Nesprin-2 Giant (NUANCE) maintains nuclear envelope architecture and composition in skin

Depletion of SUN1/2 induces heterochromatin accrual in mesenchymal stem cells during adipogenesis

Critical to the mechano-regulation of mesenchymal stem cells (MSC), Linker of the Nucleoskeleton and Cytoskeleton (LINC) complex transduces cytoskeletal forces to the nuclei. The LINC complex contains outer nuclear membrane Nesprin proteins that associate with the cytoskeleton and their inner nuclear membrane couplers, SUN proteins. Here we tested the hypothesis that severing of the LINC complex-mediated cytoskeletal connections may have different effects on chromatin organization and MSC differentiation than those due to ablation of SUN proteins. In cells cultured under adipogenic conditions, interrupting LINC complex function through dominant-negative KASH domain expression (dnKASH) increased adipogesis while heterochromatin H3K27 and H3K9 methylation was unaltered. In contrast, SUN1/2 depletion inhibited adipogenic gene expression and fat droplet formation; as well the anti-adipogenic effect of SUN1/2 depletion was accompanied by increased H3K9me3, which was enriched on Adipoq, silencing this fat locus. We conclude that releasing the nucleus from cytoskeletal constraints via dnKASH accelerates adipogenesis while depletion of SUN1/2 increases heterochromatin accrual on adipogenic genes in a fashion independent of LINC complex function. Therefore, while these two approaches both disable LINC complex functions, their divergent effects on the epigenetic landscape indicate they cannot be used interchangeably to study mechanical regulation of cell differentiation.

Inhibition of PDIs Downregulates Core LINC Complex Proteins, Promoting the Invasiveness of MDA-MB-231 Breast Cancer Cells in Confined Spaces In Vitro

Eukaryotic cells tether the nucleoskeleton to the cytoskeleton via a conserved molecular bridge, called the LINC complex. The core of the LINC complex comprises SUN-domain and KASH-domain proteins that directly associate within the nuclear envelope lumen. Intra- and inter-chain disulphide bonds, along with KASH-domain protein interactions, both contribute to the tertiary and quaternary structure of vertebrate SUN-domain proteins. The significance of these bonds and the role of PDIs (protein disulphide isomerases) in LINC complex biology remains unclear. Reducing and non-reducing SDS-PAGE analyses revealed a prevalence of SUN2 homodimers in non-tumorigenic breast epithelia MCF10A cells, but not in the invasive triple-negative breast cancer MDA-MB-231 cell line. Furthermore, super-resolution microscopy revealed SUN2 staining alterations in MCF10A, but not in MDA-MB-231 nuclei, upon reducing agent exposure. While PDIA1 levels were similar in both cell lines, pharmacological inhibition of PDI activity in MDA-MB-231 cells led to SUN-domain protein down-regulation, as well as Nesprin-2 displacement from the nucleus. This inhibition also caused changes in perinuclear cytoskeletal architecture and lamin downregulation, and increased the invasiveness of PDI-inhibited MDA-MB-231 cells in space-restrictive in vitro environments, compared to untreated cells. These results emphasise the key roles of PDIs in regulating LINC complex biology, cellular architecture, biomechanics, and invasion.

LINC complex protein nesprin-2 has pro-apoptotic activity via Bcl-2 family proteins

The apoptotic intrinsic pathway is initiated by perforation of the mitochondrial outer membrane by the effector pro-apoptotic proteins of the Bcl-2 family, Bax and Bak. Bax and Bak need to be activated, a process facilitated by the action of BH3-only pro-apoptotic members of the Bcl-2 family. The latter either directly activates the effector proteins or antagonizes the action of pro-survival Bcl-2 family members such as Bcl-xL. The nuclear envelope is a known target of the apoptotic machinery; however, it may also act as mediator of apoptosis. We showed previously that the nuclear envelope protein nesprin-2, a component of the linker of nucleoskeleton and cytoskeleton (LINC) complex, can bind to Bax in close proximity to the mitochondria and that the binding increases in apoptotic cells. We now show that depleting nesprin-2 inhibits the apoptotic mitochondrial pathway as measured by Bax and Bak activation and cytochrome c release. This survival effect was Bcl-xL-dependent. Nesprin-2 depletion also inhibited spontaneous exposure of the N-terminus of Bak in cells lacking Bcl-xL and increased the presence of Bcl-xL and Bax in the mitochondria. These results indicate that nesprin-2 promotes Bak activation and regulates mitochondrial translocation/retrotranslocation of Bcl-2 family proteins. Our findings demonstrate a new apoptotic pathway whereby the nuclear envelope, via nesprin-2, regulates apoptosis.

The secret life of chromatin tethers

The nuclear envelope plays an essential role in organizing the genome inside of the nucleus. The inner nuclear membrane is coated with a meshwork of filamentous lamin proteins that provide a surface to organize a variety of cellular processes. A subset of nuclear lamina- and membrane-associated proteins functions as anchors to hold transcriptionally silent heterochromatin at the nuclear periphery. While most chromatin tethers are integral membrane proteins, a limited number are lamina-bound. One example is the mammalian proline-rich 14 (PRR14) protein. PRR14 is a recently characterized protein with unique function that is different from other known chromatin tethers. Here, we review our current understanding of PRR14 structure and function in organizing heterochromatin at the nuclear periphery.

Identification of epigenetic modulators as determinants of nuclear size and shape

The shape and size of the human cell nucleus is highly variable among cell types and tissues. Changes in nuclear morphology are associated with disease, including cancer, as well as with premature and normal aging. Despite the very fundamental nature of nuclear morphology, the cellular factors that determine nuclear shape and size are not well understood. To identify regulators of nuclear architecture in a systematic and unbiased fashion, we performed a high-throughput imaging-based siRNA screen targeting 867 nuclear proteins including chromatin-associated proteins, epigenetic regulators, and nuclear envelope components. Using multiple morphometric parameters, and eliminating cell cycle effectors, we identified a set of novel determinants of nuclear size and shape. Interestingly, most identified factors altered nuclear morphology without affecting the levels of lamin proteins, which are known prominent regulators of nuclear shape. In contrast, a major group of nuclear shape regulators were modifiers of repressive heterochromatin. Biochemical and molecular analysis uncovered a direct physical interaction of histone H3 with lamin A mediated via combinatorial histone modifications. Furthermore, disease-causing lamin A mutations that result in disruption of nuclear shape inhibited lamin A-histone H3 interactions. Oncogenic histone H3.3 mutants defective for H3K27 methylation resulted in nuclear morphology abnormalities. Altogether, our results represent a systematic exploration of cellular factors involved in determining nuclear morphology and they identify the interaction of lamin A with histone H3 as an important contributor to nuclear morphology in human cells.

