The nuclear-envelope protein and transcriptional repressor LAP2β interacts with HDAC3 at the nuclear periphery, and induces histone H4 deacetylation

MAPK signaling pathways and HDAC3 activity are disrupted during differentiation of emerin-null myogenic progenitor cells

Mutations in the gene encoding emerin cause Emery–Dreifuss muscular dystrophy (EDMD). Emerin is an integral inner nuclear membrane protein and a component of the nuclear lamina. EDMD is characterized by skeletal muscle wasting, cardiac conduction defects and tendon contractures. The failure to regenerate skeletal muscle is predicted to contribute to the skeletal muscle pathology of EDMD. We hypothesize that muscle regeneration defects are caused by impaired muscle stem cell differentiation. Myogenic progenitors derived from emerin-null mice were used to confirm their impaired differentiation and analyze selected myogenic molecular pathways. Emerin-null progenitors were delayed in their cell cycle exit, had decreased myosin heavy chain (MyHC) expression and formed fewer myotubes. Emerin binds to and activates histone deacetylase 3 (HDAC3). Here, we show that theophylline, an HDAC3-specific activator, improved myotube formation in emerin-null cells. Addition of the HDAC3-specific inhibitor RGFP966 blocked myotube formation and MyHC expression in wild-type and emerin-null myogenic progenitors, but did not affect cell cycle exit. Downregulation of emerin was previously shown to affect the p38 MAPK and ERK/MAPK pathways in C2C12 myoblast differentiation. Using a pure population of myogenic progenitors completely lacking emerin expression, we show that these pathways are also disrupted. ERK inhibition improved MyHC expression in emerin-null cells, but failed to rescue myotube formation or cell cycle exit. Inhibition of p38 MAPK prevented differentiation in both wild-type and emerin-null progenitors. These results show that each of these molecular pathways specifically regulates a particular stage of myogenic differentiation in an emerin-dependent manner. Thus, pharmacological targeting of multiple pathways acting at specific differentiation stages may be a better therapeutic approach in the future to rescue muscle regeneration in vivo.

Editors' choice: HDAC3, p38 MAPK and ERK signaling are altered during differentiation of myogenic progenitors lacking emerin; pharmacological activation or inhibition of these signaling proteins rescues specific stages of myogenic differentiation.

Tissue-specific NETs alter genome organization and regulation even in a heterologous system

Different cell types exhibit distinct patterns of 3D genome organization that correlate with changes in gene expression in tissue and differentiation systems. Several tissue-specific nuclear envelope transmembrane proteins (NETs) have been found to influence the spatial positioning of genes and chromosomes that normally occurs during tissue differentiation. Here we study 3 such NETs: NET29, NET39, and NET47, which are expressed preferentially in fat, muscle and liver, respectively. We found that even when exogenously expressed in a heterologous system they can specify particular genome organization patterns and alter gene expression. Each NET affected largely different subsets of genes. Notably, the liver-specific NET47 upregulated many genes in HT1080 fibroblast cells that are normally upregulated in hepatogenesis, showing that tissue-specific NETs can favor expression patterns associated with the tissue where the NET is normally expressed. Similarly, global profiling of peripheral chromatin after exogenous expression of these NETs using lamin B1 DamID revealed that each NET affected the nuclear positioning of distinct sets of genomic regions with a significant tissue-specific component. Thus NET influences on genome organization can contribute to gene expression changes associated with differentiation even in the absence of other factors and overt cellular differentiation changes.

Beyond Tethering and the LEM domain: MSCellaneous functions of the inner nuclear membrane Lem2

The nuclear envelope plays a pivotal role in the functional organization of chromatin. Various inner nuclear membrane (INM) proteins associate with transcriptionally repressed chromatin, which is often found at the nuclear periphery. A prominent example is the conserved family of LEM (LAP2-Emerin-MAN1) domain proteins that interact with DNA-binding proteins and have been proposed to mediate tethering of chromatin to the nuclear membrane. We recently reported that the fission yeast protein Lem2, a homolog of metazoan LEM proteins, contributes to perinuclear localization and silencing of heterochromatin. ¹ We demonstrate that binding and tethering of centromeric chromatin depends on the LEM domain of Lem2. Unexpectedly, this domain is dispensable for heterochromatin silencing, which is instead mediated by a different structural domain of Lem2, the MSC (MAN1-Src1 C-terminal) domain. Hence, silencing and tethering by Lem2 can be mechanistically separated. Notably, the MSC domain has multiple functions beyond heterochromatic silencing. Here we discuss the implications of these novel findings for the understanding of this conserved INM protein.

Diseases of the Nucleoskeleton

The nucleus is separated from the cytosol by the nuclear envelope, which is a double lipid bilayer composed of the outer nuclear membrane and the inner nuclear membrane. The intermediate filament proteins lamin A, lamin B, and lamin C form a network underlying the inner nuclear membrane. This proteinaceous network provides the nucleus with its strength, rigidity, and elasticity. Positioned within the inner nuclear membrane are more than 150 inner nuclear membrane proteins, many of which interact directly with lamins and require lamins for their inner nuclear membrane localization. Inner nuclear membrane proteins and the nuclear lamins define the nuclear lamina. These inner nuclear membrane proteins have tissue-specific expression and diverse functions including regulating cytoskeletal organization, nuclear architecture, cell cycle dynamics, and genomic organization. Loss or mutations in lamins and inner nuclear membrane proteins cause a wide spectrum of diseases. Here, I will review the functions of the well-studied nuclear lamina proteins and the diseases associated with loss or mutations in these proteins. © 2016 American Physiological Society. Compr Physiol 6:1655-1674, 2016.

Physiological and Pathological Aging Affects Chromatin Dynamics, Structure and Function at the Nuclear Edge

Lamins are intermediate filaments that form a complex meshwork at the inner nuclear membrane. Mammalian cells express two types of Lamins, Lamins A/C and Lamins B, encoded by three different genes, LMNA, LMNB1, and LMNB2. Mutations in the LMNA gene are associated with a group of phenotypically diverse diseases referred to as laminopathies. Lamins interact with a large number of binding partners including proteins of the nuclear envelope but also chromatin-associated factors. Lamins not only constitute a scaffold for nuclear shape, rigidity and resistance to stress but also contribute to the organization of chromatin and chromosomal domains. We will discuss here the impact of A-type Lamins loss on alterations of chromatin organization and formation of chromatin domains and how disorganization of the lamina contributes to the patho-physiology of premature aging syndromes.

