Nesprin-3 connects plectin and vimentin to the nuclear envelope of Sertoli cells but is not required for Sertoli cell function in spermatogenesis

Unraveling the nexus of nesprin in dilated cardiomyopathy: From molecular insights to therapeutic prospects

Dilated cardiomyopathy is a complex and debilitating heart disorder characterized by the enlargement and weakening of the cardiac chambers, leading to impaired contractility and heart failure. Nesprins, a family of nuclear envelope spectrin repeat proteins that include isoforms Nesprin-1/-2, are integral components of the LInker of Nucleoskeleton and Cytoskeleton complex. They facilitate the connection between the nuclear envelope and the cytoskeleton, crucial for maintaining nuclear architecture, migration and positioning, and mechanical transduction and signaling. Nesprin-1/-2 are abundantly expressed in cardiac and skeletal muscles.They have emerged as key players in the pathogenesis of dilated cardiomyopathy. Mutations in synaptic nuclear envelope-1/-2 genes encoding Nesprin-1/-2 are associated with dilated cardiomyopathy, underscoring their significance in cardiac health. This review highlights the all known cases of Nesprin-1/-2 related dilated cardiomyopathy, focusing on their interactions with the nuclear envelope, their role in mechanical transduction, and their influence on gene expression. Moreover, it delves into the underlying mechanisms through which Nesprin dysfunction disrupts nuclear-cytoskeletal coupling, leading to abnormal nuclear morphology, impaired mechanotransduction, and altered gene regulation. The exploration of Nesprin's impact on dilated cardiomyopathy offers a promising avenue for therapeutic interventions aimed at ameliorating the disease. This review provides a comprehensive overview of recent advancements in understanding the pivotal role of Nesprins in dilated cardiomyopathy research.

SUN5 interacts with nuclear membrane LaminB1 and cytoskeletal GTPase Septin12 mediating the sperm head-and-tail junction

Acephalic spermatozoa syndrome (ASS) is a severe teratospermia with decaudated, decapitated, and malformed sperm, resulting in male infertility. Nuclear envelope protein SUN5 localizes to the junction between the sperm head and tail. Mutations in the SUN5 gene have been identified most frequently (33-47%) in ASS cases, and its molecular mechanism of action is yet to be explored. In the present study, we generated Sun5 knockout mice, which presented the phenotype of ASS. Nuclear membrane protein LaminB1 and cytoskeletal GTPases Septin12 and Septin2 were identified as potential partners for interacting with SUN5 by immunoprecipitation-mass spectrometry in mouse testis. Further studies demonstrated that SUN5 connected the nucleus by interacting with LaminB1 and connected the proximal centriole by interacting with Septin12. The binding between SUN5 and Septin12 promoted their aggregation together in the sperm neck. The disruption of the LaminB1/SUN5/Septin12 complex by Sun5 deficiency caused separation of the Septin12-proximal centriole from the nucleus, leading to the breakage of the head-to-tail junction. Collectively, these data provide new insights into the pathogenesis of ASS caused by SUN5 deficiency.

Plectin Deficiency in Fibroblasts Deranges Intermediate Filament and Organelle Morphology, Migration, and Adhesion

Plectin, a highly versatile and multifunctional cytolinker, has been implicated in several multisystemic disorders. Most sequence variations in the human plectin gene (PLEC) cause epidermolysis bullosa simplex with muscular dystrophy (EBS-MD), an autosomal recessive skin-blistering disorder associated with progressive muscle weakness. In this study, we performed a comprehensive cell biological analysis of dermal fibroblasts from three different patients with EBS-MD, where PLEC expression analyses revealed preserved mRNA levels in all cases, whereas full-length plectin protein content was significantly reduced or completely absent. Downstream effects of pathogenic PLEC sequence alterations included massive bundling of vimentin intermediate filament networks, including the occurrence of ring-like nuclei-encasing filament bundles, elongated mitochondrial networks, and abnormal nuclear morphologies. We found that essential fibroblast functions such as wound healing, migration, or orientation upon cyclic stretch were significantly impaired in the cells of patients with EBS-MD. Finally, EBS-MD fibroblasts displayed reduced adhesion capacities, which could be attributed to smaller focal adhesion contacts. Our study not only emphasizes plectin's functional role in human skin fibroblasts, it also provides further insights into the understanding of EBS-MD-associated disease mechanisms.

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.

RNF26 binds perinuclear vimentin filaments to integrate ER and endolysosomal responses to proteotoxic stress

Proteotoxic stress causes profound endoplasmic reticulum (ER) membrane remodeling into a perinuclear quality control compartment (ERQC) for the degradation of misfolded proteins. Subsequent return to homeostasis involves clearance of the ERQC by endolysosomes. However, the factors that control perinuclear ER integrity and dynamics remain unclear. Here, we identify vimentin intermediate filaments as perinuclear anchors for the ER and endolysosomes. We show that perinuclear vimentin filaments engage the ER-embedded RING finger protein 26 (RNF26) at the C-terminus of its RING domain. This restricts RNF26 to perinuclear ER subdomains and enables the corresponding spatial retention of endolysosomes through RNF26-mediated membrane contact sites (MCS). We find that both RNF26 and vimentin are required for the perinuclear coalescence of the ERQC and its juxtaposition with proteolytic compartments, which facilitates efficient recovery from ER stress via the Sec62-mediated ER-phagy pathway. Collectively, our findings reveal a scaffolding mechanism that underpins the spatiotemporal integration of organelles during cellular proteostasis.

Intermediate filament dysregulation in astrocytes in the human disease model of KLHL16 mutation in giant axonal neuropathy (GAN)

Giant Axonal Neuropathy (GAN) is a pediatric neurodegenerative disease caused by KLHL16 mutations. KLHL16 encodes gigaxonin, which regulates intermediate filament (IF) turnover. Previous neuropathological studies and examination of postmortem brain tissue in the current study revealed involvement of astrocytes in GAN. To develop a clinically-relevant model, we reprogrammed skin fibroblasts from seven GAN patients to pluripotent stem cells (iPSCs), which were used to generate neural progenitor cells (NPCs), astrocytes, and brain organoids. Multiple isogenic control clones were derived via CRISPR/Cas9 gene editing of one patient line carrying the G332R gigaxonin mutation. All GAN iPSCs were deficient for gigaxonin and displayed patient-specific increased vimentin expression. GAN NPCs had lower nestin expression and fewer nestin-positive cells compared to isogenic controls, but nestin morphology was unaffected. GAN brain organoids were marked by the presence of neurofilament and GFAP aggregates. GAN iPSC-astrocytes displayed striking dense perinuclear vimentin and GFAP accumulations and abnormal nuclear morphology. In over-expression systems, GFAP oligomerization and perinuclear aggregation were augmented in the presence of vimentin. GAN patient cells with large perinuclear vimentin aggregates accumulated significantly more nuclear KLHL16 mRNA compared to cells without vimentin aggregates. As an early effector of KLHL16 mutations, vimentin may be a potential target in GAN.