Cleavage of the Jaw1 C-terminal region enhances its augmentative effect on the Ca2+ release via IP3 receptors

Jaw1 (also known as IRAG2), a tail-anchored protein with 39 carboxyl (C)-terminal amino acids, is oriented to the lumen of the endoplasmic reticulum and outer nuclear membrane. We previously reported that Jaw1, as a member of the KASH protein family, plays a role in maintaining nuclear shape via its C-terminal region. Furthermore, we recently reported that Jaw1 functions as an augmentative effector of Ca2+ release from the endoplasmic reticulum by interacting with the inositol 1,4,5-trisphosphate receptors (IP3Rs). Intriguingly, the C-terminal region is partially cleaved, meaning that Jaw1 exists in the cell in at least two forms - uncleaved and cleaved. However, the mechanism of the cleavage event and its physiological significance remain to be determined. In this study, we demonstrate that the C-terminal region of Jaw1 is cleaved after its insertion by the signal peptidase complex (SPC). Particularly, our results indicate that the SPC with the catalytic subunit SEC11A, but not SEC11C, specifically cleaves Jaw1. Furthermore, using a mutant with a defect in the cleavage event, we demonstrate that the cleavage event enhances the augmentative effect of Jaw1 on the Ca2+ release ability of IP3Rs.

The ANC-1 (Nesprin-1/2) organelle-anchoring protein functions through mitochondria to polarize axon growth in response to SLT-1

A family of giant KASH proteins, including C. elegans ANC-1 and mammalian Nesprin-1 and -2, are involved in organelle anchoring and are associated with multiple neurodevelopmental disorders including autism, bipolar disorder, and schizophrenia. However, little is known about how these proteins function in neurons. Moreover, the role of organelle anchoring in axon development is poorly understood. Here, we report that ANC-1 functions with the SLT-1 extracellular guidance cue to polarize ALM axon growth. This role for ANC-1 is specific to its longer ANC-1A and ANC-1C isoforms, suggesting that it is mechanistically distinct from previously described roles for ANC-1. We find that ANC-1 is required for the localization of a cluster of mitochondria to the base of the proximal axon. Furthermore, genetic and pharmacological studies indicate that ANC-1 functions with mitochondria to promote polarization of ALM axon growth. These observations reveal a mechanism whereby ANC-1 functions through mitochondria to polarize axon growth in response to SLT-1.

Nuclear size rectification: A potential new therapeutic approach to reduce metastasis in cancer

Research on metastasis has recently regained considerable interest with the hope that single cell technologies might reveal the most critical changes that support tumor spread. However, it is possible that part of the answer has been visible through the microscope for close to 200 years. Changes in nuclear size characteristically occur in many cancer types when the cells metastasize. This was initially discarded as contributing to the metastatic spread because, depending on tumor types, both increases and decreases in nuclear size could correlate with increased metastasis. However, recent work on nuclear mechanics and the connectivity between chromatin, the nucleoskeleton, and the cytoskeleton indicate that changes in this connectivity can have profound impacts on cell mobility and invasiveness. Critically, a recent study found that reversing tumor type-dependent nuclear size changes correlated with reduced cell migration and invasion. Accordingly, it seems appropriate to now revisit possible contributory roles of nuclear size changes to metastasis.

LINCing Senescence and Nuclear Envelope Changes

The nuclear envelope (NE) has emerged as a nexus for cellular organization, signaling, and survival. Beyond its role as a barrier to separate the nucleoplasm from the cytoplasm, the NE's role in supporting and maintaining a myriad of other functions has made it a target of study in many cellular processes, including senescence. The nucleus undergoes dramatic changes in senescence, many of which are driven by changes in the NE. Indeed, Lamin B1, a key NE protein that is consistently downregulated in senescence, has become a marker for senescence. Other NE proteins have also been shown to play a role in senescence, including LINC (linker of nucleoskeleton and cytoskeleton) complex proteins. LINC complexes span the NE, forming physical connections between the cytoplasm to the nucleoplasm. In this way, they integrate nuclear and cytoplasmic mechanical signals and are essential not only for a variety of cellular functions but are needed for cell survival. However, LINC complex proteins have been shown to have a myriad of functions in addition to forming a LINC complex, often existing as nucleoplasmic or cytoplasmic soluble proteins in a variety of isoforms. Some of these proteins have now been shown to play important roles in DNA repair, cell signaling, and nuclear shape regulation, all of which are important in senescence. This review will focus on some of these roles and highlight the importance of LINC complex proteins in senescence.

Extracellular Adhesive Cues Physically Define Nucleolar Structure and Function

Adhesive cues from the extracellular matrix (ECM) specify the size and shape of the nucleus via mechanical forces transmitted through the cytoskeleton. However, the effects of these biophysical stimuli on internal nuclear architecture and cellular responses remain poorly understood. This study investigates the direct impact of ECM adhesion on nucleolar remodeling in human keratinocytes using micropatterned substrates. Limited adhesion on small micropatterns promotes fusion of nucleoli, alongside a reduction in nuclear volume and condensation of heterochromatin. These changes in nucleolar architecture are mediated by altered chromatin biomechanics and depend on integration of the nucleus with the actin cytoskeleton. Functionally, nucleolar remodeling regulates ribogenesis and protein synthesis in keratinocytes and is associated with specific transcriptional changes in ribogenesis genes. Together, these findings demonstrate that cell shape and nuclear morphology control nucleolar structure and function and implicate the nucleolus as a key mechano-sensing element within the cell.

Regulators of tubulin polyglutamylation control nuclear shape and cilium disassembly by balancing microtubule and actin assembly

Cytoskeletal networks play an important role in regulating nuclear morphology and ciliogenesis. However, the role of microtubule (MT) post-translational modifications in nuclear shape regulation and cilium disassembly has not been explored. Here we identified a novel regulator of the tubulin polyglutamylase complex (TPGC), C11ORF49/CSTPP1, that regulates cytoskeletal organization, nuclear shape, and cilium disassembly. Mechanistically, loss of C11ORF49/CSTPP1 impacts the assembly and stability of the TPGC, which modulates long-chain polyglutamylation levels on microtubules (MTs) and thereby balances the binding of MT-associated proteins and actin nucleators. As a result, loss of TPGC leads to aberrant, enhanced assembly of MTs that penetrate the nucleus, which in turn leads to defects in nuclear shape, and disorganization of cytoplasmic actin that disrupts the YAP/TAZ pathway and cilium disassembly. Further, we showed that C11ORF49/CSTPP1-TPGC plays mechanistically distinct roles in the regulation of nuclear shape and cilium disassembly. Remarkably, disruption of C11ORF49/CSTPP1-TPGC also leads to developmental defects in vivo. Our findings point to an unanticipated nexus that links tubulin polyglutamylation with nuclear shape and ciliogenesis.

The LINC Complex Assists the Nuclear Import of Mechanosensitive Transcriptional Regulators

Mechanical forces play pivotal roles in directing cell functions and fate. To elicit gene expression, either intrinsic or extrinsic mechanical information are transmitted into the nucleus beyond the nuclear envelope via at least two distinct pathways, possibly more. The first and well-known pathway utilizes the canonical nuclear transport of mechanoresponsive transcriptional regulators through the nuclear pore complex, which is an exclusive route for macromolecular trafficking between the cytoplasm and nucleoplasm. The second pathway depends on the linker of the nucleoskeleton and cytoskeleton (LINC) complex, which is a molecular bridge traversing the nuclear envelope between the cytoskeleton and nucleoskeleton. This protein complex is a central component in mechanotransduction at the nuclear envelope that transmits mechanical information from the cytoskeleton into the nucleus to influence the nuclear structure, nuclear stiffness, chromatin organization, and gene expression. Besides the mechanical force transducing function, recent increasing evidence shows that the LINC complex plays a role in controlling nucleocytoplasmic transport of mechanoresponsive transcriptional regulators. Here we discuss recent findings regarding the contribution of the LINC complex to the regulation of intracellular localization of the most-notable mechanosensitive transcriptional regulators, β-catenin, YAP, and TAZ.