Causes and consequences of nuclear envelope alterations in tumour progression

Morphological changes in the size and shape of the nucleus are highly prevalent in cancer, but the underlying molecular mechanisms and the functional relevance remain poorly understood. Nuclear envelope proteins, which can modulate nuclear shape and organization, have emerged as key components in a variety of signalling pathways long implicated in tumourigenesis and metastasis. The expression of nuclear envelope proteins is altered in many cancers, and changes in levels of nuclear envelope proteins lamins A and C are associated with poor prognosis in multiple human cancers. In this review we highlight the role of the nuclear envelope in different processes important for tumour initiation and cancer progression, with a focus on lamins A and C. Lamin A/C controls many cellular processes with key roles in cancer, including cell invasion, stemness, genomic stability, signal transduction, transcriptional regulation, and resistance to mechanical stress. In addition, we discuss potential mechanisms mediating the changes in lamin levels observed in many cancers. A better understanding of cause-and-effect relationships between lamin expression and tumour progression could reveal important mechanisms for coordinated regulation of oncogenic processes, and indicate therapeutic vulnerabilities that could be exploited for improved patient outcome.

Anchoring a Leviathan: How the Nuclear Membrane Tethers the Genome

It is well established that the nuclear envelope has many distinct direct connections to chromatin that contribute to genome organization. The functional consequences of genome organization on gene regulation are less clear. Even less understood is how interactions of lamins and nuclear envelope transmembrane proteins (NETs) with chromatin can produce anchoring tethers that can withstand the physical forces of and on the genome. Chromosomes are the largest molecules in the cell, making megadalton protein structures like the nuclear pore complexes and ribosomes seem small by comparison. Thus to withstand strong forces from chromosome dynamics an anchoring tether is likely to be much more complex than a single protein-protein or protein-DNA interaction. Here we will briefly review known NE-genome interactions that likely contribute to spatial genome organization, postulate in the context of experimental data how these anchoring tethers contribute to gene regulation, and posit several hypotheses for the physical nature of these tethers that need to be investigated experimentally. Significantly, disruption of these anchoring tethers and the subsequent consequences for gene regulation could explain how mutations in nuclear envelope proteins cause diseases ranging from muscular dystrophy to lipodystrophy to premature aging progeroid syndromes. The two favored hypotheses for nuclear envelope protein involvement in disease are (1) weakening nuclear and cellular mechanical stability, and (2) disrupting genome organization and gene regulation. Considerable experimental support has been obtained for both. The integration of both mechanical and gene expression defects in the disruption of anchoring tethers could provide a unifying hypothesis consistent with both.

Xenopus LAP2β protein knockdown affects location of lamin B and nucleoporins and has effect on assembly of cell nucleus and cell viability

Xenopus LAP2β protein is the single isoform expressed in XTC cells. The protein localizes on heterochromatin clusters both at the nuclear envelope and inside a cell nucleus. The majority of XLAP2β fraction neither colocalizes with TPX2 protein during interphase nor can be immunoprecipitated with XLAP2β antibody. Knockdown of the XLAP2β protein expression in XTC cells by synthetic siRNA and plasmid encoded siRNA resulted in nuclear abnormalities including changes in shape of nuclei, abnormal chromatin structure, loss of nuclear envelope, mislocalization of integral membrane proteins of INM such as lamin B2, mislocalization of nucleoporins, and cell death. Based on timing of cell death, we suggest mechanism associated with nucleus reassembly or with entry into mitosis. This confirms that Xenopus LAP2 protein is essential for the maintenance of cell nucleus integrity and the process of its reassembly after mitosis.

Epigenetic modulators, modifiers and mediators in cancer aetiology and progression

This year is the tenth anniversary of the publication in this journal of a model suggesting the existence of 'tumour progenitor genes'. These genes are epigenetically disrupted at the earliest stages of malignancies, even before mutations, and thus cause altered differentiation throughout tumour evolution. The past decade of discovery in cancer epigenetics has revealed a number of similarities between cancer genes and stem cell reprogramming genes, widespread mutations in epigenetic regulators, and the part played by chromatin structure in cellular plasticity in both development and cancer. In the light of these discoveries, we suggest here a framework for cancer epigenetics involving three types of genes: 'epigenetic mediators', corresponding to the tumour progenitor genes suggested earlier; 'epigenetic modifiers' of the mediators, which are frequently mutated in cancer; and 'epigenetic modulators' upstream of the modifiers, which are responsive to changes in the cellular environment and often linked to the nuclear architecture. We suggest that this classification is helpful in framing new diagnostic and therapeutic approaches to cancer.

Non-farnesylated B-type lamin can tether chromatin inside the nucleus and its chromatin interaction requires the Ig-fold region

Lamins are thought to direct heterochromatin to the nuclear lamina (NL); however, this function of lamin has not been clearly demonstrated in vivo. To address this, we analyzed polytene chromosome morphology when artificial lamin variants were expressed in Drosophila endoreplicating cells. We found that the CaaX-motif-deleted B-type lamin Dm₀, but not A-type lamin C, was able to form a nuclear envelope-independent layer that was closely associated with chromatin. Other nuclear envelope proteins were not detected in this "ectopic lamina," and the associated chromatin showed a repressive histone modification maker but not a permissive histone modification marker nor RNA polymerase II proteins. Furthermore, deletion of the C-terminal lamin-Ig-fold domain prevents chromatin association with this ectopic lamina. Thus, non-farnesylated B-type lamin Dm₀ can form an ectopic lamina and induce changes to chromatin structure and status inside the interphase nucleus.

Global reorganization of budding yeast chromosome conformation in different physiological conditions

The organization of the genome is nonrandom and important for correct function. Specifically, the nuclear envelope plays a critical role in gene regulation. It generally constitutes a repressive environment, but several genes, including the GAL locus in budding yeast, are recruited to the nuclear periphery on activation. Here, we combine imaging and computational modeling to ask how the association of a single gene locus with the nuclear envelope influences the surrounding chromosome architecture. Systematic analysis of an entire yeast chromosome establishes that peripheral recruitment of the GAL locus is part of a large-scale rearrangement that shifts many chromosomal regions closer to the nuclear envelope. This process is likely caused by the presence of several independent anchoring points. To identify novel factors required for peripheral anchoring, we performed a genome-wide screen and demonstrated that the histone acetyltransferase SAGA and the activity of histone deacetylases are needed for this extensive gene recruitment to the nuclear periphery.

