Fluorescence fluctuation spectroscopy reveals differential SUN protein oligomerization in living cells

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.

Building and breaking mechanical bridges between the nucleus and cytoskeleton: Regulation of LINC complex assembly and disassembly

The nucleus is physically coupled to the cytoskeleton through LINC complexes, macromolecular bridges composed of SUN and KASH proteins that span the nuclear envelope. LINC complexes are involved in a wide variety of critical cellular processes. For these processes to occur, cells regulate the composition, assembly, and disassembly of LINC complexes. Here we discuss recent studies on the regulation of the SUN-KASH interaction that forms the core of the LINC complex. These new findings encompass the stages of LINC complex assembly, from the formation of SUN-KASH heterooligomers to higher-order assemblies of LINC complexes. There is also new work on how components of the LINC complex are selectively dismantled, particularly by proteasomal degradation. It is becoming increasingly clear that LINC complexes are subject to multiple layers of regulation.

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.

DYT-TOR1A dystonia: an update on pathogenesis and treatment

DYT-TOR1A dystonia is a neurological disorder characterized by involuntary muscle contractions and abnormal movements. It is a severe genetic form of dystonia caused by mutations in the TOR1A gene. TorsinA is a member of the AAA + family of adenosine triphosphatases (ATPases) involved in a variety of cellular functions, including protein folding, lipid metabolism, cytoskeletal organization, and nucleocytoskeletal coupling. Almost all patients with TOR1A-related dystonia harbor the same mutation, an in-frame GAG deletion (ΔGAG) in the last of its 5 exons. This recurrent variant results in the deletion of one of two tandem glutamic acid residues (i.e., E302/303) in a protein named torsinA [torsinA(△E)]. Although the mutation is hereditary, not all carriers will develop DYT-TOR1A dystonia, indicating the involvement of other factors in the disease process. The current understanding of the pathophysiology of DYT-TOR1A dystonia involves multiple factors, including abnormal protein folding, signaling between neurons and glial cells, and dysfunction of the protein quality control system. As there are currently no curative treatments for DYT-TOR1A dystonia, progress in research provides insight into its pathogenesis, leading to potential therapeutic and preventative strategies. This review summarizes the latest research advances in the pathogenesis, diagnosis, and treatment of DYT-TOR1A dystonia.

Disulfide bond in SUN2 regulates dynamic remodeling of LINC complexes at the nuclear envelope

The LINC complex tethers the cell nucleus to the cytoskeleton to regulate mechanical forces during cell migration, differentiation, and various diseases. The function of LINC complexes relies on the interaction between highly conserved SUN and KASH proteins that form higher-order assemblies capable of load bearing. These structural details have emerged from in vitro assembled LINC complexes; however, the principles of in vivo assembly remain obscure. Here, we report a conformation-specific SUN2 antibody as a tool to visualize LINC complex dynamics in situ. Using imaging, biochemical, and cellular methods, we find that conserved cysteines in SUN2 undergo KASH-dependent inter- and intra-molecular disulfide bond rearrangements. Disruption of the SUN2 terminal disulfide bond compromises SUN2 localization, turnover, LINC complex assembly in addition to cytoskeletal organization and cell migration. Moreover, using pharmacological and genetic perturbations, we identify components of the ER lumen as SUN2 cysteines redox state regulators. Overall, we provide evidence for SUN2 disulfide bond rearrangement as a physiologically relevant structural modification that regulates LINC complex functions.

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.

Nuclear pore complexes concentrate on Actin/LINC/Lamin nuclear lines in response to mechanical stress in a SUN1 dependent manner

Formation of robust actomyosin stress fibers (SF) in response to cell stretch plays a key role in the transfer of information from the cytoplasm into the nucleus. Actin/LINC/Lamin (ALL) nuclear lines provide mechanical linkage between the actin cytoskeleton and the lamin nucleoskeleton across the nuclear envelope. To understand the establishment of ALL lines, we used live cell imaging of cells exposed to cyclic stretch. We discovered that nuclear pore complexes (NPCs) concentrate along ALL lines that are generated in response to uniaxial cyclic stretch. The ALL-associated NPCs display increased fluorescence intensity of nucleoporins Pom121, TPR and Nup153 relative to nucleoporins that are distal to the ALL lines. Here we test the hypothesis that a LINC complex component of ALL lines, SUN1 is involved in the integration of NPCs with ALL lines. We generated CRISPR SUN1 knockdown and knockout cell lines and show that SUN1 is essential for normal integration of NPCs to ALL lines. Loss or elimination of SUN1 significantly diminishes NPC/ALL line integration, demonstrating a key role for SUN1 in the recruitment or stabilization of NPCs to a discrete subdomain of the nuclear envelope at ALL lines. This work provides new insight into the mechanism by which cells respond to mechanical force through nuclear envelope remodeling.