Nuclear Morphological Abnormalities in Cancer: A Search for Unifying Mechanisms

Irregularities in nuclear shape and/or alterations to nuclear size are a hallmark of malignancy in a broad range of cancer types. Though these abnormalities are commonly used for diagnostic purposes and are often used to assess cancer progression in the clinic, the mechanisms through which they occur are not well understood. Nuclear size alterations in cancer could potentially arise from aneuploidy, changes in osmotic coupling with the cytoplasm, and perturbations to nucleocytoplasmic transport. Nuclear shape changes may occur due to alterations to cell-generated mechanical stresses and/or alterations to nuclear structural components, which balance those stresses, such as the nuclear lamina and chromatin. A better understanding of the mechanisms underlying abnormal nuclear morphology and size may allow the development of new therapeutics to target nuclear aberrations in cancer.

Silencing of Nesprin-2 inhibits the differentiation of myofibroblasts from fibroblasts induced by mechanical stretch

Mechanical force plays a pivotal role in the pathogenesis of hypertrophic scar (HTS). Dermal fibroblasts and myofibroblasts are the key cells involved in HTS. Myofibroblasts in HTS possess different biochemical and biophysical characteristics by which myofibroblasts are often distinguished from fibroblasts. The role of mechanotransducers outside the nucleus in the pathogenesis of HTS has been reported in many studies. However, the role of Nesprin‐2 in HTS is not clear. Hence, we aim to construct a cell model of HTS and explore the role of Nesprin‐2 in this process. Myofibroblasts and fibroblasts were isolated from HTS and healthy skin tissues of the same patient. Fibroblasts were exposed to cyclic stretch with 10% magnitude and a frequency of 0.1 Hz for 3 days, 5 days, and 7 days, respectively. After the cell model was confirmed, fibroblasts transfected with siRNA targeting human Nesprin‐2 were exposed to cyclic stretch. The mechanical behaviour and biochemical reaction of the dermal fibroblasts were analysed. The stretched fibroblasts at day 5 showed the same mechanotransductive and biochemical features as unstretched myofibroblasts. Mechanical strain could induce the myofibroblasts differentiation and a cell model of HTS was established successfully at day 5. The expressions of lamin A/C, alpha‐smooth muscle actin, transforming growth factor beta 1, and collagen type I in fibroblasts were reduced by the silencing of Nesprin‐2. Mechanical strain could induce the myofibroblasts differentiation and silencing of Nesprin‐2 could block the mechanical stimulation of terminal myofibroblasts differentiation. Nesprin‐2 might be a potential target to treat the HTS.

Biallelic SYNE2 Missense Mutations Leading to Nesprin-2 Giant Hypo-Expression Are Associated with Intellectual Disability and Autism

Autism spectrum disorder (ASD) is a group of neurological and developmental disabilities characterised by clinical and genetic heterogeneity. The current study aimed to expand ASD genotyping by investigating potential associations with SYNE2 mutations. Specifically, the disease-causing variants of SYNE2 in 410 trios manifesting neurodevelopmental disorders using whole-exome sequencing were explored. The consequences of the identified variants were studied at the transcript level using quantitative polymerase chain reaction (qPCR). For validation, immunofluorescence and immunoblotting were performed to analyse mutational effects at the protein level. The compound heterozygous variants of SYNE2 (NM_182914.3:c.2483T>G; p.(Val828Gly) and NM_182914.3:c.2362G>A; p.(Glu788Lys)) were identified in a 4.5-year-old male, clinically diagnosed with autism spectrum disorder, developmental delay and intellectual disability. Both variants reside within the nesprin-2 giant spectrin repeat (SR5) domain and are predicted to be highly damaging using in silico tools. Specifically, a significant reduction of nesprin-2 giant protein levels is revealed in patient cells. SYNE2 transcription and the nuclear envelope localisation of the mutant proteins was however unaffected as compared to parental control cells. Collectively, these data provide novel insights into the cardinal role of the nesprin-2 giant in neurodevelopment and suggest that the biallelic hypomorphic SYNE2 mutations may be a new cause of intellectual disability and ASD.

Regulation of diverse nuclear shapes: pathways working independently, together

Membrane-bound organelles provide physical and functional compartmentalization of biological processes in eukaryotic cells. The characteristic shape and internal organization of these organelles is determined by a combination of multiple internal and external factors. The maintenance of the shape of nucleus, which houses the genetic material within a double membrane bilayer, is crucial for a seamless spatio-temporal control over nuclear and cellular functions. Dynamic morphological changes in the shape of nucleus facilitate various biological processes. Chromatin packaging, nuclear and cytosolic protein organization, and nuclear membrane lipid homeostasis are critical determinants of overall nuclear morphology. As such, a multitude of molecular players and pathways act together to regulate the nuclear shape. Here, we review the known mechanisms governing nuclear shape in various unicellular and multicellular organisms, including the non-spherical nuclei and non-lamin-related structural determinants. The review also touches upon cellular consequences of aberrant nuclear morphologies.

Lamin microaggregates lead to altered mechanotransmission in progerin-expressing cells

The nuclear lamina is a meshwork of intermediate filament proteins, and lamin A is the primary mechanical protein. An altered splicing of lamin A, known as progerin, causes the disease Hutchinson-Gilford progeria syndrome. Progerin-expressing cells have altered nuclear shapes and stiffened nuclear lamina with microaggregates of progerin. Here, progerin microaggregate inclusions in the lamina are shown to lead to cellular and multicellular dysfunction. We show with Comsol simulations that stiffened inclusions causes redistribution of normally homogeneous forces, and this redistribution is dependent on the stiffness difference and relatively independent of inclusion size. We also show mechanotransmission changes associated with progerin expression in cells under confinement and cells under external forces. Endothelial cells expressing progerin do not align properly with patterning. Fibroblasts expressing progerin do not align properly to applied cyclic force. Combined, these studies show that altered nuclear lamina mechanics and microstructure impacts cytoskeletal force transmission through the cell.

Culturing Keratinocytes on Biomimetic Substrates Facilitates Improved Epidermal Assembly In Vitro

Mechanotransduction is defined as the ability of cells to sense mechanical stimuli from their surroundings and translate them into biochemical signals. Epidermal keratinocytes respond to mechanical cues by altering their proliferation, migration, and differentiation. In vitro cell culture, however, utilises tissue culture plastic, which is significantly stiffer than the in vivo environment. Current epidermal models fail to consider the effects of culturing keratinocytes on plastic prior to setting up three-dimensional cultures, so the impact of this non-physiological exposure on epidermal assembly is largely overlooked. In this study, primary keratinocytes cultured on plastic were compared with those grown on 4, 8, and 50 kPa stiff biomimetic hydrogels that have similar mechanical properties to skin. Our data show that keratinocytes cultured on biomimetic hydrogels exhibited major changes in cellular architecture, cell density, nuclear biomechanics, and mechanoprotein expression, such as specific Linker of Nucleoskeleton and Cytoskeleton (LINC) complex constituents. Mechanical conditioning of keratinocytes on 50 kPa biomimetic hydrogels improved the thickness and organisation of 3D epidermal models. In summary, the current study demonstrates that the effects of extracellular mechanics on keratinocyte cell biology are significant and therefore should be harnessed in skin research to ensure the successful production of physiologically relevant skin models.

