Structure of Sad1-UNC84 homology (SUN) domain defines features of molecular bridge in nuclear envelope

Behind the stoNE wall: A fervent activity for nuclear lipids

The four main types of biomolecules are nucleic acids, proteins, carbohydrates and lipids. The knowledge about their respective interactions is as important as the individual understanding of each of them. However, while, for example, the interaction of proteins with the other three groups is extensively studied, that of nucleic acids and lipids is, in comparison, very poorly explored. An iconic paradigm of physical (and likely functional) proximity between DNA and lipids is the case of the genomic DNA in eukaryotes: enclosed within the nucleus by two concentric lipid bilayers, the wealth of implications of this interaction, for example in genome stability, remains underassessed. Nuclear lipid-related phenotypes have been observed for 50 years, yet in most cases kept as mere anecdotical descriptions. In this review, we will bring together the evidence connecting lipids with both the nuclear envelope and the nucleoplasm, and will make critical analyses of these descriptions. Our exploration establishes a scenario in which lipids irrefutably play a role in nuclear homeostasis.

Identification of nuclear membrane SUN proteins and components associated with wheat fungal stress responses

In eukaryotes, the nuclear membrane that encapsulates genomic DNA is composed of an inner nuclear membrane (INM), an outer nuclear membrane (ONM), and a perinuclear space. SUN proteins located in the INM and KASH proteins in the ONM form the SUN-KASH NM-bridge, which functions as the junction of the nucleocytoplasmic complex junction. Proteins containing the SUN domain showed the highest correlation with differentially accumulated proteins (DAPs) in the wheat response to fungal stress. To understand the characteristics of SUN and its associated proteins in wheat responding to pathogen stress, here we investigated and comprehensive analyzed SUN- and KASH-related proteins among the DAPs under fungi infection based on their conserved motifs. In total, four SUN proteins, one WPP domain-interacting protein (WIP), four WPP domain-interacting tail-anchored proteins (WIT), two WPP proteins and one Ran GTPase activating protein (RanGAP) were identified. Following transient expression of Nicotiana benthamiana, TaSUN2, TaRanGAP2, TaWIT1 and TaWIP1 were identified as nuclear membrane proteins, while TaWPP1 and TaWPP2 were expressed in both the nucleus and cell membrane. RT-qPCR analysis demonstrated that the transcription of TaSUN2, TaRanGAP2 and TaWPP1 were strongly upregulated in response to fungal infection. Furthermore, using the bimolecular fluorescence complementation, the luciferase complementation and a nuclear and split-ubiquitin-based membrane yeast two-hybrid systems, we substantiated the interaction between TaSUN2 and TaWIP1, as well as TaWIP1/WIT1 and TaWPP1/WPP2. Silencing of TaSUN2, TaRanGAP2 and TaWPP1 in wheat leaves promoted powdery mildew infection and hyphal growth, and reduced the expression of TaBRI1, TaBAK1 and Ta14-3-3, indicating that these NM proteins play a positive role in resistance to fungal stress. Our study reveals the characteristics of NM proteins and propose the preliminary construction of SUN-WIP-WPP-RanGAP complex in wheat, which represents a foundation for detail elucidating their functions in wheat in future.

The Drosophila Nesprin-1 homolog MSP300 is required for muscle autophagy and proteostasis

Nesprin proteins, which are components of the linker of nucleoskeleton and cytoskeleton (LINC) complex, are located within the nuclear envelope and play prominent roles in nuclear architecture. For example, LINC complex proteins interact with both chromatin and the cytoskeleton. Here, we report that the Drosophila Nesprin MSP300 has an additional function in autophagy within larval body wall muscles. RNAi-mediated MSP300 knockdown in larval body wall muscles resulted in defects in the contractile apparatus, muscle degeneration and defective autophagy. In particular, MSP300 knockdown caused accumulation of cytoplasmic aggregates that contained poly-ubiquitylated cargo, as well as the autophagy receptor ref(2)P (the fly homolog of p62 or SQSTM) and Atg8a. Furthermore, MSP300 knockdown larvae expressing an mCherry-GFP-tagged Atg8a transgene exhibited aberrant persistence of the GFP signal within these aggregates, indicating failure of autophagosome maturation. These autophagy deficits were similar to those exhibited by loss of the endoplasmic reticulum (ER) fusion protein Atlastin (Atl), raising the possibility that Atl and MSP300 might function in the same pathway. In support of this possibility, we found that a GFP-tagged MSP300 protein trap exhibited extensive localization to the ER. Alteration of ER-directed MSP300 might abrogate important cytoskeletal contacts necessary for autophagosome completion.

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.

Life at the crossroads: the nuclear LINC complex and vascular mechanotransduction

Vascular endothelial cells line the inner surface of all blood vessels, where they are exposed to polarized mechanical forces throughout their lifespan. Both basal substrate interactions and apical blood flow-induced shear stress regulate blood vessel development, remodeling, and maintenance of vascular homeostasis. Disruption of these interactions leads to dysfunction and vascular pathologies, although how forces are sensed and integrated to affect endothelial cell behaviors is incompletely understood. Recently the endothelial cell nucleus has emerged as a prominent force-transducing organelle that participates in vascular mechanotransduction, via communication to and from cell-cell and cell-matrix junctions. The LINC complex, composed of SUN and nesprin proteins, spans the nuclear membranes and connects the nuclear lamina, the nuclear envelope, and the cytoskeleton. Here we review LINC complex involvement in endothelial cell mechanotransduction, describe unique and overlapping functions of each LINC complex component, and consider emerging evidence that two major SUN proteins, SUN1 and SUN2, orchestrate a complex interplay that extends outward to cell-cell and cell-matrix junctions and inward to interactions within the nucleus and chromatin. We discuss these findings in relation to vascular pathologies such as Hutchinson-Gilford progeria syndrome, a premature aging disorder with cardiovascular impairment. More knowledge of LINC complex regulation and function will help to understand how the nucleus participates in endothelial cell force sensing and how dysfunction leads to cardiovascular disease.