A-type lamins bind both hetero- and euchromatin, the latter being regulated by lamina-associated polypeptide 2 alpha

Lamins are components of the peripheral nuclear lamina and interact with heterochromatic genomic regions, termed lamina-associated domains (LADs). In contrast to lamin B1 being primarily present at the nuclear periphery, lamin A/C also localizes throughout the nucleus, where it associates with the chromatin-binding protein lamina-associated polypeptide (LAP) 2 alpha. Here, we show that lamin A/C also interacts with euchromatin, as determined by chromatin immunoprecipitation of euchromatin- and heterochromatin-enriched samples. By way of contrast, lamin B1 was only found associated with heterochromatin. Euchromatic regions occupied by lamin A/C overlap with those bound by LAP2alpha, and lack of LAP2alpha in LAP2alpha-deficient cells shifts binding of lamin A/C toward more heterochromatic regions. These alterations in lamin A/C-chromatin interactions correlate with changes in epigenetic histone marks in euchromatin but do not significantly affect gene expression. Loss of lamin A/C in heterochromatic regions in LAP2alpha-deficient cells, however, correlated with increased gene expression. Our data show a novel role of nucleoplasmic lamin A/C and LAP2alpha in regulating euchromatin.

Histones and histone modifications in perinuclear chromatin anchoring: from yeast to man

It is striking that within a eukaryotic nucleus, the genome can assume specific spatiotemporal distributions that correlate with the cell's functional states. Cell identity itself is determined by distinct sets of genes that are expressed at a given time. On the level of the individual gene, there is a strong correlation between transcriptional activity and associated histone modifications. Histone modifications act by influencing the recruitment of non-histone proteins and by determining the level of chromatin compaction, transcription factor binding, and transcription elongation. Accumulating evidence also shows that the subnuclear position of a gene or domain correlates with its expression status. Thus, the question arises whether this spatial organization results from or determines a gene's chromatin status. Although the association of a promoter with the inner nuclear membrane (INM) is neither necessary nor sufficient for repression, the perinuclear sequestration of heterochromatin is nonetheless conserved from yeast to man. How does subnuclear localization influence gene expression? Recent work argues that the common denominator between genome organization and gene expression is the modification of histones and in some cases of histone variants. This provides an important link between local chromatin structure and long-range genome organization in interphase cells. In this review, we will evaluate how histones contribute to the latter, and discuss how this might help to regulate genes crucial for cell differentiation.

Control of heterochromatin localization and silencing by the nuclear membrane protein Lem2

Transcriptionally silent chromatin localizes to the nuclear periphery, which provides a special microenvironment for gene repression. A variety of nuclear membrane proteins interact with repressed chromatin, yet the functional role of these interactions remains poorly understood. Here, we show that, in Schizosaccharomyces pombe, the nuclear membrane protein Lem2 associates with chromatin and mediates silencing and heterochromatin localization. Unexpectedly, we found that these functions can be separated and assigned to different structural domains within Lem2, excluding a simple tethering mechanism. Chromatin association and tethering of centromeres to the periphery are mediated by the N-terminal LEM (LAP2-Emerin-MAN1) domain of Lem2, whereas telomere anchoring and heterochromatin silencing require exclusively its conserved C-terminal MSC (MAN1-Src1 C-terminal) domain. Particularly, silencing by Lem2 is epistatic with the Snf2/HDAC (histone deacetylase) repressor complex SHREC at telomeres, while its necessity can be bypassed by deleting Epe1, a JmjC protein with anti-silencing activity. Furthermore, we found that loss of Lem2 reduces heterochromatin association of SHREC, which is accompanied by increased binding of Epe1. This reveals a critical function of Lem2 in coordinating these antagonistic factors at heterochromatin. The distinct silencing and localization functions mediated by Lem2 suggest that these conserved LEM-containing proteins go beyond simple tethering to play active roles in perinuclear silencing.

The redundancy of the mammalian heterochromatic compartment

Two chromatin compartments are present in most mammalian cells; the first contains primarily euchromatic, early replicating chromatin and the second, primarily late-replicating heterochromatin, which is the subject of this review. Heterochromatin is concentrated in three intranuclear regions: the nuclear periphery, the perinucleolar space and in pericentromeric bodies. We review recent evidence demonstrating that the heterochromatic compartment is critically involved in global nuclear organization and the maintenance of genome stability, and discuss models regarding how this compartment is formed and maintained. We also evaluate our understanding of how heterochromatic sequences (herein named heterochromatic associated regions (HADs)) might be tethered within these regions and review experiments that reveal the stochastic nature of individual HAD positioning within the compartment. These investigations suggest a substantial level of functional redundancy within the heterochromatic compartment.

Nuclear membrane diversity: underlying tissue-specific pathologies in disease?

Human 'laminopathy' diseases result from mutations in genes encoding nuclear lamins or nuclear envelope (NE) transmembrane proteins (NETs). These diseases present a seeming paradox: the mutated proteins are widely expressed yet pathology is limited to specific tissues. New findings suggest tissue-specific pathologies arise because these widely expressed proteins act in various complexes that include tissue-specific components. Diverse mechanisms to achieve NE tissue-specificity include tissue-specific regulation of the expression, mRNA splicing, signaling, NE-localization and interactions of potentially hundreds of tissue-specific NETs. New findings suggest these NETs underlie tissue-specific NE roles in cytoskeletal mechanics, cell-cycle regulation, signaling, gene expression and genome organization. This view of the NE as 'specialized' in each cell type is important to understand the tissue-specific pathology of NE-linked diseases.

Olfactory receptor genes expressed in distinct lineages are sequestered in different nuclear compartments

The olfactory system translates a vast array of volatile chemicals into diverse odor perceptions and innate behaviors. Odor detection in the mouse nose is mediated by 1,000 different odorant receptors (ORs) and 14 trace amine-associated receptors (TAARs). ORs are used in a combinatorial manner to encode the unique identities of myriad odorants. However, some TAARs appear to be linked to innate responses, raising questions about regulatory mechanisms that might segregate OR and TAAR expression in appropriate subsets of olfactory sensory neurons (OSNs). Here, we report that OSNs that express TAARs comprise at least two subsets that are biased to express TAARs rather than ORs. The two subsets are further biased in Taar gene choice and their distribution within the sensory epithelium, with each subset preferentially expressing a subgroup of Taar genes within a particular spatial domain in the epithelium. Our studies reveal one mechanism that may regulate the segregation of Olfr (OR) and Taar expression in different OSNs: the sequestration of Olfr and Taar genes in different nuclear compartments. Although most Olfr genes colocalize near large central heterochromatin aggregates in the OSN nucleus, Taar genes are located primarily at the nuclear periphery, coincident with a thin rim of heterochromatin. Taar-expressing OSNs show a shift of one Taar allele away from the nuclear periphery. Furthermore, examination of hemizygous mice with a single Taar allele suggests that the activation of a Taar gene is accompanied by an escape from the peripheral repressive heterochromatin environment to a more permissive interior chromatin environment.

