The carboxyl-terminal nucleoplasmic region of MAN1 exhibits a DNA binding winged helix domain

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

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

Lamins and chromatin join forces

The intricate interplay between lamins and chromatin underpins the structural integrity and functional organization of the eukaryotic nucleus. Lamins, type V intermediate filament proteins, form a robust meshwork beneath the inner nuclear membrane that is crucial for sustaining nuclear architecture through interactions with lamin-associated domains (LADs). LADs are predominantly heterochromatic regions in which compacted chromatin is enriched at the nuclear periphery, interacting with lamins and lamin-associated proteins. Disruptions of these interactions are implicated in a spectrum of diseases, including laminopathies, cancer, and age-related pathologies, highlighting the importance of lamin-LAD interactions. Thus, a detailed understanding of lamin-chromatin interactions may provide new insights into chromatin organization and shed light on the mechanism behind certain disease states. Here, we discuss the current state of knowledge of lamin-chromatin interactions from a biochemical and structural point of view.

Unveiling the roles of LEMD proteins in cellular processes

Proteins localized in the inner nuclear membrane (INM) engage in various fundamental cellular processes via their interactions with outer nuclear membrane (ONM) proteins and nuclear lamina. LAP2-emerin-MAN1 domain (LEMD) family proteins, predominantly positioned in the INM, participate in the maintenance of INM functions, including the reconstruction of the nuclear envelope during mitosis, mechanotransduction, and gene transcriptional modulation. Malfunction of LEMD proteins leads to severe tissue-restricted diseases, which may manifest as fatal deformities and defects. In this review, we summarize the significant roles of LEMD proteins in cellular processes, explains the mechanisms of LEMD protein-related diseases, and puts forward questions in less-explored areas like details in tissue-restricted phenotypes. It intends to sort out previous works about LEMD proteins and pave way for future researchers who might discover deeper mechanisms of and better treatment strategies for LEMD protein-related diseases.

Preserving Genome Integrity: Unveiling the Roles of ESCRT Machinery

The endosomal sorting complex required for transport (ESCRT) machinery is composed of an articulated architecture of proteins that assemble at multiple cellular sites. The ESCRT machinery is involved in pathways that are pivotal for the physiology of the cell, including vesicle transport, cell division, and membrane repair. The subunits of the ESCRT I complex are mainly responsible for anchoring the machinery to the action site. The ESCRT II subunits function to bridge and recruit the ESCRT III subunits. The latter are responsible for finalizing operations that, independently of the action site, involve the repair and fusion of membrane edges. In this review, we report on the data related to the activity of the ESCRT machinery at two sites: the nuclear membrane and the midbody and the bridge linking cells in the final stages of cytokinesis. In these contexts, the machinery plays a significant role for the protection of genome integrity by contributing to the control of the abscission checkpoint and to nuclear envelope reorganization and correlated resilience. Consistently, several studies show how the dysfunction of the ESCRT machinery causes genome damage and is a codriver of pathologies, such as laminopathies and cancer.

Cleavage of the pseudoprotease iRhom2 by the signal peptidase complex reveals an ER-to-nucleus signaling pathway

iRhoms are pseudoprotease members of the rhomboid-like superfamily and are cardinal regulators of inflammatory and growth factor signaling; they function primarily by recognizing transmembrane domains of their clients. Here, we report a mechanistically distinct nuclear function of iRhoms, showing that both human and mouse iRhom2 are non-canonical substrates of signal peptidase complex (SPC), the protease that removes signal peptides from secreted proteins. Cleavage of iRhom2 generates an N-terminal fragment that enters the nucleus and modifies the transcriptome, in part by binding C-terminal binding proteins (CtBPs). The biological significance of nuclear iRhom2 is indicated by elevated levels in skin biopsies of patients with psoriasis, tylosis with oesophageal cancer (TOC), and non-epidermolytic palmoplantar keratoderma (NEPPK); increased iRhom2 cleavage in a keratinocyte model of psoriasis; and nuclear iRhom2 promoting proliferation of keratinocytes. Overall, this work identifies an unexpected SPC-dependent ER-to-nucleus signaling pathway and demonstrates that iRhoms can mediate nuclear signaling.