The crystal structure of SUN1-KASH6 reveals an asymmetric LINC complex architecture compatible with nuclear membrane insertion

The LINC complex transmits cytoskeletal forces into the nucleus to control the structure and movement of nuclear contents. It is formed of nuclear SUN and cytoplasmic KASH proteins, which interact within the nuclear lumen, immediately below the outer nuclear membrane. However, the symmetrical location of KASH molecules within SUN-KASH complexes in previous crystal structures has been difficult to reconcile with the steric requirements for insertion of their immediately upstream transmembrane helices into the outer nuclear membrane. Here, we report the crystal structure of the SUN-KASH complex between SUN1 and JAW1/LRMP (KASH6) in an asymmetric 9:6 configuration. This intertwined assembly involves two distinct KASH conformations such that all six KASH molecules emerge on the same molecular surface. Hence, they are ideally positioned for insertion of upstream sequences into the outer nuclear membrane. Thus, we report a SUN-KASH complex architecture that appears to be directly compatible with its biological role.

Dynamic regulation of LINC complex composition and function across tissues and contexts

The concept of mechanotransduction to the nucleus through a direct force transmission mechanism has fascinated cell biologists for decades. Central to such a mechanism is the linker of nucleoskeleton and cytoskeleton (LINC) complex, which spans the nuclear envelope to couple the cytoplasmic cytoskeleton to the nuclear lamina. In reality, there is not one LINC complex identity, but instead, a family of protein configurations of varied composition that exert both shared and unique functions. Regulated expression of LINC complex components, splice variants, and mechanoresponsive protein turnover mechanisms together shape the complement of LINC complex forms present in a given cell type. Disrupting specific gene(s) encoding LINC complex components therefore gives rise to a range of organismal defects. Moreover, evidence suggests that the mechanical environment remodels LINC complexes, providing a feedback mechanism by which cellular context influences the integration of the nucleus into the cytoskeleton. In particular, evidence for crosstalk between the nuclear and cytoplasmic intermediate filament networks communicated through the LINC complex represents an emerging theme in this active area of ongoing investigation.

SUN1/2 controls macrophage polarization via modulating nuclear size and stiffness

Alteration of the size and stiffness of the nucleus triggered by environmental cues are thought to be important for eukaryotic cell fate and function. However, it remains unclear how context-dependent nuclear remodeling occurs and reprograms gene expression. Here we identify the nuclear envelope proteins SUN1/2 as mechano-regulators of the nucleus during M1 polarization of the macrophage. Specifically, we show that LPS treatment decreases the protein levels of SUN1/2 in a CK2-βTrCP-dependent manner to shrink and soften the nucleus, therefore altering the chromatin accessibility for M1-associated gene expression. Notably, the transmembrane helix of SUN1/2 is solely required and sufficient for the nuclear mechano-remodeling. Consistently, SUN1/2 depletion in macrophages facilitates their phagocytosis, tissue infiltration, and proinflammatory cytokine production, thereby boosting the antitumor immunity in mice. Thus, our study demonstrates that, in response to inflammatory cues, SUN1/2 proteins act as mechano-regulators to remodel the nucleus and chromatin for M1 polarization of the macrophage.

Nesprin-1: novel regulator of striated muscle nuclear positioning and mechanotransduction

Nesprins (nuclear envelope spectrin repeat proteins) are multi-isomeric scaffolding proteins. Giant nesprin-1 and -2 localise to the outer nuclear membrane, interact with SUN (Sad1p/UNC-84) domain-containing proteins at the inner nuclear membrane to form the LInker of Nucleoskeleton and Cytoskeleton (LINC) complex, which, in association with lamin A/C and emerin, mechanically couples the nucleus to the cytoskeleton. Despite ubiquitous expression of nesprin giant isoforms, pathogenic mutations in nesprin-1 and -2 are associated with tissue-specific disorders, particularly related to striated muscle such as dilated cardiomyopathy and Emery-Dreifuss muscular dystrophy. Recent evidence suggests this muscle-specificity might be attributable in part, to the small muscle specific isoform, nesprin-1α2, which has a novel role in striated muscle function. Our current understanding of muscle-specific functions of nesprin-1 and its isoforms will be summarised in this review to provide insight into potential pathological mechanisms of nesprin-related muscle disease and may inform potential targets of therapeutic modulation.

Molecular insights into LINC complex architecture through the crystal structure of a luminal trimeric coiled-coil domain of SUN1

The LINC complex, consisting of interacting SUN and KASH proteins, mechanically couples nuclear contents to the cytoskeleton. In meiosis, the LINC complex transmits microtubule-generated forces to chromosome ends, driving the rapid chromosome movements that are necessary for synapsis and crossing over. In somatic cells, it defines nuclear shape and positioning, and has a number of specialised roles, including hearing. Here, we report the X-ray crystal structure of a coiled-coiled domain of SUN1's luminal region, providing an architectural foundation for how SUN1 traverses the nuclear lumen, from the inner nuclear membrane to its interaction with KASH proteins at the outer nuclear membrane. In combination with light and X-ray scattering, molecular dynamics and structure-directed modelling, we present a model of SUN1's entire luminal region. This model highlights inherent flexibility between structured domains, and raises the possibility that domain-swap interactions may establish a LINC complex network for the coordinated transmission of cytoskeletal forces.

In Depth Topological Analysis of Arabidopsis Mid-SUN Proteins and Their Interaction with the Membrane-Bound Transcription Factor MaMYB

Mid-SUN proteins are a neglected family of conserved type III membrane proteins of ancient origin with representatives in plants, animals, and fungi. Previous higher plant studies have associated them with functions at the nuclear envelope and the endoplasmic reticulum (ER). In this study, high-resolution confocal light microscopy is used to explore the localisation of SUN3 and SUN4 in the perinuclear region, to explore topology, and to study the role of mid-SUNs on endoplasmic reticulum morphology. The role of SUN3 in the ER is reinforced by the identification of a protein interaction between SUN3 and the ER membrane-bound transcription factor maMYB. The results highlight the importance of mid-SUNs as functional components of the ER and outer nuclear membrane.