Networking in the nucleus: a spotlight on LEM-domain proteins

Proteins resident in the inner nuclear membrane and underlying nuclear lamina form a network that regulates nuclear functions. This review highlights a prominent family of nuclear lamina proteins that carries the LAP2-emerin-MAN1-domain (LEM-D). LEM-D proteins share an ability to bind lamins and tether repressive chromatin at the nuclear periphery. The importance of this family is underscored by findings that loss of individual LEM-D proteins causes progressive, tissue-restricted diseases, known as laminopathies. Diverse functions of LEM-D proteins are linked to interactions with unique and overlapping partners including signal transduction effectors, transcription factors and architectural proteins. Recent investigations suggest that LEM-D proteins form hubs within the nuclear lamina that integrate external signals important for tissue homeostasis and maintenance of progenitor cell populations.

Nuclear envelope protein Lem2 is required for mouse development and regulates MAP and AKT kinases

The nuclear lamina, along with associated nuclear membrane proteins, is a nexus for regulating signaling in the nucleus. Numerous human diseases arise from mutations in lamina proteins, and experimental models for these disorders have revealed aberrant regulation of various signaling pathways. Previously, we reported that the inner nuclear membrane protein Lem2, which is expressed at high levels in muscle, promotes the differentiation of cultured myoblasts by attenuating ERK signaling. Here, we have analyzed mice harboring a disrupted allele for the Lem2 gene (Lemd2). No gross phenotypic defects were seen in heterozygotes, although muscle regeneration induced by cardiotoxin was delayed. By contrast, homozygous Lemd2 knockout mice died by E11.5. Although many normal morphogenetic hallmarks were observed in E10.5 knockout embryos, most tissues were substantially reduced in size. This was accompanied by activation of multiple MAP kinases (ERK1/2, JNK, p38) and AKT. Knockdown of Lem2 expression in C2C12 myoblasts also led to activation of MAP kinases and AKT. These findings indicate that Lemd2 plays an essential role in mouse embryonic development and that it is involved in regulating several signaling pathways. Since increased MAP kinase and AKT/mTORC signaling is found in other animal models for diseases linked to nuclear lamina proteins, LEMD2 should be considered to be another candidate gene for human disease.

Directed targeting of chromatin to the nuclear lamina is mediated by chromatin state and A-type lamins

Nuclear organization has been implicated in regulating gene activity. Recently, large developmentally regulated regions of the genome dynamically associated with the nuclear lamina have been identified. However, little is known about how these lamina-associated domains (LADs) are directed to the nuclear lamina. We use our tagged chromosomal insertion site system to identify small sequences from borders of fibroblast-specific variable LADs that are sufficient to target these ectopic sites to the nuclear periphery. We identify YY1 (Ying-Yang1) binding sites as enriched in relocating sequences. Knockdown of YY1 or lamin A/C, but not lamin A, led to a loss of lamina association. In addition, targeted recruitment of YY1 proteins facilitated ectopic LAD formation dependent on histone H3 lysine 27 trimethylation and histone H3 lysine di- and trimethylation. Our results also reveal that endogenous loci appear to be dependent on lamin A/C, YY1, H3K27me3, and H3K9me2/3 for maintenance of lamina-proximal positioning.

Lamins: nuclear intermediate filament proteins with fundamental functions in nuclear mechanics and genome regulation

Lamins are intermediate filament proteins that form a scaffold, termed nuclear lamina, at the nuclear periphery. A small fraction of lamins also localize throughout the nucleoplasm. Lamins bind to a growing number of nuclear protein complexes and are implicated in both nuclear and cytoskeletal organization, mechanical stability, chromatin organization, gene regulation, genome stability, differentiation, and tissue-specific functions. The lamin-based complexes and their specific functions also provide insights into possible disease mechanisms for human laminopathies, ranging from muscular dystrophy to accelerated aging, as observed in Hutchinson-Gilford progeria and atypical Werner syndromes.

Phenotype-Dependent Coexpression Gene Clusters: Application to Normal and Premature Ageing

Hutchinson Gilford progeria syndrome (HGPS) is a rare genetic disease with symptoms of aging at a very early age. Its molecular basis is not entirely clear, although profound gene expression changes have been reported, and there are some known and other presumed overlaps with normal aging process. Identification of genes with agingor HGPS-associated expression changes is thus an important problem. However, standard regression approaches are currently unsuitable for this task due to limited sample sizes, thus motivating development of alternative approaches. Here, we report a novel iterative multiple regression approach that leverages co-expressed gene clusters to identify gene clusters whose expression co-varies with age and/or HGPS. We have applied our approach to novel RNA-seq profiles in fibroblast cell cultures at three different cellular ages, both from HGPS patients and normal samples. After establishing the robustness of our approach, we perform a comparative investigation of biological processes underlying normal aging and HGPS. Our results recapitulate previously known processes underlying aging as well as suggest numerous unique processes underlying aging and HGPS. The approach could also be useful in detecting phenotype-dependent co-expression gene clusters in other contexts with limited sample sizes.

NET23/STING promotes chromatin compaction from the nuclear envelope

Changes in the peripheral distribution and amount of condensed chromatin are observed in a number of diseases linked to mutations in the lamin A protein of the nuclear envelope. We postulated that lamin A interactions with nuclear envelope transmembrane proteins (NETs) that affect chromatin structure might be altered in these diseases and so screened thirty-one NETs for those that promote chromatin compaction as determined by an increase in the number of chromatin clusters of high pixel intensity. One of these, NET23 (also called STING, MITA, MPYS, ERIS, Tmem173), strongly promoted chromatin compaction. A correlation between chromatin compaction and endogenous levels of NET23/STING was observed for a number of human cell lines, suggesting that NET23/STING may contribute generally to chromatin condensation. NET23/STING has separately been found to be involved in innate immune response signaling. Upon infection cells make a choice to either apoptose or to alter chromatin architecture to support focused expression of interferon genes and other response factors. We postulate that the chromatin compaction induced by NET23/STING may contribute to this choice because the cells expressing NET23/STING eventually apoptose, but the chromatin compaction effect is separate from this as the condensation was still observed when cells were treated with Z-VAD to block apoptosis. NET23/STING-induced compacted chromatin revealed changes in epigenetic marks including changes in histone methylation and acetylation. This indicates a previously uncharacterized nuclear role for NET23/STING potentially in both innate immune signaling and general chromatin architecture.