Intermediate filament dysregulation and astrocytopathy in the human disease model of KLHL16 mutation in giant axonal neuropathy (GAN)

Giant Axonal Neuropathy (GAN) is a pediatric neurodegenerative disease caused by KLHL16 mutations. KLHL16 encodes gigaxonin, a regulator of intermediate filament (IF) protein turnover. Previous neuropathological studies and our own examination of postmortem GAN brain tissue in the current study revealed astrocyte involvement in GAN. To study the underlying mechanisms, we reprogrammed skin fibroblasts from seven GAN patients carrying different KLHL16 mutations to iPSCs. Isogenic controls with restored IF phenotypes were derived via CRISPR/Cas9 editing of one patient carrying a homozygous missense mutation (G332R). Neural progenitor cells (NPCs), astrocytes, and brain organoids were generated through directed differentiation. All GAN iPSC lines were deficient for gigaxonin, which was restored in the isogenic control. GAN iPSCs displayed patient-specific increased vimentin expression, while GAN NPCs had decreased nestin expression compared to isogenic control. The most striking phenotypes were observed in GAN iPSC-astrocytes and brain organoids, which exhibited dense perinuclear IF accumulations and abnormal nuclear morphology. GAN patient cells with large perinuclear vimentin aggregates accumulated nuclear KLHL16 mRNA. In over-expression studies, GFAP oligomerization and perinuclear aggregation were potentiated in the presence of vimentin. As an early effector of KLHL16 mutations, vimentin may serve as a potential therapeutic target in GAN.

Nuclear envelope, chromatin organizers, histones, and DNA: The many achilles heels exploited across cancers

In eukaryotic cells, the genome is organized in the form of chromatin composed of DNA and histones that organize and regulate gene expression. The dysregulation of chromatin remodeling, including the aberrant incorporation of histone variants and their consequent post-translational modifications, is prevalent across cancers. Additionally, nuclear envelope proteins are often deregulated in cancers, which impacts the 3D organization of the genome. Altered nuclear morphology, genome organization, and gene expression are defining features of cancers. With advances in single-cell sequencing, imaging technologies, and high-end data mining approaches, we are now at the forefront of designing appropriate small molecules to selectively inhibit the growth and proliferation of cancer cells in a genome- and epigenome-specific manner. Here, we review recent advances and the emerging significance of aberrations in nuclear envelope proteins, histone variants, and oncohistones in deregulating chromatin organization and gene expression in oncogenesis.

Cytoplasmic keratins couple with and maintain nuclear envelope integrity in colonic epithelial cells

Keratin intermediate filaments convey mechanical stability and protection against stress to epithelial cells. Keratins are essential for colon health, as seen in keratin 8 knockout (K8-/-) mice exhibiting a colitis phenotype. We hypothesized that keratins support the nuclear envelope and lamina in colonocytes. K8-/- colonocytes in vivo exhibit significantly decreased levels of lamins A/C, B1, and B2 in a colon-specific and cell-intrinsic manner. CRISPR/Cas9- or siRNA-mediated K8 knockdown in Caco-2 cells similarly decreased lamin levels, which recovered after reexpression of K8 following siRNA treatment. Nuclear area was not decreased, and roundness was only marginally increased in cells without K8. Down-regulation of K8 in adult K8flox/flox;Villin-CreERt2 mice following tamoxifen administration significantly decreased lamin levels at day 4 when K8 levels had reduced to 40%. K8 loss also led to reduced levels of plectin, LINC complex, and lamin-associated proteins. While keratins were not seen in the nucleoplasm without or with leptomycin B treatment, keratins were found intimately located at the nuclear envelope and complexed with SUN2 and lamin A. Furthermore, K8 loss in Caco-2 cells compromised nuclear membrane integrity basally and after shear stress. In conclusion, colonocyte K8 helps maintain nuclear envelope and lamina composition and contributes to nuclear integrity.

Plectin linkages are mechanosensitive and required for the nuclear piston mechanism of three-dimensional cell migration

Cells migrating through physiologically relevant three-dimensional (3D) substrates such as cell-derived matrix (CDM) use actomyosin and vimentin intermediate filaments to pull the nucleus forward and pressurize the front of the cell as part of the nuclear piston mechanism of 3D migration. In this study, we tested the role of the cytoskeleton cross-linking protein plectin in facilitating the movement of the nucleus through 3D matrices. We find that the interaction of F-actin and vimentin filaments in cells on 2D glass and in 3D CDM requires actomyosin contractility. Plectin also facilitated these interactions and interacts with vimentin in response to NMII contractility and substrate stiffness, suggesting that the association of plectin and vimentin is mechanosensitive. We find that this mechanosensitive plectin complex slows down 2D migration but is critical for pulling the nucleus forward and generating compartmentalized intracellular pressure in 3D CDM, as well as low-pressure lamellipodial migration in 3D collagen. Finally, plectin expression helped to polarize NMII to in front of the nucleus and to localize the vimentin network around the nucleus. Together, our data suggest that plectin cross-links vimentin and actomyosin filaments, organizes the vimentin network, and polarizes NMII to facilitate the nuclear piston mechanism of 3D cell migration.

Signaling Pathways and Protein-Protein Interaction of Vimentin in Invasive and Migration Cells: A Review

The vimentin (encoded by VIM) is one of the 70 human intermediate filaments (IFs), building highly dynamic and cell-type-specific web networks in the cytoplasm. Vim^-/- mice exhibit process defects associated with cell differentiation, which can have implications for understanding cancer and disease. This review showed recent reports from studies that unveiled vimentin intermediate filaments (VIFs) as an essential component of the cytoskeleton, followed by a description of vimentin's physiological functions and process reports in VIF signaling pathway and gene network studies. The main focus of the discussion is on vital signaling pathways associated with how VIF coordinates invasion cells and migration. The current research will open up multiple processes to research the function of VIF and other IF proteins in cellular and molecular biology, and they will lead to essential insights into different VIF levels for the invasive metastatic cancer cells. Enrich GO databases used Gene Ontology and Pathway Enrichment Analysis. Estimation with STRING online was to predict the functional and molecular interactions of proteins-protein with Cytoscape analysis to search and select the master genes. Using Cytoscape and STRING analysis, we presented eight genes, RhoA, Smad3, Akt1, Cdk2, Rock1, Rock2, Mapk1, and Mapk8, as the essential protein-protein interaction with vimentin involved in the invasion.