Genotype-Phenotype Correlations in Human Diseases Caused by Mutations of LINC Complex-Associated Genes: A Systematic Review and Meta-Summary

Mutations in genes encoding proteins associated with the linker of nucleoskeleton and cytoskeleton (LINC) complex within the nuclear envelope cause different diseases with varying phenotypes including skeletal muscle, cardiac, metabolic, or nervous system pathologies. There is some understanding of the structure of LINC complex-associated proteins and how they interact, but it is unclear how mutations in genes encoding them can cause the same disease, and different diseases with different phenotypes. Here, published mutations in LINC complex-associated proteins were systematically reviewed and analyzed to ascertain whether patterns exist between the genetic sequence variants and clinical phenotypes. This revealed LMNA is the only LINC complex-associated gene in which mutations commonly cause distinct conditions, and there are no clear genotype-phenotype correlations. Clusters of LMNA variants causing striated muscle disease are located in exons 1 and 6, and metabolic disease-associated LMNA variants are frequently found in the tail of lamin A/C. Additionally, exon 6 of the emerin gene, EMD, may be a mutation "hot-spot", and diseases related to SYNE1, encoding nesprin-1, are most often caused by nonsense type mutations. These results provide insight into the diverse roles of LINC-complex proteins in human disease and provide direction for future gene-targeted therapy development.

The KASH5 protein involved in meiotic chromosomal movements is a novel dynein activating adaptor

Dynein harnesses ATP hydrolysis to move cargo on microtubules in multiple biological contexts. Dynein meets a unique challenge in meiosis by moving chromosomes tethered to the nuclear envelope to facilitate homolog pairing essential for gametogenesis. Though processive dynein motility requires binding to an activating adaptor, the identity of the activating adaptor required for dynein to move meiotic chromosomes is unknown. We show that the meiosis-specific nuclear-envelope protein KASH5 is a dynein activating adaptor: KASH5 directly binds dynein using a mechanism conserved among activating adaptors and converts dynein into a processive motor. We map the dynein-binding surface of KASH5, identifying mutations that abrogate dynein binding in vitro and disrupt recruitment of the dynein machinery to the nuclear envelope in cultured cells and mouse spermatocytes in vivo. Our study identifies KASH5 as the first transmembrane dynein activating adaptor and provides molecular insights into how it activates dynein during meiosis.

Nuclear envelope mechanobiology: linking the nuclear structure and function

The nucleus, central to cellular activity, relies on both direct mechanical input as well as its molecular transducers to sense external stimuli and respond by regulating intra-nuclear chromatin organization that determines cell function and fate. In mesenchymal stem cells of musculoskeletal tissues, changes in nuclear structures are emerging as a key modulator of their differentiation and proliferation programs. In this review we will first introduce the structural elements of the nucleoskeleton and discuss the current literature on how nuclear structure and signaling are altered in relation to environmental and tissue level mechanical cues. We will focus on state-of-the-art techniques to apply mechanical force and methods to measure nuclear mechanics in conjunction with DNA, RNA, and protein visualization in living cells. Ultimately, combining real-time nuclear deformations and chromatin dynamics can be a powerful tool to study mechanisms of how forces affect the dynamics of genome function.

The Role of Emerin in Cancer Progression and Metastasis

It is commonly recognized in the field that cancer cells exhibit changes in the size and shape of their nuclei. These features often serve as important biomarkers in the diagnosis and prognosis of cancer patients. Nuclear size can significantly impact cell migration due to its incredibly large size. Nuclear structural changes are predicted to regulate cancer cell migration. Nuclear abnormalities are common across a vast spectrum of cancer types, regardless of tissue source, mutational spectrum, and signaling dependencies. The pervasiveness of nuclear alterations suggests that changes in nuclear structure may be crucially linked to the transformation process. The factors driving these nuclear abnormalities, and the functional consequences, are not completely understood. Nuclear envelope proteins play an important role in regulating nuclear size and structure in cancer. Altered expression of nuclear lamina proteins, including emerin, is found in many cancers and this expression is correlated with better clinical outcomes. A model is emerging whereby emerin, as well as other nuclear lamina proteins, binding to the nucleoskeleton regulates the nuclear structure to impact metastasis. In this model, emerin and lamins play a central role in metastatic transformation, since decreased emerin expression during transformation causes the nuclear structural defects required for increased cell migration, intravasation, and extravasation. Herein, we discuss the cellular functions of nuclear lamina proteins, with a particular focus on emerin, and how these functions impact cancer progression and metastasis.

Molecular models of LINC complex assembly at the nuclear envelope

Large protein complexes assemble at the nuclear envelope to transmit mechanical signals between the cytoskeleton and nucleoskeleton. These protein complexes are known as the linkers of the nucleoskeleton and cytoskeleton complexes (LINC complexes) and are formed by the interaction of SUN and KASH domain proteins in the nuclear envelope. Ample evidence suggests that SUN-KASH complexes form higher-order assemblies to withstand and transfer forces across the nuclear envelope. Herein, we present a review of recent studies over the past few years that have shed light on the mechanisms of SUN-KASH interactions, their higher order assembly, and the molecular mechanisms of force transfer across these complexes.

LINC complex regulation of genome organization and function

The regulation of genomic function is in part mediated through the physical organization and architecture of the nucleus. Disruption to nuclear organization and architecture is increasingly being recognized by its contribution to many diseases. The LINC complexes - protein structures traversing the nuclear envelope, that physically connect the nuclear interior, and hence the genome, to cytoplasmic cytoskeletal networks are an important component in the physical organization of the genome and its function. This connection, potentially allows for the constant detection of environmental mechanical stimuli, resulting in altered regulation of nuclear architecture and genome function, either directly or via the process of mechanotransduction. Here, we review the influences LINC complexes exert on genome functions and their impact on cellular/organismal health.

A molecular mechanism for LINC complex branching by structurally diverse SUN-KASH 6:6 assemblies

The Linker of Nucleoskeleton and Cytoskeleton (LINC) complex mechanically couples cytoskeletal and nuclear components across the nuclear envelope to fulfil a myriad of cellular functions, including nuclear shape and positioning, hearing, and meiotic chromosome movements. The canonical model is that 3:3 interactions between SUN and KASH proteins underlie the nucleocytoskeletal linkages provided by the LINC complex. Here, we provide crystallographic and biophysical evidence that SUN-KASH is a constitutive 6:6 complex in which two constituent 3:3 complexes interact head-to-head. A common SUN-KASH topology is achieved through structurally diverse 6:6 interaction mechanisms by distinct KASH proteins, including zinc-coordination by Nesprin-4. The SUN-KASH 6:6 interface provides a molecular mechanism for the establishment of integrative and distributive connections between 3:3 structures within a branched LINC complex network. In this model, SUN-KASH 6:6 complexes act as nodes for force distribution and integration between adjacent SUN and KASH molecules, enabling the coordinated transduction of large forces across the nuclear envelope.