NET gains and losses: the role of changing nuclear envelope proteomes in genome regulation

In recent years, our view of the nucleus has changed considerably with an increased awareness of the roles dynamic higher order chromatin structure and nuclear organization play in nuclear function. More recently, proteomics approaches have identified differential expression of nuclear lamina and nuclear envelope transmembrane (NET) proteins. Many NETs have been implicated in a range of developmental disorders as well as cell-type specific biological processes, including genome organization and nuclear morphology. While further studies are needed, it is clear that the differential nuclear envelope proteome contributes to cell-type specific nuclear identity and functions. This review discusses the importance of proteome diversity at the nuclear periphery and highlights the putative roles of NET proteins, with a focus on nuclear architecture.

Stochastic genome-nuclear lamina interactions: modulating roles of Lamin A and BAF

The nuclear lamina (NL) is thought to aid in the spatial organization of interphase chromosomes by providing an anchoring platform for hundreds of large genomic regions named lamina associated domains (LADs). Recently, a new live-cell imaging approach demonstrated directly that LAD-NL interactions are dynamic and in part stochastic. Here we discuss implications of these new findings and introduce Lamin A and BAF as potential modulators of stochastic LAD positioning.

Genome-wide analysis links emerin to neuromuscular junction activity in Caenorhabditis elegans

Background

Laminopathies are diseases characterized by defects in nuclear envelope structure. A well-known example is Emery-Dreifuss muscular dystrophy, which is caused by mutations in the human lamin A/C and emerin genes. While most nuclear envelope proteins are ubiquitously expressed, laminopathies often affect only a subset of tissues. The molecular mechanisms underlying these tissue-specific manifestations remain elusive. We hypothesize that different functional subclasses of genes might be differentially affected by defects in specific nuclear envelope components.

Results

Here we determine genome-wide DNA association profiles of two nuclear envelope components, lamin/LMN-1 and emerin/EMR-1 in adult Caenorhabditis elegans. Although both proteins bind to transcriptionally inactive regions of the genome, EMR-1 is enriched at genes involved in muscle and neuronal function. Deletion of either EMR-1 or LEM-2, another integral envelope protein, causes local changes in nuclear architecture as evidenced by altered association between DNA and LMN-1. Transcriptome analyses reveal that EMR-1 and LEM-2 are associated with gene repression, particularly of genes implicated in muscle and nervous system function. We demonstrate that emr-1, but not lem-2, mutants are sensitive to the cholinesterase inhibitor aldicarb, indicating altered activity at neuromuscular junctions.

Conclusions

We identify a class of elements that bind EMR-1 but do not associate with LMN-1, and these are enriched for muscle and neuronal genes. Our data support a redundant function of EMR-1 and LEM-2 in chromatin anchoring to the nuclear envelope and gene repression. We demonstrate a specific role of EMR-1 in neuromuscular junction activity that may contribute to Emery-Dreifuss muscular dystrophy in humans.

Induction of polyploidy by nuclear fusion mechanism upon decreased expression of the nuclear envelope protein LAP2β in the human osteosarcoma cell line U2OS

Background

Polyploidy has been recognized for many years as an important hallmark of cancer cells. Polyploid cells can arise through cell fusion, endoreplication and abortive cell cycle. The inner nuclear membrane protein LAP2β plays key roles in nuclear envelope breakdown and reassembly during mitosis, initiation of replication and transcriptional repression. Here we studied the function of LAP2β in the maintenance of cell ploidy state, a role which has not yet been assigned to this protein.

Results

By knocking down the expression of LAP2β, using both viral and non-viral RNAi approaches in osteosarcoma derived U2OS cells, we detected enlarged nuclear size, nearly doubling of DNA content and chromosomal duplications, as analyzed by fluorescent in situ hybridization and spectral karyotyping methodologies. Spectral karyotyping analyses revealed that near-hexaploid karyotypes of LAP2β knocked down cells consisted of not only seven duplicated chromosomal markers, as could be anticipated by genome duplication mechanism, but also of four single chromosomal markers. Furthermore, spectral karyotyping analysis revealed that both of two near-triploid U2OS sub-clones contained the seven markers that were duplicated in LAP2β knocked down cells, whereas the four single chromosomal markers were detected only in one of them. Gene expression profiling of LAP2β knocked down cells revealed that up to a third of the genes exhibiting significant changes in their expression are involved in cancer progression.

Conclusions

Our results suggest that nuclear fusion mechanism underlies the polyploidization induction upon LAP2β reduced expression. Our study implies on a novel role of LAP2β in the maintenance of cell ploidy status. LAP2β depleted U2OS cells can serve as a model to investigate polyploidy and aneuploidy formation by nuclear fusion mechanism and its involvement in cancerogenesis.

Significance of higher-order chromatin architecture for neuronal function and dysfunction

Recent studies in neurons indicate that the large-scale chromatin architectural framework, including chromosome territories or lamina-associated chromatin, undergoes dynamic changes that represent an emergent level of regulation of neuronal gene-expression. This phenomenon has been implicated in neuronal differentiation, long-term potentiation, seizures, and disorders of neural plasticity such as Rett syndrome and epilepsy.

Lamina-associated polypeptide (LAP)2α and other LEM proteins in cancer biology

The LEM proteins comprise a heterogeneous family of chromatin-associated proteins that share the LEM domain, a structural motif mediating interaction with the DNA associated protein, Barrier-to-Autointegration Factor (BAF). Most of the LEM proteins are integral proteins of the inner nuclear membrane and associate with the nuclear lamina, a structural scaffold of lamin intermediate filament proteins at the nuclear periphery, which is involved in nuclear mechanical functions and (hetero-)chromatin organization. A few LEM proteins, such as Lamina-associated polypeptide (LAP)2α and Ankyrin and LEM domain-containing protein (Ankle)1 lack transmembrane domains and localize throughout the nucleoplasm and cytoplasm, respectively. LAP2α has been reported to regulate cell proliferation by affecting the activity of retinoblastoma protein in tissue progenitor cells and numerous studies showed upregulation of LAP2α in cancer. Ankle1 is a nuclease likely involved in DNA damage repair pathways and single nucleotide polymorphisms in the Ankle1 gene have been linked to increased breast and ovarian cancer risk. In this review we describe potential mechanisms of the involvement of LEM proteins, particularly of LAP2α and Ankle1 in tumorigenesis and we provide evidence that LAP2α expression may be a valuable diagnostic and prognostic marker for tumor analyses.