PEAK1 Y635 phosphorylation regulates cell migration through association with Tensin3 and integrins

Integrins mediate cell adhesion by connecting the extracellular matrix to the intracellular cytoskeleton and orchestrate signal transduction in response to chemical and mechanical stimuli by interacting with many cytoplasmic proteins. We used BioID to interrogate the interactomes of β1 and β3 integrins in epithelial cells and identified PEAK1 as an interactor of the RGD-binding integrins α5β1, αVβ3, and αVβ5 in focal adhesions. We demonstrate that the interaction between integrins and PEAK1 occurs indirectly through Tensin3, requiring both the membrane-proximal NPxY motif on the integrin β tail and binding of the SH2 domain of Tensin3 to phosphorylated Tyr-635 on PEAK1. Phosphorylation of Tyr-635 is mediated by Src and regulates cell migration. Additionally, we found that Shc1 localizes in focal adhesions in a PEAK1 phosphorylated Tyr-1188-dependent fashion. Besides binding Shc1, PEAK1 also associates with a protein cluster that mediates late EGFR/Shc1 signaling. We propose a model in which PEAK1 binds Tensin3 and Shc1 to converge integrin and growth factor receptor signal transduction.

Nuclear lamin isoforms differentially contribute to LINC complex-dependent nucleocytoskeletal coupling and whole-cell mechanics

Interactions between the cell nucleus and cytoskeleton regulate cell mechanics and are facilitated by the interplay between the nuclear lamina and linker of nucleoskeleton and cytoskeleton (LINC) complexes. To date, the specific contribution of the four lamin isoforms to nucleocytoskeletal connectivity and whole-cell mechanics remains unknown. We discover that A- and B-type lamins distinctively interact with LINC complexes that bind F-actin and vimentin filaments to differentially modulate cortical stiffness, cytoplasmic stiffness, and contractility of mouse embryonic fibroblasts (MEFs). We propose and experimentally verify an integrated lamin–LINC complex–cytoskeleton model that explains cellular mechanical phenotypes in lamin-deficient MEFs. Our findings uncover potential mechanisms for cellular defects in human laminopathies and many cancers associated with mutations or modifications in lamin isoforms.

The ability of a cell to regulate its mechanical properties is central to its function. Emerging evidence suggests that interactions between the cell nucleus and cytoskeleton influence cell mechanics through poorly understood mechanisms. Here we conduct quantitative confocal imaging to show that the loss of A-type lamins tends to increase nuclear and cellular volume while the loss of B-type lamins behaves in the opposite manner. We use fluorescence recovery after photobleaching, atomic force microscopy, optical tweezer microrheology, and traction force microscopy to demonstrate that A-type lamins engage with both F-actin and vimentin intermediate filaments (VIFs) through the linker of nucleoskeleton and cytoskeleton (LINC) complexes to modulate cortical and cytoplasmic stiffness as well as cellular contractility in mouse embryonic fibroblasts (MEFs). In contrast, we show that B-type lamins predominantly interact with VIFs through LINC complexes to regulate cytoplasmic stiffness and contractility. We then propose a physical model mediated by the lamin–LINC complex that explains these distinct mechanical phenotypes (mechanophenotypes). To verify this model, we use dominant negative constructs and RNA interference to disrupt the LINC complexes that facilitate the interaction of the nucleus with the F-actin and VIF cytoskeletons and show that the loss of these elements results in mechanophenotypes like those observed in MEFs that lack A- or B-type lamin isoforms. Finally, we demonstrate that the loss of each lamin isoform softens the cell nucleus and enhances constricted cell migration but in turn increases migration-induced DNA damage. Together, our findings uncover distinctive roles for each of the four major lamin isoforms in maintaining nucleocytoskeletal interactions and cellular mechanics.

Nuclear lamins: Structure and function in mechanobiology

Nuclear lamins are type V intermediate filament proteins that polymerize into complex filamentous meshworks at the nuclear periphery and in less structured forms throughout the nucleoplasm. Lamins interact with a wide range of nuclear proteins and are involved in numerous nuclear and cellular functions. Within the nucleus, they play roles in chromatin organization and gene regulation, nuclear shape, size, and mechanics, and the organization and anchorage of nuclear pore complexes. At the whole cell level, they are involved in the organization of the cytoskeleton, cell motility, and mechanotransduction. The expression of different lamin isoforms has been associated with developmental progression, differentiation, and tissue-specific functions. Mutations in lamins and their binding proteins result in over 15 distinct human diseases, referred to as laminopathies. The laminopathies include muscular (e.g., Emery-Dreifuss muscular dystrophy and dilated cardiomyopathy), neurological (e.g., microcephaly), and metabolic (e.g., familial partial lipodystrophy) disorders as well as premature aging diseases (e.g., Hutchinson-Gilford Progeria and Werner syndromes). How lamins contribute to the etiology of laminopathies is still unknown. In this review article, we summarize major recent findings on the structure, organization, and multiple functions of lamins in nuclear and more global cellular processes.

Circulating Tumor Cells in Breast Cancer Patients: A Balancing Act between Stemness, EMT Features and DNA Damage Responses

Circulating tumor cells (CTCs) traverse vessels to travel from the primary tumor to distant organs where they adhere, transmigrate, and seed metastases. To cope with these challenges, CTCs have reached maximal flexibility to change their differentiation status, morphology, migratory capacity, and their responses to genotoxic stress caused by metabolic changes, hormones, the inflammatory environment, or cytostatic treatment. A significant percentage of breast cancer cells are defective in homologous recombination repair and other mechanisms that protect the integrity of the replication fork. To prevent cell death caused by broken forks, alternative, mutagenic repair, and bypass pathways are engaged but these increase genomic instability. CTCs, arising from such breast tumors, are endowed with an even larger toolbox of escape mechanisms that can be switched on and off at different stages during their journey according to the stress stimulus. Accumulating evidence suggests that DNA damage responses, DNA repair, and replication are integral parts of a regulatory network orchestrating the plasticity of stemness features and transitions between epithelial and mesenchymal states in CTCs. This review summarizes the published information on these regulatory circuits of relevance for the design of biomarkers reflecting CTC functions in real-time to monitor therapeutic responses and detect evolving chemoresistance mechanisms.

Push or pull: how cytoskeletal crosstalk facilitates nuclear movement through 3D environments

As cells move from two-dimensional (2D) surfaces into complex 3D environments, the nucleus becomes a barrier to movement due to its size and rigidity. Therefore, moving the nucleus is a key step in 3D cell migration. In this review, we discuss how coordination between cytoskeletal and nucleoskeletal networks is required to pull the nucleus forward through complex 3D spaces. We summarize recent migration models which utilize unique molecular crosstalk to drive nuclear migration through different 3D environments. In addition, we speculate about the role of proteins that indirectly crosslink cytoskeletal networks and the role of 3D focal adhesions and how these protein complexes may drive 3D nuclear migration.

