Active maintenance of nuclear actin by importin 9 supports transcription

Importin-9 and a TPR domain protein MpH drive periodic patterning of ciliary arrays in Tetrahymena

We explored how the number of structures is determined in an intracellular organelle series. In Tetrahymena, the oral apparatus contains three diagonal ciliary rows: M1, M2, and M3. During development, the M rows emerge by sequential segmentation of a group of basal bodies, starting with the longest and most anterior M1 and ending with the shortest and most posterior M3. The mpD-1 and mpH-1 alleles increase and decrease the number of M rows, respectively. We identify MpH as a TPR protein and MpD as an importin-9. Both proteins localize to the M rows and form concentration gradients. MpH is a row elongation factor whose loss shortens all M rows and often prevents the formation of M3. MpD limits row initiation after the emergence of M2. MpD could be a part of a negative feedback loop that limits row initiation when M1 assembly is properly advanced. We conclude that the forming oral apparatus has properties of a semi-autonomous intracellular developmental field.

Prostaglandins limit nuclear actin rod formation during Drosophila oogenesis

Expression of GFP-Actin results in nuclear actin rod formation during specific stages of Drosophila melanogaster oogenesis. Loss of prostaglandin (PG) synthesis and signaling results in an increased frequency of cells with nuclear actin rods; there are less rods per cell, but the rods are longer. These findings suggest that loss of PGs results in increased nuclear actin and are consistent with prior findings assessing the roles of PGs in modulating endogenous nuclear actin. Thus, GFP-Actin rod formation can be used as a tool to screen for new regulators of nuclear actin.

Phosphoproteomic analysis of X-ray-irradiated planarians provides novel insights into the DNA damage response

Phosphorylation plays a crucial role in the cellular response to radiation and cancer therapies, yet phosphoproteomics studies in planarians remain underexplored despite the organism's remarkable regenerative capacities. This study utilized advanced ion mobility mass spectrometry for 4D-label-free quantitative proteomics to identify phosphorylation sites associated with irradiation in planarians. A total of 33,284 phosphorylation sites from 15,505 phosphorylated peptides and 4710 unique phosphoproteins were identified. In the sub-lethal dose irradiation group, 1695 phosphoproteins with 3483 phosphorylation sites exhibited significant changes, while exposure to lethal doses of radiation led to significant changes in 2308 phosphoproteins with 6112 phosphorylation sites, including many kinases, transcription factors, and cytoskeletal proteins. Functional enrichment analysis revealed that the altered phosphoproteins were primarily involved in transcription, RNA biosynthesis, mRNA processing regulation, and spliceosomal complex assembly. Functional validation of five differentially phosphorylated proteins revealed that their depletion impaired stem cell regeneration after irradiation by disrupting DNA repair, suggesting that these proteins are critical to planarian biology and their radiation response. By identifying the phosphorylation state and specific sites of planarian proteins, our study lays the foundation for further research on protein phosphorylation in the radiation-induced DNA damage response. In addition, our findings provide preliminary insights into the role of calnexin, a protein involved in interacting with newly synthesized N-linked glycoproteins, in planarians.

Nuclear RhoA Activation Regulates Nucleus Size and DNA Content via Nuclear Activation of ROCK and pErk

RhoA is a major regulator of the actin cytoskeleton. Its function in the nucleus, however, is unclear. Fusing wildtype, fast cycling, constitutively active, and dominant negative forms of RhoA with tags promoting nuclear or cytoplasmic location and allowing specific detection, we established a platform to distinguish the functions of nuclear and cytoplasmic RhoA. Our data show that nuclear but not cytoplasmic activation of RhoA regulates DNA amount and nuclear size. This is mediated by sequential nuclear activation of the RhoA effector ROCK and Erk, a major cell cycle regulating kinase. The inhibition of ROCK or Erk activation in untransfected cells reduced DNA amounts to a similar extent, suggesting that endogenous activation levels of nuclear RhoA-ROCK-Erk signaling are sufficient for regulation. We reveal, furthermore, that GDP-bound, but not activated RhoA, translocates to the nucleus, indicating relatively separated cytoplasmic and nuclear RhoA signaling. Moreover, even the massive nuclear activation of RhoA does not cause an obvious increase in nuclear F-actin, indicating that RhoA activation is not critical for nuclear F-actin formation.

Endothelial cytoskeleton in mechanotransduction and vascular diseases

The cytoskeleton is an important structural component that regulates various aspects of cell morphology, movement, and intracellular signaling. It plays a pivotal role in the cellular response to biomechanical stimuli, particularly in endothelial cells, which are critical for vascular homeostasis and the pathogenesis of cardiovascular diseases. Mechanical forces, such as shear and tension, activate intracellular signaling cascades that regulate transcription, translation, and cellular behaviors. Despite extensive research into cytoskeletal functions, the precise mechanisms by which the cytoskeleton transduces mechanical signals remain incompletely understood. This review focuses on the role of cytoskeletal components in membrane, cytoplasm, and nucleus in mechanotransduction, with an emphasis on their structure, mechanical and biological behaviors, dynamic interactions, and response to mechanical forces. The collaboration between membrane cytoskeleton, cytoplasmic cytoskeleton, and nucleoskeleton is indispensable for endothelial cells to respond to mechanical stimuli. Understanding their mechanoresponsive mechanisms is essential for advancing therapeutic strategies for cardiovascular diseases.

Molecular characterization of the TUBG1 meshwork's influence on Cytoskeletal organization

The γ-tubulin (TUBG) meshwork is a central regulator of cellular architecture, orchestrating processes such as microtubule nucleation, mitochondrial organization, and genomic integrity. This study investigates the molecular impact of TUBG depletion on the cytoskeleton. Knockdown of TUBG using single guide RNA disrupted microtubule, vimentin, and lamin B networks while simultaneously reinforcing actin filaments structures. These findings suggest that actin reinforcement may act as a compensatory response to the broader disruption of cytoskeletal integrity. Expression of N-terminal (TUBG1-335) or C-terminal (TUBG334-451) fragments of TUBG1 partially restored these networks, with the C-terminal fragment demonstrating greater effectiveness reestablishing microtubule integrity. Both fragments stabilized vimentin filaments and the nuclear envelope, underscoring TUBG's dual structural and regulatory roles across multiple cytoskeletal systems. This study highlights the critical hubbing properties of TUBG in coordinating cytoskeletal integrity and its potential as a therapeutic target in cytoskeleton-related disorders.