In Vitro Synthesis and Reconstitution Using Mammalian Cell-Free Lysates Enables the Systematic Study of the Regulation of LINC Complex Assembly

Understanding the structure and structure-function relationships of membrane proteins is a fundamental problem in biomedical research. Given the difficulties inherent to performing mechanistic biochemical and biophysical studies of membrane proteins in vitro, we previously developed a facile HeLa cell-based cell-free expression (CFE) system that enables the efficient reconstitution of full-length (FL) functional inner nuclear membrane Sad1/UNC-84 (SUN) proteins (i.e., SUN1 and SUN2) in supported lipid bilayers. Here, we provide evidence that suggests that the reconstitution of CFE-synthesized FL membrane proteins in supported lipid bilayers occurs primarily through the fusion of endoplasmic reticulum-derived microsomes present within our CFE reactions with our supported lipid bilayers. In addition, we demonstrate the ease with which our synthetic biology platform can be used to investigate the impact of the chemical environment on the ability of CFE-synthesized FL SUN proteins reconstituted in supported lipid bilayers to interact with the luminal domain of the KASH protein nesprin-2. Moreover, we use our platform to study the molecular requirements for the homo- and heterotypic interactions between SUN1 and SUN2. Finally, we show that our platform can be used to simultaneously reconstitute three different CFE-synthesized FL membrane proteins in a single supported lipid bilayer. Overall, these results establish our HeLa cell-based CFE and supported lipid bilayer reconstitution platform as a powerful tool for performing mechanistic dissections of the oligomerization and function of FL membrane proteins in vitro. While our platform is not a substitute for cell-based studies, it does provide important mechanistic insights into the biology of difficult-to-study membrane proteins.

Combining Fluorescence Fluctuations and Photobleaching to Quantify Surface Density

We have established a self-calibrated method, called pbFFS for photobleaching fluctuation fluorescence spectroscopy, which aims to characterize molecules or particles labeled with an unknown distribution of fluorophores. Using photobleaching as a control parameter, pbFFS provides information on the distribution of fluorescent labels and a reliable estimation of the absolute density or concentration of these molecules. We present a complete theoretical derivation of the pbFFS approach and experimentally apply it to measure the surface density of a monolayer of fluorescently tagged streptavidin molecules, which can be used as a base platform for biomimetic systems. The surface density measured by pbFFS is consistent with the results of spectroscopic ellipsometry, a standard surface technique. However, pbFFS has two main advantages: it enables in situ characterization (no dedicated substrates are required) and can be applied to low masses of adsorbed molecules, which we demonstrate here by quantifying the density of biotin-Atto molecules that bind to the streptavidin layer. In addition to molecules immobilized on a surface, we also applied pbFFS to molecules diffusing in solution, to confirm the distribution of fluorescent labels found on a surface. Hence, pbFFS provides a set of tools for investigating the molecules labeled with a variable number of fluorophores, with the aim of quantifying either the number of molecules or the distribution of fluorescent labels, the latter case being especially relevant for oligomerization studies.

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

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

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.

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.