Nuclear mechanics in cancer

Despite decades of research, cancer metastasis remains an incompletely understood process that is as complex as it is devastating. In recent years, there has been an increasing push to investigate the biomechanical aspects of tumorigenesis, complementing the research on genetic and biochemical changes. In contrast to the high genetic variability encountered in cancer cells, almost all metastatic cells are subject to the same physical constraints as they leave the primary tumor, invade surrounding tissues, transit through the circulatory system, and finally infiltrate new tissues. Advances in live cell imaging and other biophysical techniques, including measurements of subcellular mechanics, have yielded stunning new insights into the physics of cancer cells. While much of this research has been focused on the mechanics of the cytoskeleton and the cellular microenvironment, it is now emerging that the mechanical properties of the cell nucleus and its connection to the cytoskeleton may play a major role in cancer metastasis, as deformation of the large and stiff nucleus presents a substantial obstacle during the passage through the dense interstitial space and narrow capillaries. Here, we present an overview of the molecular components that govern the mechanical properties of the nucleus, and we discuss how changes in nuclear structure and composition observed in many cancers can modulate nuclear mechanics and promote metastatic processes. Improved insights into this interplay between nuclear mechanics and metastatic progression may have powerful implications in cancer diagnostics and therapy and may reveal novel therapeutic targets for pharmacological inhibition of cancer cell invasion.

Emerin in health and disease

Emery-Dreifuss muscular dystrophy (EDMD) is caused by mutations in the genes encoding emerin, lamins A and C and FHL1. Additional EDMD-like syndromes are caused by mutations in nesprins and LUMA. This review will specifically focus on emerin function and the current thinking for how loss or mutations in emerin cause EDMD. Emerin is a well-conserved, ubiquitously expressed protein of the inner nuclear membrane. Emerin has been shown to have diverse functions, including the regulation of gene expression, cell signaling, nuclear structure and chromatin architecture. This review will focus on the relationships between these functions and the EDMD disease phenotype. Additionally it will highlight open questions concerning emerin's roles in cell and nuclear biology and disease.

Tissue specificity in the nuclear envelope supports its functional complexity

Nuclear envelope links to inherited disease gave the conundrum of how mutations in near-ubiquitous proteins can yield many distinct pathologies, each focused in different tissues. One conundrum-resolving hypothesis is that tissue-specific partner proteins mediate these pathologies. Such partner proteins may have now been identified with recent proteome studies determining nuclear envelope composition in different tissues. These studies revealed that the majority of the total nuclear envelope proteins are tissue restricted in their expression. Moreover, functions have been found for a number these tissue-restricted nuclear envelope proteins that fit with mechanisms proposed to explain how the nuclear envelope could mediate disease, including defects in mechanical stability, cell cycle regulation, signaling, genome organization, gene expression, nucleocytoplasmic transport, and differentiation. The wide range of functions to which these proteins contribute is consistent with not only their involvement in tissue-specific nuclear envelope disease pathologies, but also tissue evolution.

Nuclear lamins: making contacts with promoters

The nuclear lamina guards the genome and in many ways contributes to regulating nuclear function. Increasing evidence indicates that the lamina dynamically interacts with chromatin mainly through large repressive domains, and recent data suggest that at least some of the lamin-genome contacts may be developmentally significant. In an attempt to provide an additional meaning to lamin-genome contacts, a recent study characterized the association of gene promoters with A-type lamins in progenitor and differentiated cells. Here, we discuss how A-type lamins interact with spatially defined promoter regions, and the relationship between these interactions, associated chromatin marks and gene expression outputs. We discuss the impact of A-type lamins on nucleus-wide and local chromatin organization. We also address how lamin-promoter interactions are redistributed during differentiation of adipocyte progenitors into adipocytes. Finally, we propose a model of lineage-specific "unlocking" of developmentally regulated loci and its significance in cellular differentiation.

Emerin and histone deacetylase 3 (HDAC3) cooperatively regulate expression and nuclear positions of MyoD, Myf5, and Pax7 genes during myogenesis

The spatial organization of chromatin is critical in establishing cell-type dependent gene expression programs. The inner nuclear membrane protein emerin has been implicated in regulating global chromatin architecture. We show emerin associates with genomic loci of muscle differentiation promoting factors in murine myogenic progenitors, including Myf5 and MyoD. Prior to their transcriptional activation Myf5 and MyoD loci localized to the nuclear lamina in proliferating progenitors and moved to the nucleoplasm upon transcriptional activation during differentiation. The Pax7 locus, which is transcribed in proliferating progenitors, localized to the nucleoplasm and Pax7 moved to the nuclear lamina upon repression during differentiation. Localization of Myf5, MyoD, and Pax7 to the nuclear lamina and proper temporal expression of these genes required emerin and HDAC3. Interestingly, activation of HDAC3 catalytic activity rescued both Myf5 localization to the nuclear lamina and its expression. Collectively, these data support a model whereby emerin facilitates repressive chromatin formation at the nuclear lamina by activating the catalytic activity of HDAC3 to regulate the coordinated spatiotemporal expression of myogenic differentiation genes.