Plant nuclear envelope as a hub connecting genome organization with regulation of gene expression

Eukaryotic cells organize their genome within the nucleus with a double-layered membrane structure termed the nuclear envelope (NE) as the physical barrier. The NE not only shields the nuclear genome but also spatially separates transcription from translation. Proteins of the NE including nucleoskeleton proteins, inner nuclear membrane proteins, and nuclear pore complexes have been implicated in interacting with underlying genome and chromatin regulators to establish a higher-order chromatin architecture. Here, I summarize recent advances in the knowledge of NE proteins that are involved in chromatin organization, gene regulation, and coordination of transcription and mRNA export. These studies support an emerging view of plant NE as a central hub that contributes to chromatin organization and gene expression in response to various cellular and environmental cues.

The inner nuclear membrane protein Lem2 coordinates RNA degradation at the nuclear periphery

Transcriptionally silent chromatin often localizes to the nuclear periphery. However, whether the nuclear envelope (NE) is a site for post-transcriptional gene repression is not well understood. Here we demonstrate that Schizosaccharomycespombe Lem2, an NE protein, regulates nuclear-exosome-mediated RNA degradation. Lem2 deletion causes accumulation of RNA precursors and meiotic transcripts and de-localization of an engineered exosome substrate from the nuclear periphery. Lem2 does not directly bind RNA but instead interacts with the exosome-targeting MTREC complex and its human homolog PAXT to promote RNA recruitment. This pathway acts largely independently of nuclear bodies where exosome factors assemble. Nutrient availability modulates Lem2 regulation of meiotic transcripts, implying that this pathway is environmentally responsive. Our work reveals that multiple spatially distinct degradation pathways exist. Among these, Lem2 coordinates RNA surveillance of meiotic transcripts and non-coding RNAs by recruiting exosome co-factors to the nuclear periphery.

The Braun lab shows that the conserved nuclear membrane protein Lem2 interacts with the MTREC complex of the nuclear-exosome pathway to promote recruitment and degradation of ncRNAs and meiotic transcripts at the nuclear periphery in Schizosaccharomycespombe.

PNET2 is a component of the plant nuclear lamina and is required for proper genome organization and activity

The interaction between chromatin and the nuclear lamina (NL) is intrinsically important to the establishment of three-dimensional chromatin architecture and spatiotemporal regulation of gene expression. However, critical regulators involved in this process are poorly understood in plants. Here, we report that Arabidopsis PNET2 and its two homologs are bona fide inner nuclear membrane proteins and integral components of the NL. PNET2s physically interact with the plant nucleoskeleton and engage nucleosome-enriched chromatin at the nuclear periphery. Loss of all three PNET2s leads to severely disrupted growth and development, concomitant activation of abiotic and biotic stress responses, and ultimate lethality in Arabidopsis. The pent2 triple mutant also displays drastic transcriptome changes accompanied by a globally altered chromatin architecture revealed by HiC analysis. Our study identified PNET2 as an inner nuclear membrane (INM) component of the NL, which associates with chromatin and play a critical role in orchestrating gene expression and chromatin organization in plants.

The Diverse Cellular Functions of Inner Nuclear Membrane Proteins

The nuclear compartment is delimited by a specialized expanded sheet of the endoplasmic reticulum (ER) known as the nuclear envelope (NE). Compared to the outer nuclear membrane and the contiguous peripheral ER, the inner nuclear membrane (INM) houses a unique set of transmembrane proteins that serve a staggering range of functions. Many of these functions reflect the exceptional position of INM proteins at the membrane-chromatin interface. Recent research revealed that numerous INM proteins perform crucial roles in chromatin organization, regulation of gene expression, genome stability, and mediation of signaling pathways into the nucleus. Other INM proteins establish mechanical links between chromatin and the cytoskeleton, help NE remodeling, or contribute to the surveillance of NE integrity and homeostasis. As INM proteins continue to gain prominence, we review these advancements and give an overview on the functional versatility of the INM proteome.

Heh2/Man1 may be an evolutionarily conserved sensor of NPC assembly state

Integral membrane proteins of the Lap2-emerin-MAN1 (LEM) family have emerged as important components of the inner nuclear membrane (INM) required for the functional and physical integrity of the nuclear envelope. However, like many INM proteins, there is limited understanding of the biochemical interaction networks that enable LEM protein function. Here, we show that Heh2/Man1 can interact with major scaffold components of the nuclear pore complex (NPC), specifically the inner ring complex (IRC), in evolutionarily distant yeasts. Although an N-terminal domain is required for Heh2 targeting to the INM, we demonstrate that more stable interactions with the NPC are mediated by a C-terminal winged helix (WH) domain, thus decoupling INM targeting and NPC binding. Inhibiting Heh2's interactions with the NPC by deletion of the Heh2 WH domain leads to NPC clustering. Interestingly, Heh2's association with NPCs can also be disrupted by knocking out several outer ring nucleoporins. Thus, Heh2's interaction with NPCs depends on the structural integrity of both major NPC scaffold complexes. We propose a model in which Heh2 acts as a sensor of NPC assembly state, which may be important for NPC quality control mechanisms and the segregation of NPCs during cell division.