Reducing Cofilin dosage makes embryos resilient to heat stress

In addition to regulating the actin cytoskeleton, Cofilin also senses and responds to environmental stress. Cofilin can promote cell survival or death depending on context. Yet, many aspects of Cofilin's role in survival need clarification. Here, we show that exposing early Drosophila embryos to mild heat stress (32°C) induces a Cofilin-mediated Actin Stress Response and upregulation of heat- and ER- stress response genes. However, these responses do not alleviate the negative impacts of heat exposure. Instead, heat stressed embryos show downregulation of hundreds of developmental genes, including determinants of the embryonic body plan, and are less likely to hatch as larvae and adults. Remarkably, reducing Cofilin dosage blunts induction of all stress response pathways, mitigates downregulation of developmental genes, and completely rescues survival. Thus, Cofilin intersects with multiple stress response pathways, and modulates the transcriptomic response to heat stress. Strikingly, Cofilin knockdown emerges as a potent pro-survival manipulation for embryos.

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.

Loss of cytoplasmic actin filaments raises nuclear actin levels to drive INO80C-dependent chromosome fragmentation

Loss of cytosolic actin filaments upon TORC2 inhibition triggers chromosome fragmentation in yeast, which results from altered base excision repair of Zeocin-induced lesions. To find the link between TORC2 kinase and this yeast chromosome shattering (YCS) we performed phosphoproteomics. YCS-relevant phospho-targets included plasma membrane-associated regulators of actin polymerization, such as Las17, the yeast Wiscott-Aldrich Syndrome protein. Induced degradation of Las17 was sufficient to trigger YCS in presence of Zeocin, bypassing TORC2 inhibition. In yeast, Las17 does not act directly at damage, but instead its loss, like TORC2 inhibition, raises nuclear actin levels. Nuclear actin, in complex with Arp4, forms an essential subunit of several nucleosome remodeler complexes, including INO80C, which facilitates DNA polymerase elongation. Here we show that the genetic ablation of INO80C activity leads to partial YCS resistance, suggesting that elevated levels of nuclear G-actin may stimulate INO80C to increase DNA polymerase processivity and convert single-strand lesions into double-strand breaks.

Exploring the role of macromolecular crowding and TNFR1 in cell volume control

The excessive cosolute densities in the intracellular fluid create a physicochemical condition called macromolecular crowding (MMC). Intracellular MMC entropically maintains the biochemical thermodynamic equilibria by favoring associative reactions while hindering transport processes. Rapid cell volume shrinkage during extracellular hypertonicity elevates the MMC and disrupts the equilibria, potentially ushering cell death. Consequently, cells actively counter the hypertonic stress through regulatory volume increase (RVI) and restore the MMC homeostasis. Here, we establish fluorescence anisotropy of EGFP as a reliable tool for studying cellular MMC and explore the spatiotemporal dynamics of MMC during cell volume instabilities under multiple conditions. Our studies reveal that the actin cytoskeleton enforces spatially varying MMC levels inside adhered cells. Within cell populations, MMC is uncorrelated with nuclear DNA content but anti-correlated with the cell spread area. Although different cell lines have statistically similar MMC distributions, their responses to extracellular hypertonicity vary. The intensity of the extracellular hypertonicity determines a cell's ability for RVI, which correlates with nuclear factor kappa beta (NFkB) activation. Pharmacological inhibition and knockdown experiments reveal that tumor necrosis factor receptor 1 (TNFR1) initiates the hypertonicity-induced NFkB signaling and RVI. At severe hypertonicities, the elevated MMC amplifies cytoplasmic microviscosity and hinders receptor interacting protein kinase 1 (RIPK1) recruitment at the TNFR1 complex, incapacitating the TNFR1-NFkB signaling and consequently, RVI. Together, our studies unveil the involvement of TNFR1-NFkB signaling in modulating RVI and demonstrate the pivotal role of MMC in determining cellular osmoadaptability.

Nuclear actin is a critical regulator of Drosophila female germline stem cell maintenance

Nuclear actin has been implicated in regulating cell fate, differentiation, and cellular reprogramming. However, its roles in development and tissue homeostasis remain largely unknown. Here we uncover the role of nuclear actin in regulating stemness using Drosophila ovarian germline stem cells (GSCs) as a model. We find that the localization and structure of nuclear actin is dynamic in the early germ cells. Nuclear actin recognized by anti-actin C4 is found in both the nucleoplasm and nucleolus of GSCs. The polymeric nucleoplasmic C4 pool is lost after the 2-cell stage, whereas the monomeric nucleolar pool persists to the 8-cell stage, suggesting that polymeric nuclear actin may contribute to stemness. To test this idea, we overexpressed nuclear targeted actin constructs to alter nuclear actin polymerization states in the GSCs and early germ cells. Increasing monomeric nuclear actin, but not polymerizable nuclear actin, causes GSC loss that ultimately results in germline loss. This GSC loss is rescued by simultaneous overexpression of monomeric and polymerizable nuclear actin. Together these data reveal that GSC maintenance requires polymeric nuclear actin. This polymeric nuclear actin likely plays numerous roles in the GSCs, as increasing monomeric nuclear actin disrupts nuclear architecture causing nucleolar hypertrophy, distortion of the nuclear lamina, and heterochromatin reorganization; all factors critical for GSC maintenance and function. These data provide the first evidence that nuclear actin, and in particular, its ability to polymerize, are critical for stem cell function and tissue homeostasis in vivo.

Small spaces, big problems: The abnormal nucleoplasm of micronuclei and its consequences

Micronuclei (MN) form from missegregated chromatin that recruits its own nuclear envelope during mitotic exit and are a common consequence of chromosomal instability. MN are unstable due to errors in nuclear envelope organization and frequently rupture, leading to loss of compartmentalization, loss of nuclear functions, and major changes in genome stability and gene expression. However, recent work found that, even prior to rupture, nuclear processes can be severely defective in MN, which may contribute to rupture-associated defects and have lasting consequences for chromatin structure and function. In this review we discuss work that highlights nuclear function defects in intact MN, including their mechanisms and consequences, and how biases in chromosome missegregation into MN may affect the penetrance of these defects. Illuminating the nuclear environment of MN demonstrates that MN formation alone has major consequences for both the genome and cell and provides new insight into how nuclear content is regulated.

Nuclear actin assembly is an integral part of decidualization in human endometrial stromal cells

Decidualization of the human endometrium is critical for establishing pregnancy and is entailed by differentiation of endometrial stromal cells (ESCs) into decidual cells. During decidualization, the actin cytoskeleton is dynamically reorganized for the ESCs' morphological and functional changes. Although actin dynamically alters its polymerized state upon external stimuli not only in the cytoplasm, but also in the nucleus, nuclear actin dynamics during decidualization have not been elucidated. Here, we show that nuclear actin was specifically assembled during decidualization of human ESCs. This decidualization-specific formation of nuclear actin filaments was disassembled following the withdrawal of the decidualization stimulus, suggesting its reversible process. Mechanistically, RNA-seq analyses revealed that the forced disassembly of nuclear actin resulted in the suppression of decidualization, accompanied with the abnormal upregulation of cell proliferation genes, leading to incomplete cell cycle arrest. CCAAT/enhancer-binding protein beta (C/EBPβ), an important regulator for decidualization, was responsible for downregulation of the nuclear actin exporter, thus accelerating nuclear actin accumulation and its assembly for decidualization. Taken together, we demonstrate that decidualization-specific nuclear actin assembly induces cell cycle arrest for establishing the decidualized state of ESCs. We propose that not only the cytoplasmic actin, but also nuclear actin dynamics profoundly affect decidualization process in humans for ensuring pregnancy.