The Role of the LINC Complex in Sperm Development and Function

The LINC (LInker of Nucleoskeleton and Cytoskeleton) complex is localized within the nuclear envelope and consists of SUN (Sad1/UNc84 homology domain-containing) proteins located in the inner nuclear membrane and KASH (Klarsicht/Anc1/Syne1 homology domain-containing) proteins located in the outer nuclear membrane, hence linking nuclear with cytoplasmic structures. While the nucleoplasm-facing side acts as a key player for correct pairing of homolog chromosomes and rapid chromosome movements during meiosis, the cytoplasm-facing side plays a pivotal role for sperm head development and proper acrosome formation during spermiogenesis. A further complex present in spermatozoa is involved in head-to-tail coupling. An intact LINC complex is crucial for the production of fertile sperm, as mutations in genes encoding for complex proteins are known to be associated with male subfertility in both mice and men. The present review provides a comprehensive overview on our current knowledge of LINC complex subtypes present in germ cells and its central role for male reproduction. Future studies on distinct LINC complex components are an absolute requirement to improve the diagnosis of idiopathic male factor infertility and the outcome of assisted reproduction.

Chromosome-nuclear envelope tethering - a process that orchestrates homologue pairing during plant meiosis?

During prophase I of meiosis, homologous chromosomes pair, synapse and exchange their genetic material through reciprocal homologous recombination, a phenomenon essential for faithful chromosome segregation. Partial sequence identity between non-homologous and heterologous chromosomes can also lead to recombination (ectopic recombination), a highly deleterious process that rapidly compromises genome integrity. To avoid ectopic exchange, homology recognition must be extended from the narrow position of a crossover-competent double-strand break to the entire chromosome. Here, we review advances on chromosome behaviour during meiotic prophase I in higher plants, by integrating centromere- and telomere dynamics driven by cytoskeletal motor proteins, into the processes of homologue pairing, synapsis and recombination. Centromere-centromere associations and the gathering of telomeres at the onset of meiosis at opposite nuclear poles create a spatially organised and restricted nuclear state in which homologous DNA interactions are favoured but ectopic interactions also occur. The release and dispersion of centromeres from the nuclear periphery increases the motility of chromosome arms, allowing meiosis-specific movements that disrupt ectopic interactions. Subsequent expansion of interstitial synapsis from numerous homologous interactions further corrects ectopic interactions. Movement and organisation of chromosomes, thus, evolved to facilitate the pairing process, and can be modulated by distinct stages of chromatin associations at the nuclear envelope and their collective release.

Mislocalization of cone nuclei impairs cone function in mice

The nuclei of cone photoreceptors are located on the apical side of the outer nuclear layer (ONL) in vertebrate retinas. However, the functional role of this evolutionarily conserved localization of cone nuclei is unknown. We previously showed that Linkers of the Nucleoskeleton to the Cytoskeleton (LINC complexes) are essential for the apical migration of cone nuclei during development. Here, we developed an efficient genetic strategy to disrupt cone LINC complexes in mice. Experiments with animals from both sexes revealed that disrupting cone LINC complexes resulted in mislocalization of cone nuclei to the basal side of ONL in mouse retina. This, in turn, disrupted cone pedicle morphology, and appeared to reduce the efficiency of synaptic transmission from cones to bipolar cells. Although we did not observe other developmental or phototransduction defects in cones with mislocalized nuclei, their dark adaptation was impaired, consistent with a deficiency in chromophore recycling. These findings demonstrate that the apical localization of cone nuclei in the ONL is required for the timely dark adaptation and efficient synaptic transmission in cone photoreceptors.

Differentiating Luminal and Membrane-Associated Nuclear Envelope Proteins

The nuclear envelope (NE) consists of two concentric nuclear membranes separated by the lumen, an ∼40-nm-wide fluid layer. NE proteins are implicated in important cellular processes ranging from gene expression to nuclear positioning. Although recent progress has been achieved in quantifying the assembly states of NE proteins in their native environment with fluorescence fluctuation spectroscopy, these studies raised questions regarding the association of NE proteins with nuclear membranes during the assembly process. Monitoring the interaction of proteins with membranes is important because the binding event is often associated with conformational changes that are critical to cellular signaling pathways. Unfortunately, the close physical proximity of both membranes poses a severe experimental challenge in distinguishing luminal and membrane-associated NE proteins. This study seeks to address this problem by introducing new, to our knowledge, fluorescence-based assays that overcome the restrictions imposed by the NE environment. We found that luminal proteins violate the Stokes-Einstein relation, which eliminates a straightforward use of protein mobility as a marker of membrane association within the NE. However, a surprising anomaly in the temperature-dependent mobility of luminal proteins was observed, which was developed into an assay for distinguishing between soluble and membrane-bound NE proteins. We further introduced a second independent tool for distinguishing both protein populations by harnessing the previously reported undulations of the nuclear membranes. These membrane undulations introduce local volume changes that produce an additional fluorescence fluctuation signal for luminal, but not for membrane-bound, proteins. After testing both methods using simple model systems, we apply the two assays to investigate a previously proposed model of membrane association for the luminal domain of SUN2, a constituent protein of the linker of nucleoskeleton and cytoskeleton complex. Finally, we investigate the effect of C- and N-terminal tagging of the luminal ATPase torsinA on its ability to associate with nuclear membranes.

SUN/KASH interactions facilitate force transmission across the nuclear envelope

LINC complexes (Linker of Nucleoskeleton and Cytoskeleton), consisting of inner nuclear membrane SUN (Sad1, UNC-84) proteins and outer nuclear membrane KASH (Klarsicht, ANC-1, and Syne Homology) proteins, are essential for nuclear positioning, cell migration and chromosome dynamics. To test the in vivo functions of conserved interfaces revealed by crystal structures, Cain et al used a combination of Caenorhabditis elegans genetics, imaging in cultured NIH 3T3 fibroblasts, and Molecular Dynamic simulations, to study SUN-KASH interactions. Conserved aromatic residues at the -7 position of the C-termini of KASH proteins and conserved disulfide bonds in LINC complexes play important roles in force transmission across the nuclear envelope. Other properties of LINC complexes, such as the helices preceding the SUN domain, the longer coiled-coils spanning the perinuclear space and higher-order organization may also function to transmit mechanical forces generated by the cytoskeleton across the nuclear envelope.