O-Linked β-N-acetylglucosamine (O-GlcNAc) regulates emerin binding to barrier to autointegration factor (BAF) in a chromatin- and lamin B-enriched "niche"

Emerin, a membrane component of nuclear "lamina" networks with lamins and barrier to autointegration factor (BAF), is highly O-GlcNAc-modified ("O-GlcNAcylated") in mammalian cells. Mass spectrometry analysis revealed eight sites of O-GlcNAcylation, including Ser-53, Ser-54, Ser-87, Ser-171, and Ser-173. Emerin O-GlcNAcylation was reduced ~50% by S53A or S54A mutation in vitro and in vivo. O-GlcNAcylation was reduced ~66% by the triple S52A/S53A/S54A mutant, and S173A reduced O-GlcNAcylation of the S52A/S53A/S54A mutant by ~30%, in vivo. We separated two populations of emerin, A-type lamins and BAF; one population solubilized easily, and the other required sonication and included histones and B-type lamins. Emerin and BAF associated only in histone- and lamin-B-containing fractions. The S173D mutation specifically and selectively reduced GFP-emerin association with BAF by 58% and also increased GFP-emerin hyper-phosphorylation. We conclude that β-N-acetylglucosaminyltransferase, an essential enzyme, controls two regions in emerin. The first region, defined by residues Ser-53 and Ser-54, flanks the LEM domain. O-GlcNAc modification at Ser-173, in the second region, is proposed to promote emerin association with BAF in the chromatin/lamin B "niche." These results reveal direct control of a conserved LEM domain nuclear lamina component by β-N-acetylglucosaminyltransferase, a nutrient sensor that regulates cell stress responses, mitosis, and epigenetics.

Hutchinson-Gilford progeria syndrome through the lens of transcription

Lamins are nuclear intermediate filaments. In addition to their structural roles, they are implicated in basic nuclear functions such as chromatin organization, DNA replication, transcription, DNA repair, and cell-cycle progression. Mutations in human LMNA gene cause several diseases termed laminopathies. One of the laminopathic diseases is Hutchinson-Gilford progeria syndrome (HGPS), which is caused by a spontaneous mutation and characterized by premature aging. HGPS phenotypes share certain similarities with several apparently comparable medical conditions, such as aging and atherosclerosis, with the conspicuous absence of neuronal degeneration and cancer rarity during the short lifespan of the patients. Cell lines from HGPS patients are characterized by multiple nuclear defects, which include abnormal morphology, altered histone modification patterns, and increased DNA damage. These cell lines provide insight into the molecular pathways including senescence that require lamins A and B1. Here, we review recent data on HGPS phenotypes through the lens of transcriptional deregulation caused by lack of functional lamin A, progerin accumulation, and lamin B1 silencing.

Novel higher-order epigenetic regulation of the Bdnf gene upon seizures

Studies in cultured cells have demonstrated the existence of higher-order epigenetic mechanisms, determining the relationship between expression of the gene and its position within the cell nucleus. It is unknown, whether such mechanisms operate in postmitotic, highly differentiated cell types, such as neurons in vivo. Accordingly, we examined whether the intranuclear positions of Bdnf and Trkb genes, encoding the major neurotrophin and its receptor respectively, change as a result of neuronal activity, and what functional consequences such movements may have. In a rat model of massive neuronal activation upon kainate-induced seizures we found that elevated neuronal expression of Bdnf is associated with its detachment from the nuclear lamina, and translocation toward the nucleus center. In contrast, the position of stably expressed Trkb remains unchanged after seizures. Our study demonstrates that activation-dependent architectural remodeling of the neuronal cell nucleus in vivo contributes to activity-dependent changes in gene expression in the brain.

Partners and post-translational modifications of nuclear lamins

Nuclear intermediate filament networks formed by A- and B-type lamins are major components of the nucleoskeleton that are required for nuclear structure and function, with many links to human physiology. Mutations in lamins cause diverse human diseases ('laminopathies'). At least 54 partners interact with human A-type lamins directly or indirectly. The less studied human lamins B1 and B2 have 23 and seven reported partners, respectively. These interactions are likely to be regulated at least in part by lamin post-translational modifications. This review summarizes the binding partners and post-translational modifications of human lamins and discusses their known or potential implications for lamin function.

Building silent compartments at the nuclear periphery: a recurrent theme

In eukaryotes, the genetic material is stored in the nucleus, which is enclosed in a double lipid bilayer, the nuclear envelope (NE). It protects the genome from physical stress and separates it from the rest of the cell. On top of this physical function, growing evidence shows that the nuclear periphery contributes to the 3D organization of the genome. In turn, tridimensional organization of chromatin in the nuclear space influences genome expression. Here we review recent findings on the function of this physical barrier in gene repression and latest models on how silent subnuclear compartments at the NE are built in yeast as well as in the nematode C. elegans and mammalian cells; trying to draw parallels between the three systems.

Considering discrete protein pools when measuring the dynamics of nuclear membrane proteins

Measuring dynamics of nuclear proteins is complicated by the fact that many DNA- and chromatin-binding proteins have separate nucleoplasmic and nuclear membrane pools with distinct mobilities. Moreover, when measuring recoveries in FRAP experiments, it is important to be aware that the continuous transport of new protein through the nuclear pore complexes means that fluorescence recovery comes from both dynamic exchange of protein already within the nucleus and newly imported protein. Here we describe fluorescence recovery after photobleaching and photoactivation techniques designed to track nuclear membrane proteins and some methods we have developed that may help to distinguish these various pools. A combination of these approaches with standard FRAP approaches is necessary to understand the true dynamics of nuclear proteins.

Role of histone deacetylases in gene regulation at nuclear lamina

Theoretical models suggest that gene silencing at the nuclear periphery may involve “closing” of chromatin by transcriptional repressors, such as histone deacetylases (HDACs). Here we provide experimental evidence confirming these predictions. Histone acetylation, chromatin compactness, and gene repression in lamina-interacting multigenic chromatin domains were analyzed in Drosophila S2 cells in which B-type lamin, diverse HDACs, and lamina-associated proteins were downregulated by dsRNA. Lamin depletion resulted in decreased compactness of the repressed multigenic domain associated with its detachment from the lamina and enhanced histone acetylation. Our data reveal the major role for HDAC1 in mediating deacetylation, chromatin compaction, and gene silencing in the multigenic domain, and an auxiliary role for HDAC3 that is required for retention of the domain at the lamina. These findings demonstrate the manifold and central involvement of class I HDACs in regulation of lamina-associated genes, illuminating a mechanism by which these enzymes can orchestrate normal and pathological development.

Muscle development, regeneration and laminopathies: how lamins or lamina-associated proteins can contribute to muscle development, regeneration and disease

The aim of this review article is to evaluate the current knowledge on associations between muscle formation and regeneration and components of the nuclear lamina. Lamins and their partners have become particularly intriguing objects of scientific interest since it has been observed that mutations in genes coding for these proteins lead to a wide range of diseases called laminopathies. For over the last 10 years, various laboratories worldwide have tried to explain the pathogenesis of these rare disorders. Analyses of the distinct aspects of laminopathies resulted in formulation of different hypotheses regarding the mechanisms of the development of these diseases. In the light of recent discoveries, A-type lamins—the main building blocks of the nuclear lamina—together with other key elements, such as emerin, LAP2α and nesprins, seem to be of great importance in the modulation of various signaling pathways responsible for cellular differentiation and proliferation.