Zonula occludens 2 and Cell-Cell Contacts Are Required for Normal Nuclear Shape in Epithelia

MAGUK protein ZO-2 is present at tight junctions (TJs) and nuclei. In MDCK ZO-2 knockdown (KD) cells, nuclei exhibit an irregular shape with lobules and indentations. This condition correlates with an increase in DNA double strand breaks, however cells are not senescent and instead become resistant to UV-induced senescence. The irregular nuclear shape is also observed in isolated cells and in those without TJs, due to the lack of extracellular calcium. The aberrant nuclear shape of ZO-2 KD cells is not accompanied by a reduced expression of lamins A/C and B and lamin B receptors. Instead, it involves a decrease in constitutive and facultative heterochromatin, and microtubule instability that is restored with docetaxel. ZO-2 KD cells over-express SUN-1 that crosses the inner nuclear membrane and connects the nucleoskeleton of lamin A to nesprins, which traverse the outer nuclear membrane. Nesprins-3 and -4 that indirectly bind on their cytoplasmic face to vimentin and microtubules, respectively, are also over-expressed in ZO-2 KD cells, whereas vimentin is depleted. SUN-1 and lamin B1 co-immunoprecipitate with ZO-2, and SUN-1 associates to ZO-2 in a pull-down assay. Our results suggest that ZO-2 forms a complex with SUN-1 and lamin B1 at the inner nuclear membrane, and that ZO-2 and cell–cell contacts are required for a normal nuclear shape.

Identifying Plectin Isoform Functions through Animal Models

Plectin, a high-molecular-weight cytoskeletal linker protein, binds with high affinity to intermediate filaments of all types and connects them to junctional complexes, organelles, and inner membrane systems. In addition, it interacts with actomyosin structures and microtubules. As a multifunctional protein, plectin has been implicated in several multisystemic diseases, the most common of which is epidermolysis bullosa simplex with muscular dystrophy (EBS-MD). A great part of our knowledge about plectin’s functional diversity has been gained through the analysis of a unique collection of transgenic mice that includes a full (null) knockout (KO), several tissue-restricted and isoform-specific KOs, three double KOs, and two knock-in lines. The key molecular features and pathological phenotypes of these mice will be discussed in this review. In summary, the analysis of the different genetic models indicated that a functional plectin is required for the proper function of striated and simple epithelia, cardiac and skeletal muscle, the neuromuscular junction, and the vascular endothelium, recapitulating the symptoms of humans carrying plectin mutations. The plectin-null line showed severe skin and muscle phenotypes reflecting the importance of plectin for hemidesmosome and sarcomere integrity; whereas the ablation of individual isoforms caused a specific phenotype in myofibers, basal keratinocytes, or neurons. Tissue-restricted ablation of plectin rendered the targeted cells less resilient to mechanical stress. Studies based on animal models other than the mouse, such as zebrafish and C. elegans, will be discussed as well.

Plectin-Mediated Intermediate Filament Functions: Why Isoforms Matter

This essay focuses on the role of plectin and its various isoforms in mediating intermediate filament (IF) network functions. It is based on previous studies that provided comprehensive evidence for a concept where plectin acts as an IF recruiter, and plectin-mediated IF networking and anchoring are key elements in IF function execution. Here, plectin’s global role as modulator of IF functionality is viewed from different perspectives, including the mechanical stabilization of IF networks and their docking platforms, contribution to cellular viscoelasticity and mechanotransduction, compartmentalization and control of the actomyosin machinery, connections to the microtubule system, and mechanisms and specificity of isoform targeting. Arguments for IF networks and plectin acting as mutually dependent partners are also given. Lastly, a working model is presented that describes a unifying mechanism underlying how plectin–IF networks mechanically control and propagate actomyosin-generated forces, affect microtubule dynamics, and contribute to mechanotransduction.

From the Matrix to the Nucleus and Back: Mechanobiology in the Light of Health, Pathologies, and Regeneration of Oral Periodontal Tissues

Among oral tissues, the periodontium is permanently subjected to mechanical forces resulting from chewing, mastication, or orthodontic appliances. Molecularly, these movements induce a series of subsequent signaling processes, which are embedded in the biological concept of cellular mechanotransduction (MT). Cell and tissue structures, ranging from the extracellular matrix (ECM) to the plasma membrane, the cytosol and the nucleus, are involved in MT. Dysregulation of the diverse, fine-tuned interaction of molecular players responsible for transmitting biophysical environmental information into the cell's inner milieu can lead to and promote serious diseases, such as periodontitis or oral squamous cell carcinoma (OSCC). Therefore, periodontal integrity and regeneration is highly dependent on the proper integration and regulation of mechanobiological signals in the context of cell behavior. Recent experimental findings have increased the understanding of classical cellular mechanosensing mechanisms by both integrating exogenic factors such as bacterial gingipain proteases and newly discovered cell-inherent functions of mechanoresponsive co-transcriptional regulators such as the Yes-associated protein 1 (YAP1) or the nuclear cytoskeleton. Regarding periodontal MT research, this review offers insights into the current trends and open aspects. Concerning oral regenerative medicine or weakening of periodontal tissue diseases, perspectives on future applications of mechanobiological principles are discussed.

Plectin ensures intestinal epithelial integrity and protects colon against colitis

Plectin, a highly versatile cytolinker protein, provides tissues with mechanical stability through the integration of intermediate filaments (IFs) with cell junctions. Here, we hypothesize that plectin-controlled cytoarchitecture is a critical determinant of the intestinal barrier function and homeostasis. Mice lacking plectin in an intestinal epithelial cell (IEC; Ple^ΔIEC) spontaneously developed colitis characterized by extensive detachment of IECs from the basement membrane (BM), increased intestinal permeability, and inflammatory lesions. Moreover, plectin expression was reduced in the colons of ulcerative colitis (UC) patients and negatively correlated with the severity of colitis. Mechanistically, plectin deficiency in IECs led to aberrant keratin filament (KF) network organization and the formation of dysfunctional hemidesmosomes (HDs) and intercellular junctions. In addition, the hemidesmosomal α6β4 integrin (Itg) receptor showed attenuated association with KFs, and protein profiling revealed prominent downregulation of junctional constituents. Consistent with the effects of plectin loss in the intestinal epithelium, plectin-deficient IECs exhibited remarkably reduced mechanical stability and limited adhesion capacity in vitro. Feeding mice with a low-residue liquid diet that reduced mechanical stress and antibiotic treatment successfully mitigated epithelial damage in the Ple^ΔIEC colon.