SUNny Ways: The Role of the SUN-Domain Protein Mps3 Bridging Yeast Nuclear Organization and Lipid Homeostasis

Mps3 is a SUN (Sad1-UNC-84) domain-containing protein that is located in the inner nuclear membrane (INM). Genetic screens with multiple Mps3 mutants have suggested that distinct regions of Mps3 function in relative isolation and underscore the broad involvement of Mps3 in multiple pathways including mitotic spindle formation, telomere maintenance, and lipid metabolism. These pathways have largely been characterized in isolation, without a holistic consideration for how key regulatory events within one pathway might impinge on other aspects of biology at the nuclear membrane. Mps3 is uniquely positioned to function in these multiple pathways as its N- terminus is in the nucleoplasm, where it is important for telomere anchoring at the nuclear periphery, and its C-terminus is in the lumen, where it has links with lipid metabolic processes. Emerging work suggests that the role of Mps3 in nuclear organization and lipid homeostasis are not independent, but more connected. For example, a failure in regulating Mps3 levels through the cell cycle leads to nuclear morphological abnormalities and loss of viability, suggesting a link between the N-terminal domain of Mps3 and nuclear envelope homeostasis. We will highlight work suggesting that Mps3 is pivotal factor in communicating events between the nucleus and the lipid bilayer.

Nesprin-1-alpha2 associates with kinesin at myotube outer nuclear membranes, but is restricted to neuromuscular junction nuclei in adult muscle

Nesprins, nuclear envelope spectrin-repeat proteins encoded by the SYNE1 and SYNE2 genes, are involved in localization of nuclei. The short isoform, nesprin-1-alpha2, is required for relocation of the microtubule organizer function from centromeres to the nuclear rim during myogenesis. Using specific antibodies, we now show that both nesprin-1-alpha2 and nesprin-1-giant co-localize with kinesin at the junctions of concatenated nuclei and at the outer poles of nuclear chains in human skeletal myotubes. In adult muscle, nesprin-1-alpha2 was found, together with kinesin, only on nuclei associated with neuromuscular junctions, whereas all adult cardiomyocyte nuclei expressed nesprin-1-alpha2. In a proteomics study, kinesin heavy and light chains were the only significant proteins in myotube extracts pulled down by nesprin-1-alpha2, but not by a mutant lacking the highly-conserved STAR domain (18 amino-acids, including the LEWD motif). The results support a function for nesprin-1-alpha2 in the specific localization of skeletal muscle nuclei mediated by kinesins and suggest that its primary role is at the outer nuclear membrane.

Quantitative modeling of self-oligomerization of proteins in the nuclear envelope by fluorescence fluctuation analysis

Analysis of fluorescence fluctuation data through the time-shifted mean-segmented Q (tsMSQ) analysis method has recently been shown to successfully identify protein oligomerization and mobility in the nuclear envelope by properly accounting for local volume fluctuations of the nuclear envelope within living cells. However, by its nature, tsMSQ produces correlated data which poses unique challenges for applying goodness of fit tests and obtaining parameter uncertainties from individual measurements. In this paper, we overcome these challenges by introducing bootstrap tsMSQ which involves randomly resampling the fluorescence intensity data to eliminate the correlations in the tsMSQ data. This analysis technique was verified in both the cytoplasm and the lumen of the nuclear envelope with well-characterized proteins that served as model systems. Uncertainties and goodness of fit tests of individual measurements were compared to estimates obtained from sampling multiple experiments. We further applied bootstrapping to fluorescence fluctuation data of the luminal domain of the SUN domain-containing protein 2 in order to characterize its self-oligomerization within the nuclear envelope. Analysis of the concentration-dependent brightness suggests a monomer-trimer transition of the protein.

Measuring similarity between gene interaction profiles

Background

Gene and protein interaction data are often represented as interaction networks, where nodes stand for genes or gene products and each edge stands for a relationship between a pair of gene nodes. Commonly, that relationship within a pair is specified by high similarity between profiles (vectors) of experimentally defined interactions of each of the two genes with all other genes in the genome; only gene pairs that interact with similar sets of genes are linked by an edge in the network. The tight groups of genes/gene products that work together in a cell can be discovered by the analysis of those complex networks.

Results

We show that the choice of the similarity measure between pairs of gene vectors impacts the properties of networks and of gene modules detected within them. We re-analyzed well-studied data on yeast genetic interactions, constructed four genetic networks using four different similarity measures, and detected gene modules in each network using the same algorithm. The four networks induced different numbers of putative functional gene modules, and each similarity measure induced some unique modules. In an example of a putative functional connection suggested by comparing genetic interaction vectors, we predict a link between SUN-domain proteins and protein glycosylation in the endoplasmic reticulum.

Conclusions

The discovery of molecular modules in genetic networks is sensitive to the way of measuring similarity between profiles of gene interactions in a cell. In the absence of a formal way to choose the “best” measure, it is advisable to explore the measures with different mathematical properties, which may identify different sets of connections between genes.

Electronic supplementary material

The online version of this article (10.1186/s12859-019-3024-x) contains supplementary material, which is available to authorized users.

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

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

Impact statement

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

Mechanosensing at the Nuclear Envelope by Nuclear Pore Complex Stretch Activation and Its Effect in Physiology and Pathology

Cell fate is correlated to mechanotransduction, in which forces transmitted by the cytoskeleton filaments alter the nuclear shape, affecting transcription factor import/export, cells transcription activity and chromatin distribution. There is in fact evidence that stem cells cultured in 3D environments mimicking the native niche are able to maintain their stemness or modulate their cellular function. However, the molecular and biophysical mechanisms underlying cellular mechanosensing are still largely unclear. The propagation of mechanical stimuli via a direct pathway from cell membrane integrins to SUN proteins residing in the nuclear envelop has been demonstrated, but we suggest that the cells’ fate is mainly affected by the force distribution at the nuclear envelope level, where the SUN protein transmits the stimuli via its mechanical connection to several cell structures such as chromatin, lamina and the nuclear pore complex (NPC). In this review, we analyze the NPC structure and organization, which have not as yet been fully investigated, and its plausible involvement in cell fate. NPC is a multiprotein complex that spans the nuclear envelope, and is involved in several key cellular processes such as bidirectional nucleocytoplasmic exchange, cell cycle regulation, kinetochore organization, and regulation of gene expression. As several connections between the NPC and the nuclear envelope, chromatin and other transmembrane proteins have been identified, it is reasonable to suppose that nuclear deformations can alter the NPC structure. We provide evidence that the transmission of mechanical forces may significantly affects the basket conformation via the Nup153-SUN1 connection, both altering the passage of molecules through it and influencing the state of chromatin packing. Finally, we review the known correlations between a pathological NPC structure and diseases such as cancer, autoimmune disease, aging and laminopathies.

