as well as proteins that are normally involved in controlling the actin cytoskeleton. These discoveries have raised the possibility that nuclear actin performs its multi task activities through tight interactions with different sets of proteins. This high degree of promiscuity in the spectrum of protein-to-protein interactions correlates well with the conformational plasticity of actin and the ability to undergo regulated changes in its polymerization states. Several of the factors involved in controlling head-to-tail actin polymerization have been shown to be in the nucleus where they seem to regulate gene activity. By focusing on the multiple tasks performed by actin and actin-binding proteins, possible models of how actin dynamics controls the different phases of the RNA polymerase II transcription cycle are being identified.

Actin nucleators in the nucleus: an emerging theme

Actin is an integral component of the cytoskeleton, forming a plethora of macromolecular structures that mediate various cellular functions. The formation of such structures relies on the ability of actin monomers to associate into polymers, and this process is regulated by actin nucleation factors. These factors use monomeric actin pools at specific cellular locations, thereby permitting rapid actin filament formation when required. It has now been established that actin is also present in the nucleus, where it is implicated in chromatin remodelling and the regulation of eukaryotic gene transcription. Notably, the presence of typical actin filaments in the nucleus has not been demonstrated directly. However, studies in recent years have provided evidence for the nuclear localisation of actin nucleation factors that promote cytoplasmic actin polymerisation. Their localisation to the nucleus suggests that these proteins mediate collaboration between the cytoskeleton and the nucleus, which might be dependent on their ability to promote actin polymerisation. The nature of this cooperation remains enigmatic and it will be important to elucidate the physiological relevance of the link between cytoskeletal actin networks and nuclear events. This Commentary explores the current evidence for the nuclear roles of actin nucleation factors. Furthermore, the implication of actin-associated proteins in relaying exogenous signals to the nucleus, particularly in response to cellular stress, will be considered.

Neuronal profilin isoforms are addressed by different signalling pathways

Profilins are prominent regulators of actin dynamics. While most mammalian cells express only one profilin, two isoforms, PFN1 and PFN2a are present in the CNS. To challenge the hypothesis that the expression of two profilin isoforms is linked to the complex shape of neurons and to the activity-dependent structural plasticity, we analysed how PFN1 and PFN2a respond to changes of neuronal activity. Simultaneous labelling of rodent embryonic neurons with isoform-specific monoclonal antibodies revealed both isoforms in the same synapse. Immunoelectron microscopy on brain sections demonstrated both profilins in synapses of the mature rodent cortex, hippocampus and cerebellum. Both isoforms were significantly more abundant in postsynaptic than in presynaptic structures. Immunofluorescence showed PFN2a associated with gephyrin clusters of the postsynaptic active zone in inhibitory synapses of embryonic neurons. When cultures were stimulated in order to change their activity level, active synapses that were identified by the uptake of synaptotagmin antibodies, displayed significantly higher amounts of both isoforms than non-stimulated controls. Specific inhibition of NMDA receptors by the antagonist APV in cultured rat hippocampal neurons resulted in a decrease of PFN2a but left PFN1 unaffected. Stimulation by the brain derived neurotrophic factor (BDNF), on the other hand, led to a significant increase in both synaptic PFN1 and PFN2a. Analogous results were obtained for neuronal nuclei: both isoforms were localized in the same nucleus, and their levels rose significantly in response to KCl stimulation, whereas BDNF caused here a higher increase in PFN1 than in PFN2a. Our results strongly support the notion of an isoform specific role for profilins as regulators of actin dynamics in different signalling pathways, in excitatory as well as in inhibitory synapses. Furthermore, they suggest a functional role for both profilins in neuronal nuclei.

Nuclear mechanics in disease

Over the past two decades, the biomechanical properties of cells have emerged as key players in a broad range of cellular functions, including migration, proliferation, and differentiation. Although much of the attention has focused on the cytoskeletal networks and the cell's microenvironment, relatively little is known about the contribution of the cell nucleus. Here, we present an overview of the structural elements that determine the physical properties of the nucleus and discuss how changes in the expression of nuclear components or mutations in nuclear proteins can not only affect nuclear mechanics but also modulate cytoskeletal organization and diverse cellular functions. These findings illustrate that the nucleus is tightly integrated into the surrounding cellular structure. Consequently, changes in nuclear structure and composition are highly relevant to normal development and physiology and can contribute to many human diseases, such as muscular dystrophy, dilated cardiomyopathy, (premature) aging, and cancer.

Chromobility: the rapid movement of chromosomes in interphase nuclei

There are an increasing number of studies reporting the movement of gene loci and whole chromosomes to new compartments within interphase nuclei. Some of the movements can be rapid, with relocation of parts of the genome within less than 15 min over a number of microns. Some of these studies have also revealed that the activity of motor proteins such as actin and myosin are responsible for these long-range movements of chromatin. Within the nuclear biology field, there remains some controversy over the presence of an active nuclear acto-myosin motor in interphase nuclei. However, both actin and myosin isoforms are localized to the nucleus, and there is a requirement for rapid and directed movements of genes and whole chromosomes and evidence for the involvement of motor proteins in this relocation. The presence of nuclear motors for chromatin movement is thus an important and timely debate to have.

Nuclear actin and lamins in viral infections

Lamins are the best characterized cytoskeletal components of the cell nucleus that help to maintain the nuclear shape and participate in diverse nuclear processes including replication or transcription. Nuclear actin is now widely accepted to be another cytoskeletal protein present in the nucleus that fulfills important functions in the gene expression. Some viruses replicating in the nucleus evolved the ability to interact with and probably utilize nuclear actin for their replication, e.g., for the assembly and transport of capsids or mRNA export. On the other hand, lamins play a role in the propagation of other viruses since nuclear lamina may represent a barrier for virions entering or escaping the nucleus. This review will summarize the current knowledge about the roles of nuclear actin and lamins in viral infections.