Regulation of Precise DNA Repair by Nuclear Actin Polymerization: A Chance for Improving Gene Therapy?

Although more difficult to detect than in the cytoplasm, it is now clear that actin polymerization occurs in the nucleus and that it plays a role in the specific processes of the nucleus such as transcription, replication, and DNA repair. A number of studies suggest that nuclear actin polymerization is promoting precise DNA repair by homologous recombination, which could potentially be of help for precise genome editing and gene therapy. This review summarizes the findings and describes the challenges and chances in the field.

Chromatin plasticity in mechanotransduction

Living organisms can detect and respond to physical forces at the cellular level. The pathways that transmit these forces to the nucleus allow cells to react quickly and consistently to environmental changes. Mechanobiology involves the interaction between physical forces and biological processes and is crucial for driving embryonic development and adapting to environmental cues during adulthood. Molecular studies have shown that cells can sense mechanical signals directly through membrane receptors linked to the cytoskeleton or indirectly through biochemical cascades that can influence gene expression for environmental adaptation. This review will explore the role of epigenetic modifications, emphasizing the 3D genome architecture and nuclear structures as responders to mechanical stimuli, which ensure cellular memory and adaptability. Understanding how mechanical cues are transduced and regulate cell functioning, governing processes such as cell programming and reprogramming, is essential for advancing our knowledge of human diseases.

Single-nucleus multi-omic profiling of polyploid heart nuclei identifies fusion-derived cardiomyocytes in the human heart

Understanding the mechanisms of polyploidization in cardiomyocytes is crucial for advancing strategies to stimulate myocardial regeneration. Although endoreplication has long been considered the primary source of polyploid human cardiomyocytes, recent animal work suggests the potential for cardiomyocyte fusion. Moreover, the effects of polyploidization on the genomic-transcriptomic repertoire of human cardiomyocytes have not been studied previously. We applied single-nuclei whole genome sequencing, single nuclei RNA sequencing, and multiome ATAC + gene expression (from the same nuclei) techniques to nuclei isolated from 11 healthy hearts. Utilizing post-zygotic non-inherited somatic mutations occurring during development as "endogenous barcodes," to reconstruct lineage relationships of polyploid cardiomyocytes. Of 482 cardiomyocytes from multiple healthy donor hearts 75.7% can be sorted into several developmental clades marked by one or more somatic single-nucleotide variants (SNVs). At least ~10% of tetraploid cardiomyocytes contain cells from distinct clades, indicating fusion of lineally distinct cells, whereas 60% of higher-ploidy cardiomyocytes contain fused cells from distinct clades. Combined snRNA-seq and snATAC-seq revealed transcriptome and chromatin landscapes of polyploid cardiomyocytes distinct from diploid cardiomyocytes, and show some higher-ploidy cardiomyocytes with transcriptional signatures suggesting fusion between cardiomyocytes and endothelial and fibroblast cells. These observations provide the first evidence for cell and nuclear fusion of human cardiomyocytes, raising the possibility that cell fusion may contribute to developing or maintaining polyploid cardiomyocytes in the human heart.

Nuclear actin structure regulates chromatin accessibility

Polymerized β-actin may provide a structural basis for chromatin accessibility and actin transport into the nucleus can guide mesenchymal stem cell (MSC) differentiation. Using MSC, we show that using CK666 to inhibit Arp2/3 directed secondary actin branching results in decreased nuclear actin structure, and significantly alters chromatin access measured with ATACseq at 24 h. The ATAC-seq results due to CK666 are distinct from those caused by cytochalasin D (CytoD), which enhances nuclear actin structure. In addition, nuclear visualization shows Arp2/3 inhibition decreases pericentric H3K9me3 marks. CytoD, alternatively, induces redistribution of H3K27me3 marks centrally. Such alterations in chromatin landscape are consistent with differential gene expression associated with distinctive differentiation patterns. Further, knockdown of the non-enzymatic monomeric actin binding protein, Arp4, leads to extensive chromatin unpacking, but only a modest increase in transcription, indicating an active role for actin-Arp4 in transcription. These data indicate that dynamic actin remodeling can regulate chromatin interactions.

A phenotypic screening approach to target p60AmotL2-expressing invasive cancer cells

BACKGROUND

Tumor cells have the ability to invade and form small clusters that protrude into adjacent tissues, a phenomenon that is frequently observed at the periphery of a tumor as it expands into healthy tissues. The presence of these clusters is linked to poor prognosis and has proven challenging to treat using conventional therapies. We previously reported that p60AmotL2 expression is localized to invasive colon and breast cancer cells. In vitro, p60AmotL2 promotes epithelial cell invasion by negatively impacting E-cadherin/AmotL2-related mechanotransduction.

METHODS

Using epithelial cells transfected with inducible p60AmotL2, we employed a phenotypic drug screening approach to find compounds that specifically target invasive cells. The phenotypic screen was performed by treating cells for 72 h with a library of compounds with known antitumor activities in a dose-dependent manner. After assessing cell viability using CellTiter-Glo, drug sensitivity scores for each compound were calculated. Candidate hit compounds with a higher drug sensitivity score for p60AmotL2-expressing cells were then validated on lung and colon cell models, both in 2D and in 3D, and on colon cancer patient-derived organoids. Nascent RNA sequencing was performed after BET inhibition to analyse BET-dependent pathways in p60AmotL2-expressing cells.

RESULTS

We identified 60 compounds that selectively targeted p60AmotL2-expressing cells. Intriguingly, these compounds were classified into two major categories: Epidermal Growth Factor Receptor (EGFR) inhibitors and Bromodomain and Extra-Terminal motif (BET) inhibitors. The latter consistently demonstrated antitumor activity in human cancer cell models, as well as in organoids derived from colon cancer patients. BET inhibition led to a shift towards the upregulation of pro-apoptotic pathways specifically in p60AmotL2-expressing cells.

CONCLUSIONS

BET inhibitors specifically target p60AmotL2-expressing invasive cancer cells, likely by exploiting differences in chromatin accessibility, leading to cell death. Additionally, our findings support the use of this phenotypic strategy to discover novel compounds that can exploit vulnerabilities and specifically target invasive cancer cells.