Molecular Insights into the Mechanisms of SUN1 Oligomerization in the Nuclear Envelope

The LINC complex is found in a wide variety of organisms and is formed by the transluminal interaction between outer- and inner-nuclear-membrane KASH and SUN proteins, respectively. Most extensively studied are SUN1 and SUN2 proteins, which are widely expressed in mammals. Although SUN1 and SUN2 play functionally redundant roles in several cellular processes, more recent studies have revealed diverse and distinct functions for SUN1. While several recent in vitro structural studies have revealed the molecular details of various fragments of SUN2, no such structural information is available for SUN1. Herein, we conduct a systematic analysis of the molecular relationships between SUN1 and SUN2, highlighting key similarities and differences that could lead to clues into their distinct functions. We use a wide range of computational tools, including multiple sequence alignments, homology modeling, molecular docking, and molecular dynamic simulations, to predict structural differences between SUN1 and SUN2, with the goal of understanding the molecular mechanisms underlying SUN1 oligomerization in the nuclear envelope. Our simulations suggest that the structural model of SUN1 is stable in a trimeric state and that SUN1 trimers can associate through their SUN domains to form lateral complexes. We also ask whether SUN1 could adopt an inactive monomeric conformation as seen in SUN2. Our results imply that the KASH binding domain of SUN1 is also inhibited in monomeric SUN1 but through weaker interactions than in monomeric SUN2.

Medicago LINC Complexes Function in Nuclear Morphology, Nuclear Movement, and Root Nodule Symbiosis

Nuclear movement is involved in cellular and developmental processes across eukaryotic life, often driven by Linker of Nucleoskeleton and Cytoskeleton (LINC) complexes, which bridge the nuclear envelope (NE) via the interaction of Klarsicht/ANC-1/Syne-1 Homology (KASH) and Sad1/UNC-84 (SUN) proteins. Arabidopsis (Arabidopsis thaliana) LINC complexes are involved in nuclear movement and positioning in several cell types. Observations since the 1950s have described targeted nuclear movement and positioning during symbiosis initiation between legumes and rhizobia, but it has not been established whether these movements are functional or incidental. Here, we identify and characterize LINC complexes in the model legume Medicago truncatula We show that LINC complex characteristics such as NE localization, dependence of KASH proteins on SUN protein binding for NE enrichment, and direct SUN-KASH binding are conserved between plant species. Using a SUN dominant-negative strategy, we demonstrate that LINC complexes are necessary for proper nuclear shaping and movement in Medicago root hairs, and are important for infection thread initiation and nodulation.

Chain reaction: LINC complexes and nuclear positioning

Nuclear positioning plays an essential role in defining cell architecture and behaviour in both development and disease, and nuclear location frequently adjusts according to internal and external cues. For instance, during periods of migration in many cell types, the nucleus may be actively repositioned behind the microtubule-organising centre. Nuclear movement, for the most part, is dependent upon coupling of the cytoskeleton to the nuclear periphery. This is accomplished largely through SUN and KASH domain proteins, which together assemble to form LINC (linker of the nucleoskeleton and cytoskeleton) complexes spanning the nuclear envelope. SUN proteins of the inner nuclear membrane provide a connection to nuclear structures while acting as a tether for outer nuclear membrane KASH proteins. The latter contain binding sites for diverse cytoskeletal components. Recent publications highlight new aspects of LINC complex regulation revealing that the interplay between SUN and KASH partners can strongly influence how the nucleus functionally engages with different branches of the cytoskeleton.

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.

Molecular evolution of SUN-domain containing proteins in diverse plant species and their expression profiling in response to developmental and perturbation stimuli

SUN (Sad1/UNC-84) domain-containing proteins are highly conserved throughout evolution. They are localized to the inner membrane of the nuclear envelope and are involved in nuclear migration and nucleoskeleton formation. In the present study, a genome-wide investigation was performed in three dicotyledonous (Arabidopsis thaliana, Glycine max and Medicago truncatula) and three monocotyledonous (Oryza sativa, Zea mays and Sorghum bicolor) plants. A total of 56 SUN proteins encoded by 30 genes were identified. Based on their length, transmembrane topology, conserved domains and phylogenetic relationships, they could be divided into two previously defined groups- Cter-SUN and mid-SUN proteins. Expression of these genes was analyzed in different developmental stages, tissues and various unfavorable conditions such as salinity, drought, and hormonal treatment. Analyses indicated that the expression of SUN1/2 transcripts are ubiquitous; that of SUN3/4 are development/tissue regulated, and SUN5 are inflorescence stage-specific. This study provides an initial framework for the characterization and functional validation of the plant SUN family.

SUN2: A potential therapeutic target in cancer

The incidence of cancer is increasing at an alarming rate despite recent advances in prevention strategies, diagnostics and therapeutics for various types of cancer. The identification of novel biomarkers to aid in prognosis and treatment for cancer is urgently required. Uncontrolled proliferation and dysregulated apoptosis are characteristics exhibited by cancer cells in the initiation of various types of cancer. Notably, aberrant expression of crucial oncogenes or cancer suppressors is a defining event in cancer occurrence. Research has demonstrated that SAD1/UNC84 domain protein-2 (SUN2) serves a suppressive role in breast cancer, atypical teratoid/rhabdoid tumors and lung cancer progression. Furthermore, SUN2 inhibits cancer cell proliferation, migration and promotes apoptosis. Recent reports have also shown that SUN2 serves prominent roles in resistance to the excessive DNA damage that destabilizes the genome and promotes cancer development, and these functions of SUN2 are critical for evading initiation of cancer. Additionally, increasing evidence has demonstrated that SUN2 is involved in maintaining cell nuclear structure and appears to be a central component for organizing the natural nuclear architecture in cancer cells. The focus of the present review is to provide an overview on the pharmacological functions of SUN2 in cancers. These findings suggest that SUN2 may serve as a promising therapeutic target and novel predictive marker in various types of cancer.