The Nesprin-1/-2 ortholog ANC-1 regulates organelle positioning in C. elegans independently from its KASH or actin-binding domains

KASH proteins in the outer nuclear membrane comprise the cytoplasmic half of linker of nucleoskeleton and cytoskeleton (LINC) complexes that connect nuclei to the cytoskeleton. Caenorhabditis elegans ANC-1, an ortholog of Nesprin-1/2, contains actin-binding and KASH domains at opposite ends of a long spectrin-like region. Deletion of either the KASH or calponin homology (CH) domains does not completely disrupt nuclear positioning, suggesting neither KASH nor CH domains are essential. Deletions in the spectrin-like region of ANC-1 led to significant defects, but only recapitulated the null phenotype in combination with mutations in the transmembrane (TM) span. In anc-1 mutants, the endoplasmic reticulum ER, mitochondria, and lipid droplets were unanchored, moving throughout the cytoplasm. The data presented here support a cytoplasmic integrity model where ANC-1 localizes to the ER membrane and extends into the cytoplasm to position nuclei, ER, mitochondria, and other organelles in place.

Nuclear Envelope and Nuclear Pore Complexes in Neurodegenerative Diseases-New Perspectives for Therapeutic Interventions

Transport of proteins, transcription factors, and other signaling molecules between the nucleus and cytoplasm is necessary for signal transduction. The study of these transport phenomena is particularly challenging in neurons because of their highly polarized structure. The bidirectional exchange of molecular cargoes across the nuclear envelope (NE) occurs through nuclear pore complexes (NPCs), which are aqueous channels embedded in the nuclear envelope. The NE and NPCs regulate nuclear transport but are also emerging as relevant regulators of chromatin organization and gene expression. The alterations in nuclear transport are regularly identified in affected neurons associated with human neurodegenerative diseases. This review presents insights into the roles played by nuclear transport defects in neurodegenerative disease, focusing primarily on NE proteins and NPCs. The subcellular mislocalization of proteins might be a very desirable means of therapeutic intervention in neurodegenerative disorders.

Integrated genomic analysis identifies a genetic mutation model predicting response to immune checkpoint inhibitors in melanoma

Several biomarkers such as tumor mutation burden (TMB), neoantigen load (NAL), programmed cell‐death receptor 1 ligand (PD‐L1) expression, and lactate dehydrogenase (LDH) have been developed for predicting response to immune checkpoint inhibitors (ICIs) in melanoma. However, some limitations including the undefined cut‐off value, poor uniformity of test platform, and weak reliability of prediction have restricted the broad application in clinical practice. In order to identify a clinically actionable biomarker and explore an effective strategy for prediction, we developed a genetic mutation model named as immunotherapy score (ITS) for predicting response to ICIs therapy in melanoma, based on whole‐exome sequencing data from previous studies. We observed that patients with high ITS had better durable clinical benefit and survival outcomes than patients with low ITS in three independent cohorts, as well as in the meta‐cohort. Notably, the prediction capability of ITS was more robust than that of TMB. Remarkably, ITS was not only an independent predictor of ICIs therapy, but also combined with TMB or LDH to better predict response to ICIs than any single biomarker. Moreover, patients with high ITS harbored the immunotherapy‐sensitive characteristics including high TMB and NAL, ultraviolet light damage, impaired DNA damage repair pathway, arrested cell cycle signaling, and frequent mutations in NF1 and SERPINB3/4. Overall, these findings deserve prospective investigation in the future and may help guide clinical decisions on ICIs therapy for patients with melanoma.

ATR is essential for preservation of cell mechanics and nuclear integrity during interstitial migration

ATR responds to mechanical stress at the nuclear envelope and mediates envelope-associated repair of aberrant topological DNA states. By combining microscopy, electron microscopic analysis, biophysical and in vivo models, we report that ATR-defective cells exhibit altered nuclear plasticity and YAP delocalization. When subjected to mechanical stress or undergoing interstitial migration, ATR-defective nuclei collapse accumulating nuclear envelope ruptures and perinuclear cGAS, which indicate loss of nuclear envelope integrity, and aberrant perinuclear chromatin status. ATR-defective cells also are defective in neuronal migration during development and in metastatic dissemination from circulating tumor cells. Our findings indicate that ATR ensures mechanical coupling of the cytoskeleton to the nuclear envelope and accompanying regulation of envelope-chromosome association. Thus the repertoire of ATR-regulated biological processes extends well beyond its canonical role in triggering biochemical implementation of the DNA damage response.

The nucleus is a mechanically stiff organelle of the cell and the DNA damage response protein ATR can localize to the nuclear envelope upon mechanical stress. Here, the authors show that ATR may contribute to the integrity of the nuclear envelope and may play a role in cell migration.

PD-L1/PD-1 axis as a potent therapeutic target in breast cancer

Although both the incidence and the mortality rate of breast cancer is rising, there is no potent and practical option for the treatment of these patients, particularly in advanced stages. One of the most critical challenges for treatment is the presence of complicated and extensive tumor escape mechanisms in the tumor microenvironment. Immune checkpoint molecules are of the main immunosuppressive mechanisms used by cancerous cells to block anti-cancer immune responses. Among these molecules, PD-1 (Programmed cell death) and PD-L1 (programmed cell death-ligand 1) have been considered as worthy therapeutic targets for breast cancer therapy. In this review, we intend to discuss the immunobiology and signaling of the PD-1/PD-L1 axis and highlight its importance as a worthy therapeutic target in breast cancer. We believe that the prognostic value of PD-L1 depends on the breast cancer subtype. Moreover, the combination of PD-1/PD-L1 targeting with immune-stimulating vaccines can be considered as an effective therapeutic strategy in breast cancer.

Recapitulation of molecular regulators of nuclear motion during cell migration

Cell migration is a highly orchestrated cellular event that involves physical interactions of diverse subcellular components. The nucleus as the largest and stiffest organelle in the cell not only maintains genetic functionality, but also actively changes its morphology and translocates through dynamic formation of nucleus-bound contractile stress fibers. Nuclear motion is an active and essential process for successful cell migration and nucleus self-repairs in response to compression and extension forces in complex cell microenvironment. This review recapitulates molecular regulators that are crucial for nuclear motility during cell migration and highlights recent advances in nuclear deformation-mediated rupture and repair processes in a migrating cell.