The vimentin cytoskeleton: when polymer physics meets cell biology

The proper functions of tissues depend on the ability of cells to withstand stress and maintain shape. Central to this process is the cytoskeleton, comprised of three polymeric networks: F-actin, microtubules, and intermediate filaments (IFs). IF proteins are among the most abundant cytoskeletal proteins in cells; yet they remain some of the least understood. Their structure and function deviate from those of their cytoskeletal partners, F-actin and microtubules. IF networks show a unique combination of extensibility, flexibility and toughness that confers mechanical resilience to the cell. Vimentin is an IF protein expressed in mesenchymal cells. This review highlights exciting new results on the physical biology of vimentin intermediate filaments and their role in allowing whole cells and tissues to cope with stress.

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.

Spatial organization and crosstalk of vimentin and actin stress fibers regulate the osteogenic differentiation of human adipose-derived stem cells

Human adipose-derived stem cells (hASCs) are ideal seed cells for tissue engineering due to their multidirectional differentiation potential. Microfilaments, microtubules, and intermediate filaments are responsible for supporting the intracellular space. Vimentin, a type III intermediate filament protein that is specifically expressed in cells of mesenchymal origin, can function as a scaffold and endow cells with tension and shear stress resistance. Actin stress fibers (ASF) act as an important physical device in stress signal transduction, providing stiffness for cells, and promoting osteogenesis. Through direct physical contact, cross-linkers, and spatial interactions, vimentin and actin networks exist as intersecting entities. Spatial interactions occur in the overlapping area of cytoskeleton subsystems, which could affect cell morphology, cell mechanics, and cell fate. However, how does the spatial organization between the cytoskeletal subsystems changed during osteogenesis, especially between vimentin and ASF, is still not understood, and its mechanism effect on cell fate remains unclear. In our study, WB experiment was used to detect the expression changes in Vimentin, ASF, and other proteins. Cells were reconstructed by three-dimensional scanning with fluorescence microscope, and the spatial thickness of vimentin and ASF cytoskeletons and the thickness of the overlapping area between them were calculated, respectively, so as to observe the spatial reorganization of vimentin and ASF in cells. Cytochalasin D (an inhibitor of actin polymerization) and vimentin upregulated/downregulated cells were used to verify the change in the spatial organization between vimentin and ASF and its influence on osteogenesis. Then, heat shock protein 27 (HSP27) was downregulated to illuminate the regulatory mechanisms of spatial organization between vimentin and ASF during osteogenesis. The amounts and the spatial positions of vimentin and actin stress fiber exhibited opposite trends during osteogenesis. Through controlling the anchor sites on the nucleus, intermediate filaments vimentin can reduce the spatial proportion of actin stress fibers, which can be regulated by HSP27. In addition, depolymerization of actin stress fibers lead to lower osteogenic differentiation ability, resulting in osteogenesis and lipogenesis existed simultaneously, that can be resisted by vimentin. Our data indicate that the spatial reorganization of vimentin and actin stress fibers is a key factor in the regulation of the differentiation state of hASCs. And their spatial overlapping area is detrimental to hASCs osteogenesis, providing a new perspective for further exploring the mechanism underlying hASCs osteogenesis.

Mechanical and Non-Mechanical Functions of Filamentous and Non-Filamentous Vimentin

Intermediate filaments (IFs) formed by vimentin are less understood than their cytoskeletal partners, microtubules and F-actin, but the unique physical properties of IFs, especially their resistance to large deformations, initially suggest a mechanical function. Indeed, vimentin IFs help regulate cell mechanics and contractility, and in crowded 3D environments they protect the nucleus during cell migration. Recently, a multitude of studies, often using genetic or proteomic screenings show that vimentin has many non-mechanical functions within and outside of cells. These include signaling roles in wound healing, lipogenesis, sterol processing, and various functions related to extracellular and cell surface vimentin. Extracellular vimentin is implicated in marking circulating tumor cells, promoting neural repair, and mediating the invasion of host cells by viruses, including SARS-CoV, or bacteria such as Listeria and Streptococcus. These findings underscore the fundamental role of vimentin in not only cell mechanics but also a range of physiological functions. Also see the video abstract here https://youtu.be/YPfoddqvz-g.

Deformation Microscopy for Dynamic Intracellular and Intranuclear Mapping of Mechanics with High Spatiotemporal Resolution

Structural heterogeneity is a hallmark of living cells that drives local mechanical properties and dynamic cellular responses. However, the robust quantification of intracellular mechanics is lacking from conventional methods. Here, we describe the development of deformation microscopy, which leverages conventional imaging and an automated hyperelastic warping algorithm to investigate strain history, deformation dynamics, and changes in structural heterogeneity within the interior of cells and cell nuclei. Using deformation microscopy, we found that partial or complete disruption of LINC complexes in cardiomyocytes in vitro and lamin A/C deficiency in myocytes in vivo abrogate dominant tensile loading in the nuclear interior. We also found that cells cultured on stiff substrates or in hyperosmotic conditions displayed abnormal strain burden and asymmetries at interchromatin regions, which are associated with active transcription. Deformation microscopy represents a foundational approach toward intracellular elastography, with the potential utility to provide mechanistic and quantitative insights in diverse mechanobiological applications.

Ghosh et al. show that deformation microscopy, a technique based on image analysis and mechanics, reveals deformation dynamics and structural heterogeneity changes for several applications and at multiple scales, including tissues, cells, and nuclei. They reveal how the disruption of nuclear proteins and pathological conditions abrogate mechanical strain in the nuclear interior.

Ataxia-telangiectasia mutated coordinates the ovarian DNA repair and atresia-initiating response to phosphoramide mustard

Ataxia-telangiectasia-mutated (ATM) protein recognizes and repairs DNA double strand breaks through activation of cell cycle checkpoints and DNA repair proteins. Atm gene mutations increase female reproductive cancer risk. Phosphoramide mustard (PM) induces ovarian DNA damage and destroys primordial follicles, and pharmacological ATM inhibition prevents PM-induced follicular depletion. Wild-type (WT) C57BL/6 or Atm+/- mice were dosed once intraperitoneally with sesame oil (95%) or PM (25 mg/kg) in the proestrus phase of the estrous cycle and ovaries harvested 3 days thereafter. Atm+/- mice spent ~25% more time in diestrus phase than WT. Liquid chromatography with tandem mass spectrometry (LC-MS/MS) on ovarian protein was performed and bioinformatically analyzed. Relative to WT, Atm+/- mice had 64 and 243 proteins increased or decreased in abundance, respectively. In WT mice, PM increased 162 and decreased 20 proteins. In Atm+/- mice, 173 and 37 proteins were increased and decreased, respectively, by PM. Exportin-2 (XPO2) was localized to granulosa cells of all follicle stages and was 7.2-fold greater in Atm+/- than WT mice. Cytoplasmic FMR1-interacting protein 1 was 6.8-fold lower in Atm+/- mice and was located in the surface epithelium with apparent translocation to the ovarian medulla post-PM exposure. PM induced γH2AX, but fewer γH2AX-positive foci were identified in Atm+/- ovaries. Similarly, cleaved caspase-3 was lower in the Atm+/- PM-treated, relative to WT mice. These findings support ATM involvement in ovarian DNA repair and suggest that ATM functions to regulate ovarian atresia.