Nuclear actin dynamics and functions at a glance

Actin is well known for its cytoskeletal functions, where it helps to control and maintain cell shape and architecture, as well as regulating cell migration and intracellular cargo transport, among others. However, actin is also prevalent in the nucleus, where genome-regulating roles have been described, including it being part of chromatin-remodeling complexes. More recently, with the help of advances in microscopy techniques and specialized imaging probes, direct visualization of nuclear actin filament dynamics has helped elucidate new roles for nuclear actin, such as in cell cycle regulation, DNA replication and repair, chromatin organization and transcriptional condensate formation. In this Cell Science at a Glance article, we summarize the known signaling events driving the dynamic assembly of actin into filaments of various structures within the nuclear compartment for essential genome functions. Additionally, we highlight the physiological role of nuclear F-actin in meiosis and early embryonic development.

Actin associates with actively elongating genes and binds directly to the Cdk9 subunit of P-TEFb

Nuclear actin has been demonstrated to be essential for optimal transcription, but the molecular mechanisms and direct binding partner for actin in the RNA polymerase complex have remained unknown. By using purified proteins in a variety of biochemical assays, we demonstrate a direct and specific interaction between monomeric actin and Cdk9, the kinase subunit of the positive transcription elongation factor b required for RNA polymerase II pause-release. This interaction efficiently prevents actin polymerization, is not dependent on kinase activity of Cdk9, and is not involved with releasing positive transcription elongation factor b from its inhibitor 7SK snRNP complex. Supporting the specific role for actin in the elongation phase of transcription, chromatin immunoprecipitation followed by deep sequencing (ChIP-seq) reveals that actin interacts with genes only upon their active transcription elongation. This study therefore provides novel insights into the mechanisms by which actin facilitates the transcription process.

A bipartite NLS motif mediates the nuclear import of Drosophila moesin

The ERM protein family, which consists of three closely related proteins in vertebrates, ezrin, radixin, and moesin (ERM), is an ancient and important group of cytoplasmic actin-binding and organizing proteins. With their FERM domain, ERMs bind various transmembrane proteins and anchor them to the actin cortex through their C-terminal F-actin binding domain, thus they are major regulators of actin dynamics in the cell. ERMs participate in many fundamental cellular processes, such as phagocytosis, microvilli formation, T-cell activation and tumor metastasis. We have previously shown that, besides its cytoplasmic activities, the single ERM protein of Drosophila melanogaster, moesin, is also present in the cell nucleus, where it participates in gene expression and mRNA export. Here we study the mechanism by which moesin enters the nucleus. We show that the nuclear import of moesin is an NLS-mediated, active process. The nuclear localization sequence of the moesin protein is an evolutionarily highly conserved, conventional bipartite motif located on the surface of the FERM domain. Our experiments also reveal that the nuclear import of moesin does not require PIP2 binding or protein activation, and occurs in monomeric form. We propose, that the balance between the phosphorylated and non-phosphorylated protein pools determines the degree of nuclear import of moesin.

Biphasic regulation of epigenetic state by matrix stiffness during cell reprogramming

We investigate how matrix stiffness regulates chromatin reorganization and cell reprogramming and find that matrix stiffness acts as a biphasic regulator of epigenetic state and fibroblast-to-neuron conversion efficiency, maximized at an intermediate stiffness of 20 kPa. ATAC sequencing analysis shows the same trend of chromatin accessibility to neuronal genes at these stiffness levels. Concurrently, we observe peak levels of histone acetylation and histone acetyltransferase (HAT) activity in the nucleus on 20 kPa matrices, and inhibiting HAT activity abolishes matrix stiffness effects. G-actin and cofilin, the cotransporters shuttling HAT into the nucleus, rises with decreasing matrix stiffness; however, reduced importin-9 on soft matrices limits nuclear transport. These two factors result in a biphasic regulation of HAT transport into nucleus, which is directly demonstrated on matrices with dynamically tunable stiffness. Our findings unravel a mechanism of the mechano-epigenetic regulation that is valuable for cell engineering in disease modeling and regenerative medicine applications.

Substrate stiffness promotes vascular smooth muscle cell calcification by reducing the levels of nuclear actin monomers

BACKGROUND

Vascular calcification (VC) is a prevalent independent risk factor for adverse cardiovascular events and is associated with diabetes, hypertension, chronic kidney disease, and atherosclerosis. However, the mechanisms regulating the osteogenic differentiation of vascular smooth muscle cells (VSMC) are not fully understood.

METHODS

Using hydrogels of tuneable stiffness and lysyl oxidase-mediated stiffening of human saphenous vein ex vivo, we investigated the role of substrate stiffness in the regulation of VSMC calcification.

RESULTS

We demonstrate that increased substrate stiffness enhances VSMC osteogenic differentiation and VSMC calcification. We show that the effects of substrate stiffness are mediated via a reduction in the level of actin monomer within the nucleus. We show that in cells interacting with soft substrate, elevated levels of nuclear actin monomer repress osteogenic differentiation and calcification by repressing YAP-mediated activation of both TEA Domain transcription factor (TEAD) and RUNX Family Transcription factor 2 (RUNX2).

CONCLUSION

This work highlights for the first time the role of nuclear actin in mediating substrate stiffness-dependent VSMC calcification and the dual role of YAP-TEAD and YAP-RUNX2 transcriptional complexes.

Loss of p53 function promotes DNA damage-induced formation of nuclear actin filaments

Tumor suppressor p53 plays a central role in response to DNA damage. DNA-damaging agents modulate nuclear actin dynamics, influencing cell behaviors; however, whether p53 affects the formation of nuclear actin filaments remains unclear. In this study, we found that p53 depletion promoted the formation of nuclear actin filaments in response to DNA-damaging agents, such as doxorubicin (DOXO) and etoposide (VP16). Even though the genetic probes used for the detection of nuclear actin filaments exerted a promotive effect on actin polymerization, the detected formation of nuclear actin filaments was highly dependent on both p53 depletion and DNA damage. Whilst active p53 is known to promote caspase-1 expression, the overexpression of caspase-1 reduced DNA damage-induced formation of nuclear actin filaments in p53-depleted cells. In contrast, co-treatment with DOXO and the pan-caspase inhibitor Q-VD-OPh or the caspase-1 inhibitor Z-YVAD-FMK induced the formation of nuclear actin filament formation even in cells bearing wild-type p53. These results suggest that the p53-caspase-1 axis suppresses DNA damage-induced formation of nuclear actin filaments. In addition, we found that the expression of nLifeact-GFP, the filamentous-actin-binding peptide Lifeact fused with the nuclear localization signal (NLS) and GFP, modulated the structure of nuclear actin filaments to be phalloidin-stainable in p53-depleted cells treated with the DNA-damaging agent, altering the chromatin structure and reducing the transcriptional activity. The level of phosphorylated H2AX (γH2AX), a marker of DNA damage, in these cells also reduced upon nLifeact-GFP expression, whilst details of the functional relationship between the formation of nLifeact-GFP-decorated nuclear actin filaments and DNA repair remained to be elucidated. Considering that the loss of p53 is associated with cancer progression, the results of this study raise a possibility that the artificial reinforcement of nuclear actin filaments by nLifeact-GFP may enhance the cytotoxic effect of DNA-damaging agents in aggressive cancer cells through a reduction in gene transcription.