Lack of a Retinal Phenotype in a Syne-2/Nesprin-2 Knockout Mouse Model

Syne-2 (also known as Nesprin-2) is a member of a family of proteins that are found primarily in the outer nuclear membrane, as well as other subcellular compartments. Syne-2 contains a C-terminal KASH transmembrane domain and is part of a protein network that associates the nuclear envelope to the cytoskeleton via the binding to actin filaments. Syne-2 plays a role in nuclear migration, nuclear positioning during retinal development, and in ciliogenesis. In a previous study, we showed a connection between Syne-2 and the multifunctional scaffold protein Pericentrin (Pcnt). The elimination of the interaction of Syne-2 and Pcnt showed defects in nuclear migration and the formation of outer segments during retinal development, as well as disturbances in centrosomal migration at the beginning of ciliogenesis in general. In this study, the Syne-2 KO mouse model Nesprin-2△ABD (Syne-2^tm1Ngl, MGI) with special attention to Pcnt and ciliogenesis was analyzed. We show reduced expression of Syne-2 in the retina of the Syne-2 KO mouse but found no significant structural—and only a minor functional—phenotype. For the first time, detailed expression analyses showed an expression of a Syne-2 protein larger than 400 kDa (~750 kDa) in the Syne-2/Nesprin-2 KO mouse. In conclusion, the lack of an overt phenotype in Syne-2/Nesprin-2 KO mice suggests the usage of alternative translational start sites, producing Syne-2 splice variants with an intact Pcnt interaction site. Nevertheless, deletion of the actin-binding site in the Syne-2/Nesprin-2 KO mouse revealed a high variability in scotopic oscillatory potentials assuming a novel function of Syne-2 in synchronizing inner retinal processes.

Depletion of Nesprin-2 is associated with an embryonic lethal phenotype in mice

Nesprin-2 is a nuclear envelope component and provides a link between cytoskeletal components of the cytoplasm and the nucleoplasm. Several isoforms are generated from its gene Syne2. Loss of the largest isoform Nesprin-2 Giant in mice is associated with a skin phenotype and altered wound healing, loss of C-terminal isoforms in mice leads to cardiomyopathies and neurological defects. Here we attempted to establish mice with an inducible knockout of all Nesprin-2 isoforms by inserting shRNA encoding sequences targeting the N- and C-terminus into the ROSA26 locus of mice. This caused early embryonic death of the animals harboring the mutant allele, which was presumably due to leaky expression of the shRNAs. Mutant embryos were only observed before E13. They had an altered appearance and were smaller in size than their wild type littermates. From this we conclude that the Nesprin-2 gene function is crucial during embryonic growth, differentiation and organogenesis.

Inhibition of PP2A activity by H2O2 during mitosis disrupts nuclear envelope reassembly and alters nuclear shape

Many types of cancer cells exhibit abnormal nuclear shapes induced by various molecular changes. However, whether reactive oxygen species (ROS) induce nuclear deformation has not been fully addressed. Here, we show that hydrogen peroxide (H₂O₂) treatment induced concentration-dependent alterations in nuclear shape that were abolished by pretreatment with the antioxidant N-acetyl-L-cysteine or by catalase overexpression. Interestingly, treatment with H₂O₂ induced nuclear shape alterations significantly more frequently in mitotic cells than in asynchronous cells, suggesting that H₂O₂ mainly affects nuclear envelope disassembly and/or reassembly processes. Because protein phosphatase 2 A (PP2A) activity is reported to be involved in nuclear envelope reassembly during mitosis, we investigated the possible involvement of PP2A. Indeed, H₂O₂ reduced the activity of PP2A, an effect that was mimicked by the PP1 and PP2A inhibitor okadaic acid. Moreover, overexpression of PP2A but not PP1 or PP4 partially rescued H₂O₂-induced alterations in nuclear shape, indicating that the decrease in PP2A activity induced by H₂O₂ is specifically involved in the observed nuclear shape alterations. We further show that treatment of mitotic cells with H₂O₂ induced the mislocalization of BAF (barrier-to-autointegration factor), a substrate of PP2A, during telophase. This effect was associated with Lamin A/C mislocalization and was rescued by PP2A overexpression. Collectively, our findings suggest that H₂O₂ preferentially affects mitotic cells through PP2A inhibition, which induces the subsequent mislocalization of BAF and Lamin A/C during nuclear envelope reassembly, leading to the formation of an abnormal nuclear shape.

A class of harmful chemical compounds produces morphological abnormalities in the nucleus that may help promote tumor growth. Reactive oxygen species (ROS) are DNA- and protein-damaging molecules that originate both from environmental contaminants and as a byproduct of cellular metabolism or stress. Jae-Ho Lee and colleagues at Ajou University, Suwon, South Korea have now identified a mechanism by which ROS can disrupt the shape and structure of the nucleus. They show that ROS exposure reduces the ativity of an enzyme called PP2A, which is required for the targeted recruitment of proteins that rebuild the membrane envelope surrounding the nucleus after cell division. Perturbations in this envelope can potentially contribute to damage to the chromosomal DNA within the nucleus, creating conditions that can trigger or accelerate the process of tumorigenesis.

Chromatin's physical properties shape the nucleus and its functions

The cell nucleus encloses, organizes, and protects the genome. Chromatin maintains nuclear mechanical stability and shape in coordination with lamins and the cytoskeleton. Abnormal nuclear shape is a diagnostic marker for human diseases, and it can cause nuclear dysfunction. Chromatin mechanics underlies this link, as alterations to chromatin and its physical properties can disrupt or rescue nuclear shape. The cell can regulate nuclear shape through mechanotransduction pathways that sense and respond to extracellular cues, thus modulating chromatin compaction and rigidity. These findings reveal how chromatin's physical properties can regulate cellular function and drive abnormal nuclear morphology and dysfunction in disease.

The nucleoporin ELYS regulates nuclear size by controlling NPC number and nuclear import capacity

How intracellular organelles acquire their characteristic sizes is a fundamental question in cell biology. Given stereotypical changes in nuclear size in cancer, it is important to understand the mechanisms that control nuclear size in human cells. Using a high-throughput imaging RNAi screen, we identify and mechanistically characterize ELYS, a nucleoporin required for post-mitotic nuclear pore complex (NPC) assembly, as a determinant of nuclear size in mammalian cells. ELYS knockdown results in small nuclei, reduced nuclear lamin B2 localization, lower NPC density, and decreased nuclear import. Increasing nuclear import by importin α overexpression rescues nuclear size and lamin B2 import, while inhibiting importin α/β-mediated nuclear import decreases nuclear size. Conversely, ELYS overexpression increases nuclear size, enriches nuclear lamin B2 at the nuclear periphery, and elevates NPC density and nuclear import. Consistent with these observations, knockdown or inhibition of exportin 1 increases nuclear size. Thus, we identify ELYS as a novel positive effector of mammalian nuclear size and propose that nuclear size is sensitive to NPC density and nuclear import capacity.