The Pathogenesis and Therapies of Striated Muscle Laminopathies

Emery-Dreifuss muscular dystrophy (EDMD) is a genetic condition characterized by early contractures, skeletal muscle weakness, and cardiomyopathy. During the last 20 years, various genetic approaches led to the identification of causal genes of EDMD and related disorders, all encoding nuclear envelope proteins. By their respective localization either at the inner nuclear membrane or the outer nuclear membrane, these proteins interact with each other and establish a connection between the nucleus and the cytoskeleton. Beside this physical link, these proteins are also involved in mechanotransduction, responding to environmental cues, such as increased tension of the cytoskeleton, by the activation or repression of specific sets of genes. This ability of cells to adapt to environmental conditions is altered in EDMD. Increased knowledge on the pathophysiology of EDMD has led to the development of drug or gene therapies that have been tested on mouse models. This review proposed an overview of the functions played by the different proteins involved in EDMD and related disorders and the current therapeutic approaches tested so far.

A molecular model for LINC complex regulation: activation of SUN2 for KASH binding

Linkers of the nucleoskeleton and cytoskeleton are key molecular complexes that span the nuclear envelope (NE) and provide a direct linkage between the nucleoskeleton and cytoskeleton. Two major components of these complexes are members of the SUN and KASH protein families that interact in the perinuclear space to allow the transmission of mechanochemical signals across the NE. Structural details of the mammalian SUN domain protein SUN2 have established that SUN2 must form a trimer to bind to KASH, and that this trimerization is mediated through two predicted coiled-coil regions of the protein, CC1 and CC2, which precede the SUN domain. Recent crystallographic data suggest that CC2-SUN formed an unexpected autoinhibited monomer unable to bind to KASH. These structural insights raise the question of how full-length SUN2 transitions from a monomer to a trimer inside the NE. In this study we used a computational approach to model a fragment of SUN2 containing CC1, CC2, and the SUN domain. We observed the dynamics of these modeled structures using ∼1 μs molecular dynamics simulations and showed that the interplay between CC1 and CC2 may be sufficient for the release of CC2-SUN2 from its autoinhibited state. Additionally, using our models and gel filtration analysis, we show the involvement of an E452 residue on CC1 in the monomer–­trimer transition of SUN2. Intriguingly, mutations in this residue have been seen in muscular dystrophy–associated SUN2 variants. Finally, we propose a Ca²⁺-dependent monomer–trimer transition of SUN2.

Linker of nucleoskeleton and cytoskeleton complex proteins in cardiomyopathy

The linker of nucleoskeleton and cytoskeleton (LINC) complex couples the nuclear lamina to the cytoskeleton. The LINC complex and its associated proteins play diverse roles in cells, ranging from genome organization, nuclear morphology, gene expression, to mechanical stability. The importance of a functional LINC complex is highlighted by the large number of mutations in genes encoding LINC complex proteins that lead to skeletal and cardiac myopathies. In this review, the structure, function, and interactions between components of the LINC complex will be described. Mutations that are known to cause cardiomyopathy in patients will be discussed alongside their respective mouse models. Furthermore, future challenges for the field and emerging technologies to investigate LINC complex function will be discussed.

Differential incorporation of SUN-domain proteins into LINC complexes is coupled to gene expression

LInkers of Nucleoskeleton and Cytoskeleton (LINC) complexes, composed of SUN and KASH-domain proteins, span the nuclear envelope and physically connect the nuclear interior to cytoskeletal elements. Most human cells contain two SUN proteins, Sun1 and Sun2, and several KASH-proteins suggesting that multiple functionally distinct LINC complexes co-exist in the nuclear envelope. We show here, however, that while Sun1 and Sun2 in HeLa cells are each able to bind KASH-domains, Sun1 is more efficiently incorporated into LINC complexes under normal growth conditions. Furthermore, the balance of Sun1 and Sun2 incorporated into LINC complexes is cell type-specific and is correlated with SRF/Mkl1-dependent gene expression. In addition, we found that Sun1 has a LINC complex-independent role in transcriptional control, possibly by regulating the SRF/Mkl1 pathway. Together, these data reveal novel insights into the mechanisms of LINC complex regulation and demonstrate that Sun1 modulates gene expression independently of its incorporation into LINC complexes.

The KASH-containing isoform of Nesprin1 giant associates with ciliary rootlets of ependymal cells

Biallelic nonsense mutations of SYNE1 underlie a variable array of cerebellar and non-cerebellar pathologies of unknown molecular etiology. SYNE1 encodes multiple isoforms of Nesprin1 that associate with the nuclear envelope, with large cerebellar synapses and with ciliary rootlets of photoreceptors. Using two novel mouse models, we determined the expression pattern of Nesprin1 isoforms in the cerebellum whose integrity and functions are invariably affected by SYNE1 mutations. We further show that a giant isoform of Nesprin1 associates with the ciliary rootlets of ependymal cells that line brain ventricles and establish that this giant ciliary isoform of Nesprin1 harbors a KASH domain. Whereas cerebellar phenotypes are not recapitulated in Nes1g^STOP/STOP mice, these mice display a significant increase of ventricular volume. Together, these data fuel novel hypotheses about the molecular pathogenesis of SYNE1 mutations and support that KASH proteins may localize beyond the nuclear envelope in vivo.

Fluorescence fluctuation spectroscopy reveals differential SUN protein oligomerization in living cells

Fluorescence fluctuation spectroscopy is established as a powerful tool for quantifying protein oligomerization in the nuclear envelopes of living cells. It reveals that the SUN proteins SUN1 and SUN2 display differential oligomerization in vivo, which has important implications for LINC complex–dependent nuclear mechanotransduction.

Linker-of-nucleoskeleton-and-cytoskeleton (LINC) complexes are conserved molecular bridges within the nuclear envelope that mediate mechanical force transmission into the nucleoplasm. The core of a LINC complex is formed by a transluminal interaction between the outer and inner nuclear membrane KASH and SUN proteins, respectively. Mammals encode six KASH proteins and five SUN proteins. Recently, KASH proteins were shown to bind to the domain interfaces of trimeric SUN2 proteins in vitro. However, neither the existence of SUN2 trimers in living cells nor the extent to which other SUN proteins conform to this assembly state have been tested experimentally. Here we extend the application of fluorescence fluctuation spectroscopy to quantify SUN protein oligomerization in the nuclear envelopes of living cells. Using this approach, we demonstrate for the first time that SUN2 trimerizes in vivo and we demonstrate that the in vivo oligomerization of SUN1 is not limited to a trimer. In addition, we provide evidence to support the existence of potential regulators of SUN protein oligomerization in the nuclear envelope. The differential SUN protein oligomerization illustrated here suggests that SUN proteins may have evolved to form different assembly states in order to participate in diverse mechanotransduction events.