Chromatin meets the cytoskeleton: the importance of nuclear actin dynamics and associated motors for genome stability

The actin cytoskeleton is of fundamental importance for numerous cellular processes, including intracellular transport, cell plasticity, and cell migration. However, functions of filamentous actin (F-actin) in the nucleus remain understudied due to the comparatively low abundance of nuclear actin and the resulting experimental limitations to its visualization. Owing to recent technological advances such as super-resolution microscopy and the development of nuclear-specific actin probes, essential roles of the actin cytoskeleton in the context of genome maintenance are now emerging. In addition to the contributions of monomeric actin as a component of multiple important nuclear protein complexes, nuclear actin has been found to undergo polymerization in response to DNA damage and DNA replication stress. Consequently, nuclear F-actin plays important roles in the regulation of intra-nuclear mobility of repair and replication foci as well as the maintenance of nuclear shape, two important aspects of efficient stress tolerance. Beyond actin itself, there is accumulating evidence for the participation of multiple actin-binding proteins (ABPs) in the surveillance of genome integrity, including nucleation factors and motor proteins of the myosin family. Here we summarize recent findings highlighting the importance of actin cytoskeletal factors within the nucleus in key genome maintenance pathways.

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.

Actin filaments accumulated in the nucleus remain in the vicinity of condensing chromosomes in the zebrafish early embryo

In the cytoplasm, filamentous actin (F-actin) plays a critical role in cell regulation, including cell migration, stress fiber formation, and cytokinesis. Recent studies have shown that actin filaments that form in the nucleus are associated with diverse functions. Here, using live imaging of an F-actin-specific probe, superfolder GFP-tagged utrophin (UtrCH-sfGFP), we demonstrated the dynamics of nuclear actin in zebrafish (Danio rerio) embryos. In early zebrafish embryos up to around the high stage, UtrCH-sfGFP increasingly accumulated in nuclei during the interphase and reached a peak during the prophase. After nuclear envelope breakdown (NEBD), patches of UtrCH-sfGFP remained in the vicinity of condensing chromosomes during the prometaphase to metaphase. When zygotic transcription was inhibited by injecting α-amanitin, the nuclear accumulation of UtrCH-sfGFP was still observed at the sphere and dome stages, suggesting that zygotic transcription may induce a decrease in nuclear F-actin. The accumulation of F-actin in nuclei may contribute to proper mitotic progression of large cells with rapid cell cycles in zebrafish early embryos, by assisting in NEBD, chromosome congression, and/or spindle assembly.

The Role of non-muscle actin paralogs in cell cycle progression and proliferation

Uncontrolled cell proliferation leads to several pathologies, including cancer. Thus, this process must be tightly regulated. The cell cycle accounts for cell proliferation, and its progression is coordinated with changes in cell shape, for which cytoskeleton reorganization is responsible. Rearrangement of the cytoskeleton allows for its participation in the precise division of genetic material and cytokinesis. One of the main cytoskeletal components is filamentous actin-based structures. Mammalian cells have at least six actin paralogs, four of which are muscle-specific, while two, named β- and γ-actin, are abundantly present in all types of cells. This review summarizes the findings that establish the role of non-muscle actin paralogs in regulating cell cycle progression and proliferation. We discuss studies showing that the level of a given non-muscle actin paralog in a cell influences the cell's ability to progress through the cell cycle and, thus, proliferation. Moreover, we elaborate on the non-muscle actins' role in regulating gene transcription, interactions of actin paralogs with proteins involved in controlling cell proliferation, and the contribution of non-muscle actins to different structures in a dividing cell. The data cited in this review show that non-muscle actins regulate the cell cycle and proliferation through varying mechanisms. We point to the need for further studies addressing these mechanisms.

SUMOylation and the oncogenic E17K mutation affect AKT1 subcellular distribution and impact on Nanog-binding dynamics to chromatin in embryonic stem cells

AKT/PKB is a kinase involved in the regulation of a plethora of cell processes. Particularly, in embryonic stem cells (ESCs), AKT is crucial for the maintenance of pluripotency. Although the activation of this kinase relies on its recruitment to the cellular membrane and subsequent phosphorylation, multiple other post-translational modifications (PTMs), including SUMOylation, fine-tune its activity and target specificity. Since this PTM can also modify the localization and availability of different proteins, in this work we explored if SUMOylation impacts on the subcellular compartmentalization and distribution of AKT1 in ESCs. We found that this PTM does not affect AKT1 membrane recruitment, but it modifies the AKT1 nucleus/cytoplasm distribution, increasing its nuclear presence. Additionally, within this compartment, we found that AKT1 SUMOylation also impacts on the chromatin-binding dynamics of NANOG, a central pluripotency transcription factor. Remarkably, the oncogenic E17K AKT1 mutant produces major changes in all these parameters increasing the binding of NANOG to its targets, also in a SUMOylation dependent manner. These findings demonstrate that SUMOylation modulates AKT1 subcellular distribution, thus adding an extra layer of regulation of its function, possibly by affecting the specificity and interaction with its downstream targets.

RHOJ controls EMT-associated resistance to chemotherapy

The resistance of cancer cells to therapy is responsible for the death of most patients with cancer1. Epithelial-to-mesenchymal transition (EMT) has been associated with resistance to therapy in different cancer cells2,3. However, the mechanisms by which EMT mediates resistance to therapy remain poorly understood. Here, using a mouse model of skin squamous cell carcinoma undergoing spontaneous EMT during tumorigenesis, we found that EMT tumour cells are highly resistant to a wide range of anti-cancer therapies both in vivo and in vitro. Using gain and loss of function studies in vitro and in vivo, we found that RHOJ-a small GTPase that is preferentially expressed in EMT cancer cells-controls resistance to therapy. Using genome-wide transcriptomic and proteomic profiling, we found that RHOJ regulates EMT-associated resistance to chemotherapy by enhancing the response to replicative stress and activating the DNA-damage response, enabling tumour cells to rapidly repair DNA lesions induced by chemotherapy. RHOJ interacts with proteins that regulate nuclear actin, and inhibition of actin polymerization sensitizes EMT tumour cells to chemotherapy-induced cell death in a RHOJ-dependent manner. Together, our study uncovers the role and the mechanisms through which RHOJ acts as a key regulator of EMT-associated resistance to chemotherapy.