Nesprin-2G knockout fibroblasts exhibit reduced migration, changes in focal adhesion composition, and reduced ability to generate traction forces

The nuclear envelope protein nesprin-2G is a component of the linker of nucleoskeleton and cytoskeleton (LINC) complex and is responsible for mechanical and signaling crosstalk between the nucleus and cytoskeleton. A prior study has demonstrated that nesprin-2G knockout mice show delayed wound healing. The goal was to elucidate the mechanism underlying the delayed wound closure in this mouse model. Primary fibroblasts from wild-type and knockout neonatal mice were isolated. Knockout cells exhibited decreased focal adhesion (FA) size, number, and intensity. Consistent with this result, FA protein expression levels were decreased in knockout cells. Additionally, knockout fibroblasts displayed an abnormal actin cytoskeleton, as evidenced by loss of TAN line formation and both cytoplasmic and peri-nuclear actin staining. Using collective and single cell motility assays, it was found that knockout cells exhibited a reduction in both speed and directed migration. Traction force microscopy revealed that knockout fibroblasts generated fewer traction forces compared with WT fibroblasts. In summary, the data indicated that changes in actin organization and defects in FAs result in a reduced ability of knockout fibroblasts to generate traction forces needed for efficient motility.

Changes in the Nuclear Envelope in Laminopathies

Double-membrane-bound nucleus is the major organelle of every metazoan cell, which controls various nuclear processes like chromatin maintenance, DNA replication, transcription and nucleoskeleton-cytoskeleton coupling. Nuclear homeostasis depends on the integrity of nuclear membrane and associated proteins. Lamins, underlying the inner nuclear membrane (INM), play a crucial role in maintaining nuclear homeostasis. In this review, we have focussed on the disruption of nuclear homeostasis due to lamin A/C mutation which produces a plethora of diseases, termed as laminopathies.

Functional analyses of Pericentrin and Syne-2 interaction in ciliogenesis

Pericentrin (Pcnt) is a multifunctional scaffold protein and mutations in the human PCNT gene are associated with several diseases, including ciliopathies. Pcnt plays a crucial role in ciliary development in olfactory receptor neurons, but its function in the photoreceptor-connecting cilium is unknown. We downregulated Pcnt in the retina ex vivo and in vivo via a virus-based RNA interference approach to study Pcnt function in photoreceptors. ShRNA-mediated knockdown of Pcnt impaired the development of the connecting cilium and the outer segment of photoreceptors, and caused a nuclear migration defect. In protein interaction screens, we found that the outer nuclear membrane protein Syne-2 (also known as Nesprin-2) is an interaction partner of Pcnt in photoreceptors. Syne-2 is important for positioning murine photoreceptor cell nuclei and for centrosomal migration during early ciliogenesis. CRISPR/Cas9-mediated knockout of Syne-2 in cell culture led to an overexpression and mislocalization of Pcnt and to ciliogenesis defects. Our findings suggest that the Pcnt-Syne-2 complex is important for ciliogenesis and outer segment formation during retinal development and plays a role in nuclear migration.

Mouse models of nesprin-related diseases

Nesprins (nuclear envelope spectrin repeat proteins) are a family of multi-isomeric scaffolding proteins. Nesprins form the LInker of Nucleoskeleton-and-Cytoskeleton (LINC) complex with SUN (Sad1p/UNC84) domain-containing proteins at the nuclear envelope, in association with lamin A/C and emerin, linking the nucleoskeleton to the cytoskeleton. The LINC complex serves as both a physical linker between the nuclear lamina and the cytoskeleton and a mechanosensor. The LINC complex has a broad range of functions and is involved in maintaining nuclear architecture, nuclear positioning and migration, and also modulating gene expression. Over 80 disease-related variants have been identified in SYNE-1/2 (nesprin-1/2) genes, which result in muscular or central nervous system disorders including autosomal dominant Emery-Dreifuss muscular dystrophy, dilated cardiomyopathy and autosomal recessive cerebellar ataxia type 1. To date, 17 different nesprin mouse lines have been established to mimic these nesprin-related human diseases, which have provided valuable insights into the roles of nesprin and its scaffold LINC complex in a tissue-specific manner. In this review, we summarise the existing nesprin mouse models, compare their phenotypes and discuss the potential mechanisms underlying nesprin-associated diseases.

Luma is not essential for murine cardiac development and function

AIMS

Luma is a recently discovered, evolutionarily conserved protein expressed in mammalian heart, which is associated with the LInker of Nucleoskeleton and Cytoskeleton (LINC) complex. The LINC complex structurally integrates the nucleus and the cytoplasm and plays a critical role in mechanotransduction across the nuclear envelope. Mutations in several LINC components in both humans and mice result in various cardiomyopathies, implying they play essential, non-redundant roles. A single amino acid substitution of serine 358 to leucine (S358L) in Luma is the unequivocal cause of a distinct form of arrhythmogenic cardiomyopathy. However, the role of Luma in heart has remained obscure. In addition, it also remains to be determined how the S358L mutation in Luma leads to cardiomyopathy.

METHODS AND RESULTS

To determine the role of Luma in the heart, we first determined the expression pattern of Luma in mouse heart. Luma was sporadically expressed in cardiomyocytes throughout the heart, but was highly and uniformly expressed in cardiac fibroblasts and vascular smooth muscle cells. We also generated germline null Luma mice and discovered that germline null mutants were viable and exhibited normal cardiac function. Luma null mice also responded normally to pressure overload induced by transverse aortic constriction. In addition, localization and expression of other LINC complex components in both cardiac myocytes and fibroblasts was unaffected by global loss of Luma. Furthermore, we also generated and characterized Luma S358L knock-in mice, which displayed normal cardiac function and morphology.

CONCLUSION

Our data suggest that Luma is dispensable for murine cardiac development and function and that the Luma S358L mutation alone may not cause cardiomyopathy in mice.

Genetic Control of Left Atrial Gene Expression Yields Insights into the Genetic Susceptibility for Atrial Fibrillation

BACKGROUND

Genome-wide association studies have identified 23 loci for atrial fibrillation (AF), but the mechanisms responsible for these associations, as well as the causal genes and genetic variants, remain undefined.

METHODS

To identify the effect of common genetic variants on gene expression that might explain the mechanisms linking genome-wide association loci with AF risk, we performed RNA sequencing of left atrial appendages from a biracial cohort of 265 subjects.

RESULTS

Combining gene expression data with genome-wide single nucleotide polymorphism data, we found that approximately two-thirds of the expressed genes were regulated in cis by common genetic variants at a false discovery rate of <0.05, defined as cis-expression quantitative trait loci. Twelve of 23 reported AF genome-wide association loci displayed genome-wide significant cis-expression quantitative trait loci, at PRRX1 (chromosome 1q24), SNRNP27 (1q24), CEP68 (2p14), FKBP7 (2q31), KCNN2 (5q22), FAM13B (5q31), CAV1 (7q31), ASAH1 (8p22), MYOZ1 (10q22), C11ORF45 (11q24), TBX5 (12q24), and SYNE2 (14q23), suggesting that altered expression of these genes plays a role in AF susceptibility. Allelic expression imbalance was used as an independent method to characterize the cis-control of gene expression. One thousand two hundred forty-eight of 5153 queried genes had cis-single nucleotide polymorphisms that significantly regulated allelic expression at a false discovery rate of <0.05.

CONCLUSIONS

We provide a genome-wide catalog of the genetic control of gene expression in human left atrial appendage. These data can be used to confirm the relevance of genome-wide association loci and to direct future functional studies to identify the genes and genetic variants responsible for complex diseases such as AF.