ESCRT recruitment by the S. cerevisiae inner nuclear membrane protein Heh1 is regulated by Hub1-mediated alternative splicing

Misassembled nuclear pore complexes (NPCs) are removed by sealing off the surrounding nuclear envelope (NE), which is conducted by the endosomal sorting complexes required for transport (ESCRT) machinery. Recruitment of ESCRT proteins to the NE is mediated by the interaction between the ESCRT member Chm7 and the inner nuclear membrane protein Heh1, which belongs to the conserved LEM family. Increased ESCRT recruitment results in excessive membrane scission at damage sites but its regulation remains poorly understood. Here, we show that Hub1-mediated alternative splicing of HEH1 pre-mRNA, resulting in production of its shorter form Heh1-S, is critical for the integrity of the NE in Saccharomyces cerevisiae ESCRT-III mutants lacking Hub1 or Heh1-S display severe growth defects and accumulate improperly assembled NPCs. This depends on the interaction of Chm7 with the conserved MSC domain, which is only present in the longer variant Heh1-L. Heh1 variants assemble into heterodimers, and we demonstrate that a unique splice segment in Heh1-S suppresses growth defects associated with the uncontrolled interaction between Heh1-L and Chm7. Together, our findings reveal that Hub1-mediated splicing generates Heh1-S to regulate ESCRT recruitment to the NE.This article has an associated First Person interview with the first author of the paper.

Protein structural and mechanistic basis of progeroid laminopathies

Progeroid laminopathies are characterized by the premature appearance of certain signs of physiological aging in a subset of tissues. They are caused by mutations in genes coding for A-type lamins or lamin-binding proteins. Here, we review how different mutations causing progeroid laminopathies alter protein structure or protein-protein interactions and how these impact on mechanisms that protect cell viability and function. One group of progeroid laminopathies, which includes Hutchinson-Gilford progeria syndrome, is characterized by accumulation of unprocessed prelamin A or variants. These are caused by mutations in the A-type lamin gene (LMNA), altering prelamin A itself, or in ZMPSTE24, encoding an endoprotease involved in its processing. The abnormally expressed farnesylated proteins impact on various cellular processes that may contribute to progeroid phenotypes. Other LMNA mutations lead to the production of nonfarnesylated A-type lamin variants with amino acid substitutions in solvent-exposed hot spots located mainly in coil 1B and the immunoglobulin fold domain. Dominant missense mutations might reinforce interactions between lamin domains, thus giving rise to excessively stabilized filament networks. Recessive missense mutations in A-type lamins and barrier-to-autointegration factor (BAF) causing progeroid disorders are found at the interface between these interacting proteins. The amino acid changes decrease the binding affinity of A-type lamins for BAF, which may contribute to lamina disorganization, as well as defective repair of mechanically induced nuclear envelope rupture. Targeting these molecular alterations in A-type lamins and associated proteins identified through structural biology studies could facilitate the design of therapeutic strategies to treat patients with rare but severe progeroid laminopathies.

CHMPions of repair: Emerging perspectives on sensing and repairing the nuclear envelope barrier

Understanding how the integrity of the nuclear membranes is protected against internal and external stresses is an emergent challenge. Work reviewed here investigated the mechanisms by which losses of nuclear-cytoplasmic compartmentalization are sensed and ameliorated. Fundamental to these is spatial control over interactions between the endosomal sorting complexes required for transport machinery and LAP2-emerin-MAN1 family inner nuclear membrane proteins, which together promote nuclear envelope sealing in interphase and at the end of mitosis. We suggest that the size of the nuclear envelope hole dictates the mechanism of its repair, with larger holes requiring barrier-to-autointegration factor and the potential triggering of a postmitotic nuclear envelope reassembly pathway in interphase. We also consider why these mechanisms fail at ruptured micronuclei. Together, this work re-emphasizes the need to understand how membrane flow and local lipid metabolism help ensure that the nuclear envelope is refractory to mechanical rupture yet fluid enough to allow its essential dynamics.