Developmental control of replication timing defines a new breed of chromosomal domains with a novel mechanism of chromatin unfolding

We recently identified a set of chromosome domains that are early replicating uniquely in pluripotent cells. Their switch from early to late replication occurs just prior to germ layer commitment, associated with a stable form of gene silencing that is difficult to reverse. Here, we discuss results demonstrating that these domains are among the least sensitive regions in the genome to global digestion by either MNase or restriction enzymes. This inaccessible chromatin state persists whether these regions are in their physically distended early replicating or compact late replicating configuration, despite dramatic changes in 3D chromatin folding and long-range chromatin interactions, and despite large changes in transcriptional activity. This contrasts with the strong correlation between early replication, accessibility, transcriptional activity and open chromatin configuration that is observed genome-wide. We put these results in context with findings from other studies indicating that many structural (DNA sequence) and functional (density and activity of replication origins) properties of developmentally regulated replication timing ("switching") domains resemble properties of constitutively late replicating domains. This suggests that switching domains are a type of late replicating domain within which both replication timing and transcription are subject to unique or additional layers of control not experienced by the bulk of the genome. We predict that understanding the unusual structure of these domains will reveal a novel principle of chromosome folding.

Perinuclear positioning of the inactive human cystic fibrosis gene depends on CTCF, A-type lamins and an active histone deacetylase

The nuclear positioning of mammalian genes often correlates with their functional state. For instance, the human cystic fibrosis transmembrane conductance regulator (CFTR) gene associates with the nuclear periphery in its inactive state, but occupies interior positions when active. It is not understood how nuclear gene positioning is determined. Here, we investigated trichostatin A (TSA)-induced repositioning of CFTR in order to address molecular mechanisms controlling gene positioning. Treatment with the histone deacetylase (HDAC) inhibitor TSA induced increased histone acetylation and CFTR repositioning towards the interior within 20  min. When CFTR localized in the nuclear interior (either after TSA treatment or when the gene was active) consistent histone H3 hyperacetylation was observed at a CTCF site close to the CFTR promoter. Knockdown experiments revealed that CTCF was essential for perinuclear CFTR positioning and both, CTCF knockdown as well as TSA treatment had similar and CFTR-specific effects on radial positioning. Furthermore, knockdown experiments revealed that also A-type lamins were required for the perinuclear positioning of CFTR. Together, the results showed that CTCF, A-type lamins and an active HDAC were essential for perinuclear positioning of CFTR and these components acted on a CTCF site adjacent to the CFTR promoter. The results are consistent with the idea that CTCF bound close to the CFTR promoter, A-type lamins and an active HDAC form a complex at the nuclear periphery, which becomes disrupted upon inhibition of the HDAC, leading to the observed release of CFTR.

The nuclear envelope proteome differs notably between tissues

One hypothesis to explain how mutations in the same nuclear envelope proteins yield pathologies focused in distinct tissues is that as yet unidentified tissue-specific partners mediate the disease pathologies. The nuclear envelope proteome was recently determined from leukocytes and muscle. Here the same methodology is applied to liver and a direct comparison of the liver, muscle and leukocyte data sets is presented. At least 74 novel transmembrane proteins identified in these studies have been directly confirmed at the nuclear envelope. Within this set, RT-PCR, western blot and staining of tissue cryosections confirms that the protein complement of the nuclear envelope is clearly distinct from one tissue to another. Bioinformatics reveals similar divergence between tissues across the larger data sets. For proteins acting in complexes according to interactome data, the whole complex often exhibited the same tissue-specificity. Other tissue-specific nuclear envelope proteins identified were known proteins with functions in signaling and gene regulation. The high tissue specificity in the nuclear envelope likely underlies the complex disease pathologies and argues that all organelle proteomes warrant re-examination in multiple tissues.

DNA sequence-dependent compartmentalization and silencing of chromatin at the nuclear lamina

A large fraction of the mammalian genome is organized into inactive chromosomal domains along the nuclear lamina. The mechanism by which these lamina associated domains (LADs) are established remains to be elucidated. Using genomic repositioning assays, we show that LADs, spanning the developmentally regulated IgH and Cyp3a loci contain discrete DNA regions that associate chromatin with the nuclear lamina and repress gene activity in fibroblasts. Lamina interaction is established during mitosis and likely involves the localized recruitment of Lamin B during late anaphase. Fine-scale mapping of LADs reveals numerous lamina-associating sequences (LASs), which are enriched for a GAGA motif. This repeated motif directs lamina association and is bound by the transcriptional repressor cKrox, in a complex with HDAC3 and Lap2β. Knockdown of cKrox or HDAC3 results in dissociation of LASs/LADs from the nuclear lamina. These results reveal a mechanism that couples nuclear compartmentalization of chromatin domains with the control of gene activity.

Multiple novel functions of lamina associated polypeptide 2α in striated muscle

Lamina-associated polypeptide 2α (LAP2α) is a nucleoplasmic protein that interacts with A-type lamins and the retinoblastoma protein (pRb) and affects pRb-mediated cell cycle regulation and chromatin organization. Mutations in lamin A/C and LAP2α cause late onset striated muscle diseases, but the molecular mechanisms are poorly understood. We have recently reported on the striated muscle phenotype of LAP2α-deficient mice, revealing new unexpected roles of LAP2α. Loss of LAP2α in skeletal muscle caused an upregulated stem cell-type gene expression in muscle satellite cell progeny and their delayed myogenic differentiation in vitro. In vivo, the myofiber-associated muscle stem cell pool was increased. In addition, absence of LAP2α promoted muscle remodeling towards fast myofiber types in the soleus muscle of old animals. In cardiac tissue, deletion of LAP2α caused systolic dysfunction in young mice with an increased susceptibility for fibrosis in old animals. The functional impairment in the heart was accompanied by a deregulation of major cardiac transcription factors, GATA4 and MEF2c and activation of compensatory pathways, including the downregulation of β-adrenergic receptor signaling.Here we discuss potential functions of LAP2α in striated muscle at molecular level and how loss of these functions may cause the diverse muscle phenotypes. We propose that LAP2α serves as a transcriptional co-regulator, which controls muscle specific gene expression during muscle regeneration, muscle remodeling and stress response.