Nesprin-1/2: roles in nuclear envelope organisation, myogenesis and muscle disease

Nesprins (nuclear envelope spectrin repeat proteins) are multi-isomeric scaffolding proteins. Nesprin-1 and -2 are highly expressed in skeletal and cardiac muscles and together with SUN (Sad1p/UNC84) domain-containing proteins form the LInker of Nucleoskeleton and Cytoskeleton (LINC) complex at the nuclear envelope in association with lamin A/C and emerin. Mutations in nesprin-1/2 have been found in patients with autosomal dominant Emery-Dreifuss muscular dystrophy (EDMD) as well as dilated cardiomyopathy (DCM). Several lines of evidence indicate that compromised LINC complex function is the critical step leading to muscle disease. Here, we review recent advances in our understanding of the functions of nesprin-1/2 in the LINC complex and mechanistic insights into how mutations in nesprin-1/2 lead to nesprin-related muscle diseases, in particular DCM and EDMD.

Chromatin histone modifications and rigidity affect nuclear morphology independent of lamins

Nuclear shape and architecture influence gene localization, mechanotransduction, transcription, and cell function. Abnormal nuclear morphology and protrusions termed "blebs" are diagnostic markers for many human afflictions including heart disease, aging, progeria, and cancer. Nuclear blebs are associated with both lamin and chromatin alterations. A number of prior studies suggest that lamins dictate nuclear morphology, but the contributions of altered chromatin compaction remain unclear. We show that chromatin histone modification state dictates nuclear rigidity, and modulating it is sufficient to both induce and suppress nuclear blebs. Treatment of mammalian cells with histone deacetylase inhibitors to increase euchromatin or histone methyltransferase inhibitors to decrease heterochromatin results in a softer nucleus and nuclear blebbing, without perturbing lamins. Conversely, treatment with histone demethylase inhibitors increases heterochromatin and chromatin nuclear rigidity, which results in reduced nuclear blebbing in lamin B1 null nuclei. Notably, increased heterochromatin also rescues nuclear morphology in a model cell line for the accelerated aging disease Hutchinson-Gilford progeria syndrome caused by mutant lamin A, as well as cells from patients with the disease. Thus, chromatin histone modification state is a major determinant of nuclear blebbing and morphology via its contribution to nuclear rigidity.

Functional Analysis of LINC Complexes in the Skin

The genome in eukaryotic cells is encased by two intricate and interconnected concentric membranes, which together with the underlying nuclear lamina form the nuclear envelope (NE). Two fundamental macromolecular structures are embedded within the nuclear envelope: the nuclear pore (NPC) and the LINC complex. The former perforates the nucleus controlling biomolecule trafficking between the nucleoplasm and the cytoplasm, while the latter integrates the nucleus via the cytoskeleton to the extracellular matrix. LINC complex structural and functional integrity is of utmost importance for various fundamental cellular functions. Mechanical forces are relayed into the nuclear interior via the LINC complex, which controls lamina organization, chromosome dynamics, and genome organization and stability. Thus, LINC constituents play pivotal roles in cellular architecture including organelle positioning, cell movement, tissue assembly, organ homeostasis, and organismal aging. The LINC complex oligomeric core contains several multi-isomeric, multifunctional, and often tissue-specific proteins. Therefore, for a proper functional analysis, genetic mouse models are an invaluable resource. Herein, we focus on the LINC complex roles in the skin and describe methods that enable the successful isolation of primary embryonic fibroblast and newborn skin cells, which can be then investigated functionally in vitro.

Novel nesprin-1 mutations associated with dilated cardiomyopathy cause nuclear envelope disruption and defects in myogenesis

Nesprins-1 and -2 are highly expressed in skeletal and cardiac muscle and together with SUN (Sad1p/UNC84)-domain containing proteins and lamin A/C form the LInker of Nucleoskeleton-and-Cytoskeleton (LINC) bridging complex at the nuclear envelope (NE). Mutations in nesprin-1/2 have previously been found in patients with autosomal dominant Emery–Dreifuss muscular dystrophy (EDMD) as well as dilated cardiomyopathy (DCM). In this study, three novel rare variants (R8272Q, S8381C and N8406K) in the C-terminus of the SYNE1 gene (nesprin-1) were identified in seven DCM patients by mutation screening. Expression of these mutants caused nuclear morphology defects and reduced lamin A/C and SUN2 staining at the NE. GST pull-down indicated that nesprin-1/lamin/SUN interactions were disrupted. Nesprin-1 mutations were also associated with augmented activation of the ERK pathway in vitro and in hearts in vivo. During C2C12 muscle cell differentiation, nesprin-1 levels are increased concomitantly with kinesin light chain (KLC-1/2) and immunoprecipitation and GST pull-down showed that these proteins interacted via a recently identified LEWD domain in the C-terminus of nesprin-1. Expression of nesprin-1 mutants in C2C12 cells caused defects in myoblast differentiation and fusion associated with dysregulation of myogenic transcription factors and disruption of the nesprin-1 and KLC-1/2 interaction at the outer nuclear membrane. Expression of nesprin-1α₂ WT and mutants in zebrafish embryos caused heart developmental defects that varied in severity. These findings support a role for nesprin-1 in myogenesis and muscle disease, and uncover a novel mechanism whereby disruption of the LINC complex may contribute to the pathogenesis of DCM.

p63 Transcription Factor Regulates Nuclear Shape and Expression of Nuclear Envelope-Associated Genes in Epidermal Keratinocytes

The maintenance of a proper nuclear architecture and three-dimensional organization of the genes, enhancer elements, and transcription machinery plays an essential role in tissue development and regeneration. Here we show that in the developing skin, epidermal progenitor cells of mice lacking p63 transcription factor display alterations in the nuclear shape accompanied by a marked decrease in expression of several nuclear envelope-associated components (Lamin B1, Lamin A/C, Sun1, Nesprin-3, Plectin) compared with controls. Furthermore, chromatin immunoprecipitation-quantitative PCR assay showed enrichment of p63 on Sun1, Syne3, and Plec promoters, suggesting them as p63 targets. Alterations in the nuclei shape and expression of nuclear envelope-associated proteins were accompanied by altered distribution patterns of the repressive histone marks trimethylation on lysine 27 of histone H3, trimethylation on lysine 9 of histone H3, and heterochromatin protein 1-alpha in p63-null keratinocytes. These changes were also accompanied by downregulation of the transcriptional activity and relocation of the keratinocyte-specific gene loci away from the sites of active transcription toward the heterochromatin-enriched repressive nuclear compartments in p63-null cells. These data demonstrate functional links between the nuclear envelope organization, chromatin architecture, and gene expression in keratinocytes and suggest nuclear envelope-associated genes as important targets mediating p63-regulated gene expression program in the epidermis.

Outer nuclear membrane protein Kuduk modulates the LINC complex and nuclear envelope architecture

LINC complexes connect the inner and outer nuclear membrane (ONM) to transduce nucleocytoskeletal force. Ding et al. identify an ONM protein, Kuduk/TMEM258, which modulates the quality of LINC complexes and regulates the nuclear envelope architecture, nuclear positioning, and the development of ovarian follicles.