PIP2-Effector Protein MPRIP Regulates RNA Polymerase II Condensation and Transcription

The specific post-translational modifications of the C-terminal domain (CTD) of the Rpb1 subunit of RNA polymerase II (RNAPII) correlate with different stages of transcription. The phosphorylation of the Ser5 residues of this domain associates with the initiation condensates, which are formed through liquid-liquid phase separation (LLPS). The subsequent Tyr1 phosphorylation of the CTD peaks at the promoter-proximal region and is involved in the pause-release of RNAPII. By implementing super-resolution microscopy techniques, we previously reported that the nuclear Phosphatidylinositol 4,5-bisphosphate (PIP2) associates with the Ser5-phosphorylated-RNAPII complex and facilitates the RNAPII transcription. In this study, we identified Myosin Phosphatase Rho-Interacting Protein (MPRIP) as a novel regulator of the RNAPII transcription that recruits Tyr1-phosphorylated CTD (Tyr1P-CTD) to nuclear PIP2-containing structures. The depletion of MPRIP increases the number of the initiation condensates, indicating a defect in the transcription. We hypothesize that MPRIP regulates the condensation and transcription through affecting the association of the RNAPII complex with nuclear PIP2-rich structures. The identification of Tyr1P-CTD as an interactor of PIP2 and MPRIP further points to a regulatory role in RNAPII pause-release, where the susceptibility of the transcriptional complex to leave the initiation condensate depends on its association with nuclear PIP2-rich structures. Moreover, the N-terminal domain of MPRIP, which is responsible for the interaction with the Tyr1P-CTD, contains an F-actin binding region that offers an explanation of how nuclear F-actin formations can affect the RNAPII transcription and condensation. Overall, our findings shed light on the role of PIP2 in RNAPII transcription through identifying the F-actin binding protein MPRIP as a transcription regulator and a determinant of the condensation of RNAPII.

Prostaglandins limit nuclear actin to control nucleolar function during oogenesis

Prostaglandins (PGs), locally acting lipid signals, regulate female reproduction, including oocyte development. However, the cellular mechanisms of PG action remain largely unknown. One cellular target of PG signaling is the nucleolus. Indeed, across organisms, loss of PGs results in misshapen nucleoli, and changes in nucleolar morphology are indicative of altered nucleolar function. A key role of the nucleolus is to transcribe ribosomal RNA (rRNA) to drive ribosomal biogenesis. Here we take advantage of the robust, in vivo system of Drosophila oogenesis to define the roles and downstream mechanisms whereby PGs regulate the nucleolus. We find that the altered nucleolar morphology due to PG loss is not due to reduced rRNA transcription. Instead, loss of PGs results in increased rRNA transcription and overall protein translation. PGs modulate these nucleolar functions by tightly regulating nuclear actin, which is enriched in the nucleolus. Specifically, we find that loss of PGs results in both increased nucleolar actin and changes in its form. Increasing nuclear actin, by either genetic loss of PG signaling or overexpression of nuclear targeted actin (NLS-actin), results in a round nucleolar morphology. Further, loss of PGs, overexpression of NLS-actin or loss of Exportin 6, all manipulations that increase nuclear actin levels, results in increased RNAPI-dependent transcription. Together these data reveal PGs carefully balance the level and forms of nuclear actin to control the level of nucleolar activity required for producing fertilization competent oocytes.

Detection of Actin in Nuclear Protein Fraction Isolated from Adult Drosophila Ovary

Much evidence supports the presence of cytoskeletal elements in the nucleus; however, the exact functions of these proteins in the nucleus are still uncertain. Of the cytoskeletal proteins, the activity and biological significance of nuclear actin has been the most extensively researched. It is now clear that actin performs essential tasks both in the cytoplasm and the nucleus, and that the dynamic balance between the large cytoplasmic and the significantly smaller nuclear actin pools is maintained by robust transport mechanisms. Therefore, the compartment-specific manipulation or investigation of actin has been an enormous challenge. Here, we present a protocol for the detection of actin in isolated nuclear protein fractions from Drosophila ovaries.

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.

Hubbing the Cancer Cell

Oncogenic transformation drives adaptive changes in a growing tumor that affect the cellular organization of cancerous cells, resulting in the loss of specialized cellular functions in the polarized compartmentalization of cells. The resulting altered metabolic and morphological patterns are used clinically as diagnostic markers. This review recapitulates the known functions of actin, microtubules and the γ-tubulin meshwork in orchestrating cell metabolism and functional cellular asymmetry.

Cytoskeletal dysregulation and neurodegenerative disease: Formation, monitoring, and inhibition of cofilin-actin rods

The presence of atypical cytoskeletal dynamics, structures, and associated morphologies is a common theme uniting numerous diseases and developmental disorders. In particular, cytoskeletal dysregulation is a common cellular feature of Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease. While the numerous activators and inhibitors of dysregulation present complexities for characterizing these elements as byproducts or initiators of the disease state, it is increasingly clear that a better understanding of these anomalies is critical for advancing the state of knowledge and plan of therapeutic attack. In this review, we focus on the hallmarks of cytoskeletal dysregulation that are associated with cofilin-linked actin regulation, with a particular emphasis on the formation, monitoring, and inhibition of cofilin-actin rods. We also review actin-associated proteins other than cofilin with links to cytoskeleton-associated neurodegenerative processes, recognizing that cofilin-actin rods comprise one strand of a vast web of interactions that occur as a result of cytoskeletal dysregulation. Our aim is to present a current perspective on cytoskeletal dysregulation, connecting recent developments in our understanding with emerging strategies for biosensing and biomimicry that will help shape future directions of the field.

Regulation of nuclear actin levels and MRTF/SRF target gene expression during PC6.3 cell differentiation

Actin has important functions in both cytoplasm and nucleus of the cell, with active nuclear transport mechanisms maintaining the cellular actin balance. Nuclear actin levels are subject to regulation during many cellular processes from cell differentiation to cancer. Here we show that nuclear actin levels increase upon differentiation of PC6.3 cells towards neuron-like cells. Photobleaching experiments demonstrate that this increase is due to decreased nuclear export of actin during cell differentiation. Increased nuclear actin levels lead to decreased nuclear localization of MRTF-A, a well-established transcription cofactor of SRF. In line with MRTF-A localization, transcriptomics analysis reveals that MRTF/SRF target gene expression is first transiently activated, but then substantially downregulated during PC6.3 cell differentiation. This study therefore describes a novel cellular context, where regulation of nuclear actin is utilized to tune MRTF/SRF target gene expression during cell differentiation.