Time-shifted mean-segmented Q data of a luminal protein measured at the nuclear envelope by fluorescence fluctuation microscopy

Fluorescence fluctuation microscopy is a widely used method to determine the mobility and oligomeric state of proteins in the live cell environment. Existing analysis methods rely on statistical evaluation of data segments with the implicit assumption that no significant signal fluctuations occur on the time scale of a data segment. Recent work on extending fluorescence fluctuation methods to the nuclear envelope of living cells identified a slow fluctuation process that is associated with the undulations of the nuclear membranes, which lead to intensity fluctuations due to local volume changes at the nuclear envelope. This environment violates the above-mentioned assumption and is associated with biased evaluation of fluorescence fluctuation data by traditional analysis methods, such as the autocorrelation function. This challenge was overcome by the introduction of the time-shifted mean-segmented Q function, which relies on a sliding scale of data segment lengths. Here, we share experimental fluorescence fluctuation data taken at the nuclear envelope and demonstrate the calculation of the time-shifted mean-segmented Q function from the raw data. The data and analysis should be valuable for researchers interested in fluorescence fluctuation techniques and provides an opportunity to examine the influence of slow fluctuations on existing data analysis methods. The data is related to the research article titled "Protein oligomerization and mobility within the nuclear envelope evaluated by the time-shifted mean-segmented Q factor" [1].

Super-resolution Microscopy-based Bimolecular Fluorescence Complementation to Study Protein Complex Assembly and Co-localization

Numerous experimental approaches exist to study interactions between two subunits of a large macromolecular complex. However, most methods do not provide spatial and temporal information about binding, which are critical for dissecting the mechanism of assembly of nanosized complexes in vivo. While recent advances in super-resolution microscopy techniques have provided insights into biological structures beyond the diffraction limit, most require extensive expertise and/or special sample preparation, and it is a challenge to extend beyond binary, two color experiments. Using HyVolution, a super-resolution technique that combines confocal microscopy at sub-airy unit pinhole sizes with computational deconvolution, we achieved 140 nm resolution in both live and fixed samples with three colors, including two fluorescent proteins (mTurquoise2 and GFP) with significant spectral overlap that were distinguished by means of shifting the excitation wavelength away from common wavelengths. By combining HyVolution super-resolution fluorescence microscopy with bimolecular fluorescence complementation (SRM-BiFC), we describe a new assay capable of visualizing protein-protein interactions in vivo at sub-diffraction resolution. This method was used to improve our understanding of the ordered assembly of the Saccharomyces cerevisiae spindle pole body (SPB), a ~1 giga-Dalton heteromeric protein complex formed from 18 structural components present in multiple copies. We propose that SRM-BiFC is a powerful tool for examination of direct interactions between protein complex subunits at sub-diffraction resolution in live cells.

Identifying Heteroprotein Complexes in the Nuclear Envelope

The nucleus is delineated by the nuclear envelope (NE), which is a double membrane barrier composed of the inner and outer nuclear membranes as well as a ∼40-nm wide lumen. In addition to its barrier function, the NE acts as a critical signaling node for a variety of cellular processes, which are mediated by protein complexes within this subcellular compartment. Although fluorescence fluctuation spectroscopy is a powerful tool for characterizing protein complexes in living cells, it was recently demonstrated that conventional fluorescence fluctuation spectroscopy methods are not suitable for applications in the NE because of the presence of slow nuclear membrane undulations. We previously addressed this challenge by developing time-shifted mean-segmented Q (tsMSQ) analysis and applied it to successfully characterize protein homo-oligomerization in the NE. However, many NE complexes, such as the linker of the nucleoskeleton and cytoskeleton complex, are formed by heterotypic interactions, which single-color tsMSQ is unable to characterize. Here, we describe the development of dual-color (DC) tsMSQ to analyze NE heteroprotein complexes built from proteins that carry two spectrally distinct fluorescent labels. Experiments performed on model systems demonstrate that DC tsMSQ properly identifies heteroprotein complexes and their stoichiometry in the NE by accounting for spectral cross talk and local volume fluctuations. Finally, we applied DC tsMSQ to study the assembly of the linker of the nucleoskeleton and cytoskeleton complex, a heteroprotein complex composed of Klarsicht/ANC-1/SYNE homology and Sad1/UNC-84 (SUN) proteins, in the NE of living cells. Using DC tsMSQ, we demonstrate the ability of the SUN protein SUN2 and the Klarsicht/ANC-1/SYNE homology protein nesprin-2 to form a heterocomplex in vivo. Our results are consistent with previously published in vitro studies and demonstrate the utility of the DC tsMSQ technique for characterizing NE heteroprotein complexes.

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.

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.