Linker of nucleoskeleton and cytoskeleton (LINC) complexes spanning the nuclear envelope (NE) contribute to nucleocytoskeletal force transduction. A few NE proteins have been found to regulate the LINC complex. In this study, we identify one, Kuduk (Kud), which can reside at the outer nuclear membrane and is required for the development of Drosophila melanogaster ovarian follicles and NE morphology of myonuclei. Kud associates with LINC complex components in an evolutionarily conserved manner. Loss of Kud increases the level but impairs functioning of the LINC complex. Overexpression of Kud suppresses NE targeting of cytoskeleton-free LINC complexes. Thus, Kud acts as a quality control mechanism for LINC-mediated nucleocytoskeletal connections. Genetic data indicate that Kud also functions independently of the LINC complex. Overexpression of the human orthologue TMEM258 in Drosophila proved functional conservation. These findings expand our understanding of the regulation of LINC complexes and NE architecture.

Nuclear envelope: a new frontier in plant mechanosensing?

In animals, it is now well established that forces applied at the cell surface are propagated through the cytoskeleton to the nucleus, leading to deformations of the nuclear structure and, potentially, to modification of gene expression. Consistently, altered nuclear mechanics has been related to many genetic disorders, such as muscular dystrophy, cardiomyopathy and progeria. In plants, the integration of mechanical signals in cell and developmental biology has also made great progress. Yet, while the link between cell wall stresses and cytoskeleton is consolidated, such cortical mechanical cues have not been integrated with the nucleoskeleton. Here, we propose to take inspiration from studies on animal nuclei to identify relevant methods amenable to probing nucleus mechanics and deformation in plant cells, with a focus on microrheology. To identify potential molecular targets, we also compare the players at the nuclear envelope, namely lamina and LINC complex, in both plant and animal nuclei. Understanding how mechanical signals are transduced to the nucleus across kingdoms will likely have essential implications in development (e.g. how mechanical cues add robustness to gene expression patterns), in the nucleoskeleton-cytoskeleton nexus (e.g. how stress is propagated in turgid/walled cells), as well as in transcriptional control, chromatin biology and epigenetics.

Nesprin 1α2 is essential for mouse postnatal viability and nuclear positioning in skeletal muscle

The position of the nucleus in a cell is controlled by interactions between the linker of nucleoskeleton and cytoskeleton (LINC) complex and the cytoskeleton. Defects in nuclear positioning and abnormal aggregation of nuclei occur in many muscle diseases and correlate with muscle dysfunction. Nesprin 1, which includes multiple isoforms, is an integral component of the LINC complex, critical for nuclear positioning and anchorage in skeletal muscle, and is thought to provide an essential link between nuclei and actin. However, previous studies have yet to identify which isoform is responsible. To elucidate this, we generated a series of nesprin 1 mutant mice. We showed that the actin-binding domains of nesprin 1 were dispensable, whereas nesprin 1α2, which lacks actin-binding domains, was crucial for postnatal viability, nuclear positioning, and skeletal muscle function. Furthermore, we revealed that kinesin 1 was displaced in fibers of nesprin 1α2-knockout mice, suggesting that this interaction may play an important role in positioning of myonuclei and functional skeletal muscle.

Differential gene expression in narrow- and broad-headed European glass eels (Anguilla anguilla) points to a transcriptomic link of head shape dimorphism with growth rate and chemotaxis

One of the major challenges in evolutionary biology is to understand the mechanisms underlying morphological dimorphism and plasticity, including the genomic basis of traits and links to ecology. At the yellow eel stage of the European eel (Anguilla anguilla), two morphotypes are found: broad- and narrow-heads. This dimorphism has been linked to dietary differences, with broad-heads feeding on harder, larger prey than narrow-heads. However, recent research showed that both morphotypes could be distinguished at the glass eel stage, the nonfeeding predecessor of the yellow eel stage, implying that nondietary factors play a role in the development of this head shape dimorphism. Here, we used transcriptome profiling (RNAseq) to identify differentially expressed genes between broad- and narrow-headed glass eels. We found 260 significantly differentially expressed genes between the morphotypes, of which most were related to defence and immune responses. Interestingly, two genes involved in growth (soma and igf2) were significantly upregulated in narrow-heads, while nine genes involved in chemotaxis showed significant differential expression. Thus, we found support for the observation that head shape is associated with somatic growth, with fast-growing eels developing a narrower head. Additionally, observations in the wild have shown that slow-growers prefer freshwater, while fast-growers prefer brackish water. The differential expression of genes involved in chemotaxis seems to indicate that glass eel growth rate and habitat choice are linked. We hypothesize that two levels of segregation could take place in the European eel: first according to habitat choice and second according to feeding preference.

Control of nuclear organization by F-actin binding proteins

The regulation of nuclear shape and deformability is a key factor in controlling diverse events from embryonic development to cancer cell metastasis, but the mechanisms governing this process are still unclear. Our recent study demonstrated an unexpected role for the F-actin bundling protein fascin in controlling nuclear plasticity through a direct interaction with Nesprin-2. Nesprin-2 is a component of the LINC complex that is known to couple the F-actin cytoskeleton to the nuclear envelope. We demonstrated that fascin, which is predominantly associated with peripheral F-actin rich filopodia, binds directly to Nesprin-2 at the nuclear envelope in a range of cell types. Depleting fascin or specifically blocking the fascin-Nesprin-2 complex leads to defects in nuclear polarization, movement and cell invasion. These studies reveal a novel role for an F-actin bundling protein in control of nuclear plasticity and underline the importance of defining nuclear-associated roles for F-actin binding proteins in future.

Sex Change in Clownfish: Molecular Insights from Transcriptome Analysis

Sequential hermaphroditism is a unique reproductive strategy among teleosts that is displayed mainly in fish species living in the coral reef environment. The reproductive biology of hermaphrodites has long been intriguing; however, very little is known about the molecular pathways underlying their sex change. Here, we provide the first de novo transcriptome analyses of a hermaphrodite teleost´s undergoing sex change in its natural environment. Our study has examined relative gene expression across multiple groups-rather than just two contrasting conditions- and has allowed us to explore the differential expression patterns throughout the whole process. Our analysis has highlighted the rapid and complex genomic response of the brain associated with sex change, which is subsequently transmitted to the gonads, identifying a large number of candidate genes, some well-known and some novel, involved in the process. The present study provides strong evidence of the importance of the sex steroidogenic machinery during sex change in clownfish, with the aromatase gene playing a central role, both in the brain and the gonad. This work constitutes the first genome-wide study in a social sex-changing species and provides insights into the genetic mechanism governing social sex change and gonadal restructuring in protandrous hermaphrodites.

Implications and Assessment of the Elastic Behavior of Lamins in Laminopathies

Lamins are mechanosensitive and elastic components of the nuclear lamina that respond to external mechanical cues by altering gene regulation in a feedback mechanism. Numerous mutations in A-type lamins cause a plethora of diverse diseases collectively termed as laminopathies, the majority of which are characterized by irregularly shaped, fragile, and plastic nuclei. These nuclei are challenged to normal mechanotransduction and lead to disease phenotypes. Here, we review our current understanding of the nucleocytoskeleton coupling in mechanotransduction mediated by lamins. We also present an up-to-date understanding of the methods used to determine laminar elasticity both at the bulk and single molecule level.