A Balance Between Intermediate Filaments and Microtubules Maintains Nuclear Architecture in the Cardiomyocyte

Rationale: Mechanical forces are transduced to nuclear responses via the linkers of the nucleoskeleton and cytoskeleton (LINC) complex, which couples the cytoskeleton to the nuclear lamina and associated chromatin. While disruption of the LINC complex can cause cardiomyopathy, the relevant interactions that bridge the nucleoskeleton to cytoskeleton are poorly understood in the cardiomyocyte, where cytoskeletal organization is unique. Furthermore, while microtubules and desmin intermediate filaments associate closely with cardiomyocyte nuclei, the importance of these interactions is unknown. Objective: Here, we sought to determine how cytoskeletal interactions with the LINC complex regulate nuclear homeostasis in the cardiomyocyte. Methods and Results: To this end, we acutely disrupted the LINC complex, microtubules, actin, and intermediate filaments and assessed the consequences on nuclear morphology and genome organization in rat ventricular cardiomyocytes via a combination of super-resolution imaging, biophysical, and genomic approaches. We find that a balance of dynamic microtubules and desmin intermediate filaments is required to maintain nuclear shape and the fidelity of the nuclear envelope and lamina. Upon depletion of desmin (or nesprin [nuclear envelope spectrin repeat protein]-3, its binding partner in the LINC complex), polymerizing microtubules collapse the nucleus and drive infolding of the nuclear membrane. This results in DNA damage, a loss of genome organization, and broad transcriptional changes. The collapse in nuclear integrity is concomitant with compromised contractile function and may contribute to the pathophysiological changes observed in desmin-related myopathies. Conclusions: Disrupting the tethering of desmin to the nucleus results in a loss of nuclear homeostasis and rapid alterations to cardiomyocyte function. Our data suggest that a balance of forces imposed by intermediate filaments and microtubules is required to maintain nuclear structure and genome organization in the cardiomyocyte.

A Glance at the Nuclear Envelope Spectrin Repeat Protein 3

Nuclear envelope spectrin repeat protein 3 (nesprin-3) is an evolutionarily-conserved structural protein, widely-expressed in vertebrate cells. Along with other nesprin family members, nesprin-3 acts as an essential component of the linker of nucleoskeleton and cytoskeleton (LINC) complex. Naturally, nesprin-3 shares many functions with LINC, including the localization of various cellular structures and bridging of the nucleoskeleton and cytoskeleton, observed in vitro. When nesprin-3 was knocked down in vivo, using zebrafish and mouse models, however, the animals were minimally affected. This paradoxical observation should not limit the physiological importance of nesprin-3, as recently, nesprin-3 has reignited the interest of the research community in studies on cancer cells migration. Moreover, nesprin-3 also plays an active role in certain developmental conditions such as adipogenesis and spermatogenesis, although more studies are needed. Meanwhile, the various protein binding partners of nesprin-3 should also be emphasized, as they are necessary for maintaining the structure of nesprin-3 and enabling it to carry out its various physiological and pathological functions. Nesprin-3 promises to further our understanding of these complex cellular events. Therefore, this review will focus on nesprin-3, examining it from a genetic, structural, and functional perspective. The final part of the review will in turn address the limitations of existing research and the future perspectives for the study of nesprin-3.

Vimentin protects cells against nuclear rupture and DNA damage during migration

Mammalian cells frequently migrate through tight spaces during normal embryogenesis, wound healing, diapedesis, or in pathological situations such as metastasis. Nuclear size and shape are important factors in regulating the mechanical properties of cells during their migration through such tight spaces. At the onset of migratory behavior, cells often initiate the expression of vimentin, an intermediate filament protein that polymerizes into networks extending from a juxtanuclear cage to the cell periphery. However, the role of vimentin intermediate filaments (VIFs) in regulating nuclear shape and mechanics remains unknown. Here, we use wild-type and vimentin-null mouse embryonic fibroblasts to show that VIFs regulate nuclear shape and perinuclear stiffness, cell motility in 3D, and the ability of cells to resist large deformations. These changes increase nuclear rupture and activation of DNA damage repair mechanisms, which are rescued by exogenous reexpression of vimentin. Our findings show that VIFs provide mechanical support to protect the nucleus and genome during migration.

Vimentin Intermediate Filament Rings Deform the Nucleus During the First Steps of Adhesion

During cell spreading, cells undergo many changes to their architecture and their mechanical properties. Vimentin, as an integral part of the cell architecture, and its mechanical stability must adapt to the new state of the cell. This study focuses on the structures formed by vimentin during the first steps of cell adhesion. Very early, ball-like structures, or "knots," are seen and often vimentin filaments emerge in the shape of rings around the nucleus. Although intermediate filaments are not known to be associated to motor proteins to form contractile systems, these rings can nonetheless strongly deform the cell nucleus. In the first 6 to 12 h of adhesion, these vimentin knots and rings disappear, and the intermediate filament network returns to the state seen before detachment of the cells. As these vimentin structures are very transient in the early steps of cell spreading, they have rarely been described in the literature. However, they can also be seen during mitosis, which is an event that involves partial detachment and re-spreading of the cells. Interestingly, the turnover dynamics of vimentin are reduced in both the knots and rings, compared to vimentin in the lamellipodia. It remains to define how the force is transmitted from the ball-like structures to the rings, and to measure the impact of such strong nuclear deformation on gene expression during cell re-spreading and the rearrangement of the vimentin network.

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.

Nuclear envelope dynamics during mammalian spermatogenesis: new insights on male fertility

The production of highly specialized spermatozoa from undifferentiated spermatogonia is a strictly organized and programmed process requiring extensive restructuring of the entire cell. One of the most remarkable cellular transformations accompanying the various phases of spermatogenesis is the profound remodelling of the nuclear architecture, in which the nuclear envelope (NE) seems to be crucially involved. In recent years, several proteins from the distinct layers forming the NE (i.e. the inner and outer nuclear membranes as well as the nuclear lamina) have been associated with meiosis and/or spermiogenesis in different mammalian species. Among these are A- and B-type lamins, Dpy-19-like protein 2 (DPY19L2), lamin B receptor (LBR), lamina-associated polypeptide 1 (LAP1), LAP2/emerin/MAN1 (LEM) domain-containing proteins, spermatogenesis-associated 46 (SPATA46) and diverse elements of the linker of nucleoskeleton and cytoskeleton (LINC) complex, namely Sad-1/UNC-84 homology (SUN) and Klarsicht/ANC-1/Syne-1 homology (KASH) domain-containing proteins. Herein, we summarize the current state of the art on the cellular and subcellular distribution of NE proteins expressed during mammalian spermatogenesis, and discuss the latest research developments regarding their testis-specific functions. This review provides a comprehensive and innovative overview of the NE network as a regulatory platform and as an essential determinant of efficient meiotic chromosome recombination as well as spermiogenesis-associated nuclear remodelling and differentiation in mammalian male germline cells. Thus, this review provides important novel insights on the biological relevance of NE proteins for male fertility.