Role of KASH domain lengths in the regulation of LINC complexes

The linker of the nucleoskeleton and cytoskeleton (LINC) complex is formed by the conserved interactions between Sad-1 and UNC-84 (SUN) and Klarsicht, ANC-1, SYNE homology (KASH) domain proteins, providing a physical coupling between the nucleoskeleton and cytoskeleton that mediates the transfer of physical forces across the nuclear envelope. The LINC complex can perform distinct cellular functions by pairing various KASH domain proteins with the same SUN domain protein. For example, in Caenorhabditis elegans, SUN protein UNC-84 binds to two KASH proteins UNC-83 and ANC-1 to mediate nuclear migration and anchorage, respectively. In addition to distinct cytoplasmic domains, the luminal KASH domain also varies among KASH domain proteins of distinct functions. In this study, we combined in vivo C. elegans genetics and in silico molecular dynamics simulations to understand the relation between the length and amino acid composition of the luminal KASH domain, and the function of the SUN–KASH complex. We show that longer KASH domains can withstand and transfer higher forces and interact with the membrane through a conserved membrane proximal EEDY domain that is unique to longer KASH domains. In agreement with our models, our in vivo results show that swapping the KASH domains of ANC-1 and UNC-83, or shortening the KASH domain of ANC-1, both result in a nuclear anchorage defect in C. elegans.

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.

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.

Protein oligomerization and mobility within the nuclear envelope evaluated by the time-shifted mean-segmented Q factor

Analysis of fluorescence fluctuation experiments by the mean-segmented Q (MSQ) method was recently used to successfully characterize the oligomeric state and mobility of proteins within the nuclear envelope (NE) of living cells. However, two significant shortcomings of MSQ were recognized. Non-ideal detector behavior due to dead-time and afterpulsing as well as the lack of error analysis currently limit the potential of MSQ. This paper presents time-shifted MSQ (tsMSQ), a new formulation of MSQ that is robust with respect to dead-time and afterpulsing. In addition, a protocol for performing error analysis on tsMSQ data is introduced to assess the quality of fit models and estimate the uncertainties of fit parameters. Together, these developments significantly simplify and improve the analysis of fluorescence fluctuation data taken within the NE. To demonstrate these new developments, tsMSQ was used to characterize the oligomeric state and mobility of the luminal domains of two inner nuclear membrane SUN proteins. The results for the luminal domain of SUN2 obtained through tsMSQ without correction for non-ideal detector effects agree with a recent study that was conducted using the original MSQ formulation. Finally, tsMSQ was applied to characterize the oligomeric state and mobility of the luminal domain of the germline-restricted SUN3.

Conserved SUN-KASH Interfaces Mediate LINC Complex-Dependent Nuclear Movement and Positioning

Many nuclear positioning events involve linker of nucleoskeleton and cytoskeleton (LINC) complexes, which transmit forces generated by the cytoskeleton across the nuclear envelope. LINC complexes are formed by trans-luminal interactions between inner nuclear membrane SUN proteins and outer nuclear membrane KASH proteins, but how these interactions are regulated is poorly understood. We combine in vivo C. elegans genetics, in vitro wounded fibroblast polarization, and in silico molecular dynamics simulations to elucidate mechanisms of LINC complexes. The extension of the KASH domain by a single alanine residue or the mutation of the conserved tyrosine at -7 completely blocked the nuclear migration function of C. elegans UNC-83. Analogous mutations at -7 of mouse nesprin-2 disrupted rearward nuclear movements in NIH 3T3 cells, but did not disrupt ANC-1 in nuclear anchorage. Furthermore, conserved cysteines predicted to form a disulfide bond between SUN and KASH proteins are important for the function of certain LINC complexes, and might promote a developmental switch between nuclear migration and nuclear anchorage. Mutations of conserved cysteines in SUN or KASH disrupted ANC-1-dependent nuclear anchorage in C. elegans and Nesprin-2G-dependent nuclear movements in polarizing fibroblasts. However, the SUN cysteine mutation did not disrupt nuclear migration. Moreover, molecular dynamics simulations showed that a disulfide bond is necessary for the maximal transmission of cytoskeleton-generated forces by LINC complexes in silico. Thus, we have demonstrated functions for SUN-KASH binding interfaces, including a predicted intermolecular disulfide bond, as mechanistic determinants of nuclear positioning that may represent targets for regulation.

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.

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.

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.