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.

The increasing relevance of nuclear envelope myopathies

PURPOSE OF REVIEW

Nuclear envelope links to a wide range of disorders, including several myopathies and neuropathies over the past 2 decades, has spurred research leading to a completely changed view of this important cellular structure and its functions. However, the many functions now assigned to the nuclear envelope make it increasingly hard to determine which functions underlie these disorders.

RECENT FINDINGS

New nuclear envelope functions in genome organization, regulation and repair, signaling, and nuclear and cellular mechanics have been added to its classical barrier function. Arguments can be made for any of these functions mediating abnormality in nuclear envelope disorders and data exist supporting many. Moreover, transient and/or distal nuclear envelope connections to other cellular proteins and structures may increase the complexity of these disorders.

SUMMARY

Although the increased understanding of nuclear envelope functions has made it harder to distinguish specific causes of nuclear envelope disorders, this is because it has greatly expanded the spectrum of possible mechanisms underlying them. This change in perspective applies well beyond the known nuclear envelope disorders, potentially implicating the nuclear envelope in a much wider range of myopathies and neuropathies.

Nuclear Envelope-Associated Chromosome Dynamics during Meiotic Prophase I

Chromosome dynamics during meiotic prophase I are associated with a series of major events such as chromosomal reorganization and condensation, pairing/synapsis and recombination of the homologs, and chromosome movements at the nuclear envelope (NE). The NE is the barrier separating the nucleus from the cytoplasm and thus plays a central role in NE-associated chromosomal movements during meiosis. Previous studies have shown in various species that NE-linked chromosome dynamics are actually driven by the cytoskeleton. The linker of nucleoskeleton and cytoskeleton (LINC) complexes are important constituents of the NE that facilitate in the transfer of cytoskeletal forces across the NE to individual chromosomes. The LINCs consist of the inner and outer NE proteins Sad1/UNC-84 (SUN), and Klarsicht/Anc-1/Syne (KASH) domain proteins. Meiosis-specific adaptations of the LINC components and unique modifications of the NE are required during chromosomal movements. Nonetheless, the actual role of the NE in chromosomic dynamic movements in plants remains elusive. This review summarizes the findings of recent studies on meiosis-specific constituents and modifications of the NE and corresponding nucleoplasmic/cytoplasmic adaptors being involved in NE-associated movement of meiotic chromosomes, as well as describes the potential molecular network of transferring cytoplasm-derived forces into meiotic chromosomes in model organisms. It helps to gain a better understanding of the NE-associated meiotic chromosomal movements in plants.

Investigating LINC Complex Protein Homo-oligomerization in the Nuclear Envelopes of Living Cells Using Fluorescence Fluctuation Spectroscopy

Linkers of nucleoskeleton and cytoskeleton (LINC) complexes are conserved nuclear envelope (NE) spanning molecular bridges which mechanically integrate the nucleus with the cytoskeleton and mediate force transmission into the nucleoplasm. Despite their critical roles in fundamental cellular processes such as meiotic chromosome and nuclear positioning, the mechanism of LINC complex assembly in cells remains unclear. To begin to address this deficit, we recently developed z-scan fluorescence fluctuation spectroscopy (FFS) and brightness analysis as a method for quantifying the oligomeric states of fluorescent protein-tagged NE proteins including nesprins and SUN proteins. Since the homo-oligomerization of SUN2 is critical for its ability to interact with nesprins within the perinuclear space, the knowledge obtained through quantitative brightness experiments reveals important insights into the in vivo mechanisms of LINC complex assembly. Here we describe the procedure we use to determine the brightness of proteins in the NE of living cells. In addition to the measurement procedure, we discuss the instrumentation requirements and present the results of applying this procedure to measure the brightness of nesprin-2 and SUN2.

Interactions of Nesprin-4-Containing LINC Complexes in Outer Hair Cells Explored by BioID

As components of diverse tissues and organs, metazoan cells have to display a wide variety of specialized functions. Implementation of such functions invariably entails the establishment of tissue-specific cellular architecture (Bone and Starr, J Cell Sci 129:1951-1961, 2016). In animal cells, the nucleus is typically the largest organelle and in many respects acts as a landmark for multiple subcellular structures. For instance, in epithelial cells, the nucleus is frequently positioned close to the basal membrane via association with the cytoskeleton. Clearly such associations must be mediated by protein components of the outer nuclear membrane. One such protein is Nesprin-4, a member of the KASH domain family that is expressed in a variety of epithelial cells, including sensory outer hair cells of the inner ear. In this chapter, I describe a proximity-based biotinylation technique, BioID, that can be applied to Nesprin-4 to map its interactions at the nuclear periphery.

A synthetic biology platform for the reconstitution and mechanistic dissection of LINC complex assembly

The linker of nucleoskeleton and cytoskeleton (LINC) is a conserved nuclear envelope-spanning molecular bridge that is responsible for the mechanical integration of the nucleus with the cytoskeleton. LINC complexes are formed by a transluminal interaction between the outer and inner nuclear membrane KASH and SUN proteins, respectively. Despite recent structural insights, our mechanistic understanding of LINC complex assembly remains limited by the lack of an experimental system for its in vitro reconstitution and manipulation. Here, we describe artificial nuclear membranes (ANMs) as a synthetic biology platform based on mammalian cell-free expression for the rapid reconstitution of SUN proteins in supported lipid bilayers. We demonstrate that SUN1 and SUN2 are oriented in ANMs with solvent-exposed C-terminal KASH-binding SUN domains. We also find that SUN2 possesses a single transmembrane domain, while SUN1 possesses three. Finally, SUN protein-containing ANMs bind synthetic KASH peptides, thereby reconstituting the LINC complex core. This work represents the first in vitro reconstitution of KASH-binding SUN proteins in supported lipid bilayers using cell-free expression, which will be invaluable for testing proposed models of LINC complex assembly and its regulation.