Actin depolymerizing factor ADF7 inhibits actin bundling protein VILLIN1 to regulate root hair formation in response to osmotic stress in Arabidopsis

Actin cytoskeleton is essential for root hair formation. However, the underlying molecular mechanisms of actin dynamics in root hair formation in response to abiotic stress are largely undiscovered. Here, genetic analysis showed that actin-depolymerizing protein ADF7 and actin-bundling protein VILLIN1 (VLN1) were positively and negatively involved in root hair formation of Arabidopsis respectively. Moreover, RT-qPCR, GUS staining, western blotting, and genetic analysis revealed that ADF7 played an important role in inhibiting the expression and function of VLN1 during root hair formation. Filament actin (F-actin) dynamics observation and actin pharmacological experiments indicated that ADF7-inhibited-VLN1 pathway led to the decline of F-actin bundling and thick bundle formation, as well as the increase of F-actin depolymerization and turnover to promote root hair formation. Furthermore, the F-actin dynamics mediated by ADF7-inhibited-VLN1 pathway was associated with the reactive oxygen species (ROS) accumulation in root hair formation. Finally, ADF7-inhibited-VLN1 pathway was critical for osmotic stress-induced root hair formation. Our work demonstrates that ADF7 inhibits VLN1 to regulate F-actin dynamics in root hair formation in response to osmotic stress, providing the novel evidence on the F-actin dynamics and their molecular mechanisms in root hair formation and in abiotic stress.

Root hairs are required for plants to absorb nutrients and water. The dynamics of cytoskeleton such as actin filaments (F-actin) are necessary for the formation of root hairs, which is regulated by different kinds of cytoskeleton-binding proteins. At the same time, the dynamics of cytoskeleton are also involved in plant abiotic stress tolerance. However, there are few studies on the underlying molecular mechanisms of F-actin dynamics in root hair formation in response to abiotic stress. Actin depolymerization factor 7 (ADF7) and actin bunding protein Villin 1 (VLN1) are important actin-binding proteins in Arabidopsis. Here, we describe a pathway that ADF7 inhibits VLN1 to regulate F-actin dynamics in root hair formation in response to osmotic stress, providing a new evidence for the studies on the molecular mechanisms of F-actin dynamics in root hair formation and in plant abiotic stress tolerance.

Nuclear fascin regulates cancer cell survival

Fascin is an important regulator of F-actin bundling leading to enhanced filopodia assembly. Fascin is also overexpressed in most solid tumours where it supports invasion through control of F-actin structures at the periphery and nuclear envelope. Recently, fascin has been identified in the nucleus of a broad range of cell types but the contributions of nuclear fascin to cancer cell behaviour remain unknown. Here, we demonstrate that fascin bundles F-actin within the nucleus to support chromatin organisation and efficient DDR. Fascin associates directly with phosphorylated Histone H3 leading to regulated levels of nuclear fascin to support these phenotypes. Forcing nuclear fascin accumulation through the expression of nuclear-targeted fascin-specific nanobodies or inhibition of Histone H3 kinases results in enhanced and sustained nuclear F-actin bundling leading to reduced invasion, viability, and nuclear fascin-specific/driven apoptosis. These findings represent an additional important route through which fascin can support tumourigenesis and provide insight into potential pathways for targeted fascin-dependent cancer cell killing.

Parallel import mechanisms ensure the robust nuclear localization of actin in Drosophila

Actin, as an ancient and fundamental protein, participates in various cytoplasmic as well as nuclear functions in eukaryotic cells. Based on its manifold tasks in the nucleus, it is a reasonable assumption that the nuclear presence of actin is essential for the cell, and consequently, its nuclear localization is ensured by a robust system. However, today only a single nuclear import and a single nuclear export pathway is known which maintain the dynamic balance between cytoplasmic and nuclear actin pools. In our work, we tested the robustness of the nuclear import of actin, and investigated whether the perturbations of nuclear localization affect the viability of the whole organism. For this aim, we generated a genetic system in Drosophila, in which we rescued the lethal phenotype of the null mutation of the Actin5C gene with transgenes that express different derivatives of actin, including a Nuclear Export Signal (NES)-tagged isoform which ensures forced nuclear export of the protein. We also disrupted the SUMOylation site of actin, suggested earlier to be responsible for nuclear retention, and eliminated the activity of the single nuclear import factor dedicated to actin. We found that, individually, none of the above mentioned manipulations led to a notable reduction in nuclear actin levels and thus, fully rescued lethality. However, the NES tagging of actin, together with the knock out of its importin, significantly reduced the amount of nuclear actin and induced lethality, confirming that the presence of actin in the nucleus is essential, and thereby, over-secured. Supporting this, we identified novel nuclear importins specific to actin, which sheds light on the mechanism behind the robustness of nuclear localization of actin, and supports the idea of essentiality of its nuclear functions.

Cyclic-AMP Increases Nuclear Actin Monomer Which Promotes Proteasomal Degradation of RelA/p65 Leading to Anti-Inflammatory Effects

The second messenger, cAMP has potent immunosuppressive and anti-inflammatory actions. These have been attributed, in part, to the ability of cAMP-induced signals to interfere with the function of the proinflammatory transcription factor Nuclear Factor-kappa B (NF-κB). However, the mechanisms underlying the modulation of NF-κB activity by cAMP remain unclear. Here we demonstrate an important role for cAMP-mediated increase in nuclear actin monomer levels in inhibiting NF-κB activity. Elevated cAMP or forced expression of a nuclear localised polymerisation defective actin mutant (NLS-Actin_R62D) inhibited basal and TNFα induced mRNA levels of NF-κB-dependent genes and NF-κB-dependent reporter gene activity. Elevated cAMP or NLS-Actin_R62D did not affect NF-κB nuclear translocation but did reduce total cellular and nuclear RelA/p65 levels. Preventing the cAMP-induced increase in nuclear actin monomer, either by expressing a nuclear localised active mutant of the actin polymerising protein mDIA, silencing components of the nuclear actin import complex IPO9 and CFL1 or overexpressing the nuclear export complex XPO6, rescued RelA/p65 levels and NF-κB reporter gene activity in forskolin-stimulated cells. Elevated cAMP or NLS-Actin_R62D reduced the half-life of RelA/p65, which was reversed by the proteasome inhibitor MG132. Accordingly, forskolin stimulated association of RelA/p65 with ubiquitin affinity beads, indicating increased ubiquitination of RelA/p65 or associated proteins. Taken together, our data demonstrate a novel mechanism underlying the anti-inflammatory effects of cAMP and highlight the important role played by nuclear actin in the regulation of inflammation.

Cytoskeletal form and function in mammalian oocytes and zygotes

The actin and microtubule cytoskeletons of mammalian oocytes and zygotes exist in distinct forms at various subcellular locations. This enables each cytoskeletal system to perform vastly different functions in time and space within the same cell. In recent years, key discovery enabling tools including light-sensitive microscopy assays have helped to illuminate cytoskeletal form and function in female reproductive cell biology. New findings include unexpected participation of F-actin in oocyte chromosome segregation, oocyte specific modes of spindle self-organization as well as existence of nuclear actin polymers whose functions are only starting to emerge. Functional actin-microtubule interactions have also been identified as an important feature that supports mammalian embryo development. Other advances have revealed reproductive age-related changes in chromosome structure and dynamics that predispose mammalian eggs to aneuploidy.