Cytoskeletal control of nuclear morphology and stiffness are required for OPN-induced bone-marrow-derived mesenchymal stem cell migration

During cell migration, the movement of the nucleus must be coordinated with the cytoskeletal dynamics that influence the efficiency of cell migration. Our previous study demonstrated that osteopontin (OPN) significantly promotes the migration of bone-marrow-derived mesenchymal stem cells (BMSCs). However, the mechanism that regulates nuclear mechanics of the cytoskeleton during OPN-promoted BMSC migration remains unclear. In this study, we investigated how the actin cytoskeleton influences nuclear mechanics in BMSCs. We assessed the morphology and mechanics of the nuclei in the OPN-treated BMSCs subjected to disruption or polymerization of the actin cytoskeleton. We found that disruption of actin organization by cytochalasin D (Cyto D) resulted in a decrease in the nuclear projected area and nuclear stiffness. Stabilizing the actin assembly with jasplakinolide (JASP) resulted in an increase in the nuclear projected area and nuclear stiffness. SUN1 (Sad-1/UNC-84 1) is a component of the LINC (linker of nucleoskeleton and cytoskeleton) complex involved in the connections between the nucleus and the cytoskeleton. We found that SUN1 depletion by RNAi decreased the nuclear stiffness and OPN-promoted BMSC migration. Thus, the F-actin cytoskeleton plays an important role in determining the morphology and mechanical properties of the nucleus. We suggest that the cytoskeletal-nuclear interconnectivity through SUN1 proteins plays an important role in OPN-promoted BMSC migration.

Nesprins and Lamins in Health and Diseases of Cardiac and Skeletal Muscles

Since the discovery of the inner nuclear transmembrane protein emerin in the early 1990s, nuclear envelope (NE) components and related involvement in nuclei integrity and functionality have been highly investigated. The NE is composed of two distinct lipid bilayers described as the inner (INM) and outer (ONM) nuclear membrane. NE proteins can be specifically “integrated” in the INM (such as emerin and SUN proteins) or in the ONM such as nesprins. Additionally, flanked to the INM, the nuclear lamina, a proteinaceous meshwork mainly composed of lamins A and C completes NE composition. This network of proteins physically interplays to guarantee NE integrity and most importantly, shape the bridge between cytoplasmic cytoskeletons networks (such as microtubules and actin) and the genome, through the anchorage to the heterochromatin. The essential network driving the connection of nucleoskeleton with cytoskeleton takes place in the perinuclear space (the space between ONM and INM) with the contribution of the LINC complex (for Linker of Nucleoskeleton to Cytoskeleton), hosting KASH and SUN proteins interactions. This close interplay between compartments has been related to diverse functions from nuclear integrity, activity and positioning through mechanotransduction pathways. At the same time, mutations in NE components genes coding for proteins such as lamins or nesprins, had been associated with a wide range of congenital diseases including cardiac and muscular diseases. Although most of these NE associated proteins are ubiquitously expressed, a large number of tissue-specific disorders have been associated with diverse pathogenic mutations. Thus, diagnosis and molecular explanation of this group of diseases, commonly called “nuclear envelopathies,” is currently challenging. This review aims, first, to give a better understanding of diverse functions of the LINC complex components, from the point of view of lamins and nesprins. Second, to summarize human congenital diseases with a special focus on muscle and heart abnormalities, caused by mutations in genes coding for these two types of NE associated proteins.

Cytoplasmic Intermediate Filaments in Cell Biology

Intermediate filaments (IFs) are one of the three major elements of the cytoskeleton. Their stability, intrinsic mechanical properties, and cell type-specific expression patterns distinguish them from actin and microtubules. By providing mechanical support, IFs protect cells from external forces and participate in cell adhesion and tissue integrity. IFs form an extensive and elaborate network that connects the cell cortex to intracellular organelles. They act as a molecular scaffold that controls intracellular organization. However, IFs have been revealed as much more than just rigid structures. Their dynamics is regulated by multiple signaling cascades and appears to contribute to signaling events in response to cell stress and to dynamic cellular functions such as mitosis, apoptosis, and migration.

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.

Mouse models of nesprin-related diseases

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

Ultra-structure of the sperm head-to-tail linkage complex in the absence of the spermatid-specific LINC component SPAG4

Tight connection between sperm head and tail is crucial for the transport of the male genome and fertilization. The linkage complex, the sperm head-to-tail coupling apparatus (HTCA), originates from the centrosome and anchors to the nuclear membrane. In contrast to its ultra-structural organization, which is already well known for decades, its protein composition largely still awaits future deciphering. SUN-domain proteins are essential components of a complex that links the cytoskeleton to the peripheral nucleoskeleton, which is the nuclear lamina. Here, we studied the impact of the SUN protein SPAG4/SUN4 on the formation of the HTCA. SPAG4/SUN4 is specifically expressed in haploid male germ cells showing a polarized distribution towards the posterior pole in late spermatids that corresponds to the tail attachment site. SPAG4-deficient male mice are infertile with compromised manchette formation and malformed sperm heads. Nonetheless, sperm tails are present demonstrating dispensability of a proper manchette for their formation. Ultra-structural analyses revealed that the development of the sperm head-to-tail linkage complex in the absence of SPAG4 resembles that in the wild type. However, in SPAG4-deficient sperm, the attachment site is diminished with obvious lateral detachment of the HTCA from the nucleus. Our results thus indicate that SPAG4, albeit not essential for the formation of the HTCA per se, is, nevertheless, required for tightening the sperm head-to-tail anchorage by provoking the correct attachment of the lateral parts of the basal plate to the implantation fossa.

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.