Emerin self-assembly and nucleoskeletal coupling regulate nuclear envelope mechanics against stress

Emerin is an integral nuclear envelope protein participating in the maintenance of nuclear shape. When mutated or absent, emerin causes X-linked Emery-Dreifuss muscular dystrophy (EDMD). To define how emerin takes parts in molecular scaffolding at the nuclear envelope and helps protect the nucleus against mechanical stress, we established its nanoscale organization using single molecule tracking and super-resolution microscopy. We show that emerin monomers form localized oligomeric nanoclusters stabilized by both lamin A/C and SUN1 LINC complex. Interactions of emerin with nuclear actin and BAF additionally modulate its membrane mobility and its ability to oligomerize. In nuclei subjected to mechanical challenges, the mechanotransducing functions of emerin are coupled to changes in its oligomeric state, and the incremental self-assembly of emerin determines nuclear shape adaptation against forces. We also show that the abnormal nuclear envelope deformations induced by EDMD emerin mutants stem from an improper formation of lamin A/C and LINC complex-stabilized emerin oligomers. These findings place emerin at the center of the molecular processes that regulate nuclear shape remodeling in response to mechanical challenges.

Mechanically Induced Nuclear Shuttling of β-Catenin Requires Co-transfer of Actin

Mesenchymal stem cells (MSCs) respond to environmental forces with both cytoskeletal re-structuring and activation of protein chaperones of mechanical information, β-catenin, and yes-associated protein 1 (YAP1). To function, MSCs must differentiate between dynamic forces such as cyclic strains of extracellular matrix due to physical activity and static strains due to ECM stiffening. To delineate how MSCs recognize and respond differently to both force types, we compared effects of dynamic (200 cycles × 2%) and static (1 × 2% hold) strain on nuclear translocation of β-catenin and YAP1 at 3 hours after force application. Dynamic strain induced nuclear accumulation of β-catenin, and increased cytoskeletal actin structure and cell stiffness, but had no effect on nuclear YAP1 levels. Critically, both nuclear actin and nuclear stiffness increased along with dynamic strain-induced β-catenin transport. Augmentation of cytoskeletal structure using either static strain or lysophosphatidic acid did not increase nuclear content of β-catenin or actin, but induced robust nuclear increase in YAP1. As actin binds β-catenin, we considered whether β-catenin, which lacks a nuclear localization signal, was dependent on actin to gain entry to the nucleus. Knockdown of cofilin-1 (Cfl1) or importin-9 (Ipo9), which co-mediate nuclear transfer of G-actin, prevented dynamic strain-mediated nuclear transfer of both β-catenin and actin. In sum, dynamic strain induction of actin re-structuring promotes nuclear transport of G-actin, concurrently supporting nuclear access of β-catenin via mechanisms used for actin transport. Thus, dynamic and static strain activate alternative mechanoresponses reflected by differences in the cellular distributions of actin, β-catenin, and YAP1.

The prophase oocyte nucleus is a homeostatic G-actin buffer

Formation of healthy mammalian eggs from oocytes requires specialised F-actin structures. F-actin disruption produces aneuploid eggs, which are a leading cause of human embryo deaths, genetic disorders, and infertility. We found that oocytes contain prominent nuclear F-actin structures that are correlated with meiotic developmental capacity. We demonstrate that nuclear F-actin is a conserved feature of healthy mammalian oocytes and declines significantly with female reproductive ageing. Actin monomers used for nuclear F-actin assembly are sourced from an excess pool in the oocyte cytoplasm. Increasing monomeric G-actin transfer from the cytoplasm to the nucleus or directly enriching the nucleus with monomers leads to assembly of stable nuclear F-actin bundles that significantly restrict chromatin mobility. Conversely, reducing G-actin monomer transfer by blocking nuclear import triggers assembly of a dense cytoplasmic F-actin network that is incompatible with healthy oocyte development. Our data overall suggest that the large oocyte nucleus helps to maintain cytoplasmic F-actin organisation and that defects in this function could be linked with reproductive age-related female infertility.

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.

Nuclear Actin Dynamics in Gene Expression, DNA Repair, and Cancer

Actin is a highly conserved protein in mammals. The actin dynamics is regulated by actin-binding proteins and actin-related proteins. Nuclear actin and these regulatory proteins participate in multiple nuclear processes, including chromosome architecture organization, chromatin remodeling, transcription machinery regulation, and DNA repair. It is well known that the dysfunctions of these processes contribute to the development of cancer. Moreover, emerging evidence has shown that the deregulated actin dynamics is also related to cancer. This chapter discusses how the deregulation of nuclear actin dynamics contributes to tumorigenesis via such various nuclear events.

Organelle-inspired supramolecular nanomedicine to precisely abolish liver tumor growth and metastasis

Organelles are responsible for the efficient storage and transport of substances in living systems. A myriad of extracellular vesicles (EVs) acts as a bridge to exchange signaling molecules in cell–cell communication, and the highly dynamic tubulins and actins contribute to efficient intracellular substance transport. The inexhaustible cues of natural cargo delivery by organelles inspire researchers to explore the construction of biomimetic architectures for “smart” delivery carriers. Herein, we report a 10-hydroxycamptothecin (HCPT)-peptide conjugate HpYss that simulates the artificial EV-to-filament transformation process for precise liver cancer therapy. Under the sequential stimulus of extracellular alkaline phosphatase (ALP) and intracellular glutathione (GSH), HpYss proceeds via tandem self-assembly with a morphological transformation from nanoparticles to nanofibers. The experimental phase diagram elucidates the influence of ALP and GSH contents on the self-assembled nanostructures. In addition, the dynamic transformation of organelle-mimetic architectures that are formed by HpYss in HepG2 cells enables the efficient delivery of the anticancer drug HCPT to the nucleus, and the size–shape change from extracellular nanoparticles (50–100 nm) to intracellular nanofibers (4–9 nm) is verified to be of key importance for nuclear delivery. Nuclear targeting of HpYss amplifies apoptosis, thus significantly enhancing the inhibitory effect of HCPT (>10-fold) to HepG2 cells. Benefitting from the spatiotemporally controlled nanostructures, HpYss exhibited deep penetration, enhanced accumulation, and long-term retention in multicellular spheroid and xenograft models, potently abolishing liver tumor growth and preventing lung metastasis. We envision that our organelle-mimicking delivery strategy provides a novel paradigm for designing nanomedicine to cancer therapy.

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An organelle-inspired nanomedicine for precise liver cancer therapy is proposed.

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The delivery process mimics the transport of extracellular vesicles and filaments.

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The extra- and intracellular tandem self-assembly influence the nanostructures.

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The dynamic size–shape change of nanomedicine actuates the nuclear delivery.

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Spatiotemporally controlled nanomedicine abolishes liver tumor growth and lung metastasis.