Dealing with the nucleus during cell migration

The position of the nucleus within cells is a key event during cell migration. The movement and positioning of the nucleus strongly impacts cell migration. Notably, the last two years largely contributed to emphasise the dynamicity of the nucleus-cytoskeleton interactions that occur during cell migration. Nuclei are under continuous tension from opposing intracellular forces and its tether to the cytoskeleton can be regulated at different levels. Interestingly, it was showed how nuclear positioning is highly related to cell function. In most migrating cells, including cancer cells, the nucleus can be the rate limiting step of cell migration and is placed away from the leading edge. By contrast, leukocytes position their nucleus close to the lamellipodia at the leading edge, and the nucleus contributes to drilling through the endothelium. Differences in cell migration in 2D versus 3D environments are also evident. The mechanisms and forces at play during nuclear positioning and translocation are clearly affected by the nature of the substrate. As such nuclear positioning during cell migration can vary between cell types and environments. In this review we aim to give an overview of the latest discoveries in the field revealing how nuclear positioning is tightly regulated, not only by intrinsic nuclear properties, such as deformability, nuclear envelope content or nucleus-cytoskeleton connectivity, but also by the microenvironment.

LINC complexes and nuclear positioning

One of the characteristics of eukaryotic cells is their structural plasticity associated with the ability to carry out a broad range of complex functions, both autonomously and as components of tissues and organs. Major cellular rearrangements can be observed in various systems from meiosis in fission yeast, through dermal differentiation in nematodes, to muscle and neuronal development in vertebrates. Each of these processes involves oftentimes dramatic relocation of the nucleus within the cell. During the last decade it has become apparent that the nuclear periphery represents a nexus of cytoskeletal interactions that are involved not only in nuclear movement but also in the distribution and dissemination of mechanical forces throughout the cell. Nucleocytoskeletal coupling is mediated in large part by SUN- and KASH-domain proteins of the nuclear membranes, that together assemble to form LINC (Linker of the Nucleoskeleton and Cytoskeleton) complexes. In this review we will describe how the LINC complex repertoire contributes to nuclear positioning and chromosome dynamics in a variety of cellular contexts.

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

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

Expression of Epithelial and Mesenchymal Differentiation Markers in the Early Human Gonadal Development

Expressions of cytokeratin 8 (CK8), vimentin, nestin, and alpha-smooth-muscle-actin (alpha-SMA) were analyzed in the developing gonads of 12, 5-9 week old (W) human conceptuses by immunohistochemistry and immunofluorescence. During the investigated period, the number of CK8 positive cells increased from 56% to 92% in the gonadal surface epithelium, from 50% to 60% in the stroma, and from 23% to 42% in the medulla. In the early fetal period, the cell expression of CK8 increased in all gonadal parts, whereas primordial germ cells (PGC) remained negative. The expression of vimentin increased in the gonad stroma (gs) from 73% to 88%, and in the surface epithelium from 18% to 97% until ninth W. The medulla had the highest expression of vimentin in the seventh to eighth W (93%). Vimentin and CK8 colocalized in the somatic cells, while some PGCs showed vimentin expression only. Initially, nestin was positive in the gonad surface epithelium (8%) and stroma (52%), however during further development it decreased to 1% and 33%, respectively. In the early fetal period, the nestin positive cells decreased from 44% to 31% in the gonad medulla. Alpha-SMA was positive only in the blood vessels and mesonephros. The described pattern of expression of intermediate filaments (IF) in developing human gonads suggests their role in the control of PGC apoptosis, early differentiation of gs cells and cell migration. Both epithelial and mesenchymal origins of follicular cells and possible epithelial-to-mesenchymal transition of somatic cells is proposed. Lastly, IF intensity expression varies depending on the cell type and developmental period analyzed. Anat Rec, 300:1315-1326, 2017. © 2017 Wiley Periodicals, Inc.

Implications and Assessment of the Elastic Behavior of Lamins in Laminopathies

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

Cytoskeletal Integrators: The Spectrin Superfamily

This review discusses the spectrin superfamily of proteins that function to connect cytoskeletal elements to each other, the cell membrane, and the nucleus. The signature domain is the spectrin repeat, a 106-122-amino-acid segment comprising three α-helices. α-actinin is considered to be the ancestral protein and functions to cross-link actin filaments. It then evolved to generate spectrin and dystrophin that function to link the actin cytoskeleton to the cell membrane, as well as the spectraplakins and plakins that link cytoskeletal elements to each other and to junctional complexes. A final class comprises the nesprins, which are able to bind to the nuclear membrane. This review discusses the domain organization of the various spectrin family members, their roles in protein-protein interactions, and their roles in disease, as determined from mutations, and it also describes the functional roles of the family members as determined from null phenotypes.

Under Pressure: Mechanical Stress Management in the Nucleus

Cells are constantly adjusting to the mechanical properties of their surroundings, operating a complex mechanochemical feedback, which hinges on mechanotransduction mechanisms. Whereas adhesion structures have been shown to play a central role in mechanotransduction, it now emerges that the nucleus may act as a mechanosensitive structure. Here, we review recent advances demonstrating that mechanical stress emanating from the cytoskeleton can activate pathways in the nucleus which eventually impact both its structure and the transcriptional machinery.

Squish and squeeze-the nucleus as a physical barrier during migration in confined environments

From embryonic development to cancer metastasis, cell migration plays a central role in health and disease. It is increasingly becoming apparent that cells migrating in three-dimensional (3-D) environments exhibit some striking differences compared with their well-established 2-D counterparts. One key finding is the significant role the nucleus plays during 3-D migration: when cells move in confined spaces, the cell body and nucleus must deform to squeeze through available spaces, and the deformability of the large and relatively rigid nucleus can become rate-limiting. In this review, we highlight recent findings regarding the role of nuclear mechanics in 3-D migration, including factors that govern nuclear deformability, and emerging mechanisms by which cells apply cytoskeletal forces to the nucleus to facilitate nuclear translocation. Intriguingly, the 'physical barrier' imposed by the nucleus also impacts cytoplasmic dynamics that affect cell migration and signaling, and changes in nuclear structure resulting from the mechanical forces acting on the nucleus during 3-D migration could further alter cellular function. These findings have broad relevance to the migration of both normal and cancerous cells inside living tissues, and motivate further research into the molecular details by which cells move their nuclei, as well as the consequences of the mechanical stress on the nucleus.

Cross talk between the cytoplasm and nucleus during development and disease

Mechanotransduction is a process whereby mechanical stimuli outside the cell are sensed by components of the plasma membrane and transmitted as signals through the cytoplasm that terminate in the nucleus. The nucleus responds to these signals by altering gene expression. During mechanotransduction, complex networks of proteins are responsible for cross talk between the cytoplasm and the nucleus. These proteins include cell membrane receptors, cytoplasmic filaments, LINC complex members that bridge the nucleus and cytoplasm, and nuclear envelope proteins that connect to the chromatin. Mechanotransduction also plays a critical role in development. Furthermore, it is possible that disrupted mechanotransduction leads to changes in gene expression that underlie the pathogenic mechanisms of disease.