Structural basis for receptor-regulated SMAD recognition by MAN1

Receptor-regulated SMAD (R-SMAD: SMAD1, SMAD2, SMAD3, SMAD5 and SMAD8) proteins are key transcription factors of the transforming growth factor-β (TGF-β) superfamily of cytokines. MAN1, an integral protein of the inner nuclear membrane, is a SMAD cofactor that terminates TGF-β superfamily signals. Heterozygous loss-of-function mutations in MAN1 result in osteopoikilosis, Buschke-Ollendorff syndrome and melorheostosis. MAN1 interacts with MAD homology 2 (MH2) domains of R-SMAD proteins using its C-terminal U2AF homology motif (UHM) domain and UHM ligand motif (ULM) and facilitates R-SMAD dephosphorylation. Here, we report the structural basis for R-SMAD recognition by MAN1. The SMAD2–MAN1 and SMAD1–MAN1 complex structures show that an intramolecular UHM–ULM interaction of MAN1 forms a hydrophobic surface that interacts with a hydrophobic surface among the H2 helix, the strands β8 and β9, and the L3 loop of the MH2 domains of R-SMAD proteins. The complex structures also show the mechanism by which SMAD cofactors distinguish R-SMAD proteins that possess a highly conserved molecular surface.

Mechanical regulation of gene expression in cardiac myocytes and fibroblasts

The intact heart undergoes complex and multiscale remodelling processes in response to altered mechanical cues. Remodelling of the myocardium is regulated by a combination of myocyte and non-myocyte responses to mechanosensitive pathways, which can alter gene expression and therefore function in these cells. Cellular mechanotransduction and its downstream effects on gene expression are initially compensatory mechanisms during adaptations to the altered mechanical environment, but under prolonged and abnormal loading conditions, they can become maladaptive, leading to impaired function and cardiac pathologies. In this Review, we summarize mechanoregulated pathways in cardiac myocytes and fibroblasts that lead to altered gene expression and cell remodelling under physiological and pathophysiological conditions. Developments in systems modelling of the networks that regulate gene expression in response to mechanical stimuli should improve integrative understanding of their roles in vivo and help to discover new combinations of drugs and device therapies targeting mechanosignalling in heart disease.

An ESCRT-LEM protein surveillance system is poised to directly monitor the nuclear envelope and nuclear transport system

The integrity of the nuclear membranes coupled to the selective barrier of nuclear pore complexes (NPCs) are essential for the segregation of nucleoplasm and cytoplasm. Mechanical membrane disruption or perturbation to NPC assembly triggers an ESCRT-dependent surveillance system that seals nuclear pores: how these pores are sensed and sealed is ill defined. Using a budding yeast model, we show that the ESCRT Chm7 and the integral inner nuclear membrane (INM) protein Heh1 are spatially segregated by nuclear transport, with Chm7 being actively exported by Xpo1/Crm1. Thus, the exposure of the INM triggers surveillance with Heh1 locally activating Chm7. Sites of Chm7 hyperactivation show fenestrated sheets at the INM and potential membrane delivery at sites of nuclear envelope herniation. Our data suggest that perturbation to the nuclear envelope barrier would lead to local nuclear membrane remodeling to promote membrane sealing. Our findings have implications for disease mechanisms linked to NPC assembly and nuclear envelope integrity.

LEM domain-containing protein 3 antagonizes TGFβ-SMAD2/3 signaling in a stiffness-dependent manner in both the nucleus and cytosol

Transforming growth factor-β (TGFβ) signaling through SMAD2/3 is an important driver of pathological fibrosis in multiple organ systems. TGFβ signaling and extracellular matrix (ECM) stiffness form an unvirtuous pathological circuit in which matrix stiffness drives activation of latent TGFβ, and TGFβ signaling then drives cellular stress and ECM synthesis. Moreover, ECM stiffness also appears to sensitize cells to exogenously activated TGFβ through unknown mechanisms. Here, using human fibroblasts, we explored the effect of ECM stiffness on a putative inner nuclear membrane protein, LEM domain-containing protein 3 (LEMD3), which is physically connected to the cell's actin cytoskeleton and inhibits TGFβ signaling. We showed that LEMD3-SMAD2/3 interactions are inversely correlated with ECM stiffness and TGFβ-driven luciferase activity and that LEMD3 expression is correlated with the mechanical response of the TGFβ-driven luciferase reporter. We found that actin polymerization but not cellular stress or LEMD3-nuclear-cytoplasmic couplings were necessary for LEMD3-SMAD2/3 interactions. Intriguingly, LEMD3 and SMAD2/3 frequently interacted in the cytosol, and we discovered LEMD3 was proteolytically cleaved into protein fragments. We confirmed that a consensus C-terminal LEMD3 fragment binds SMAD2/3 in a stiffness-dependent manner throughout the cell and is sufficient for antagonizing SMAD2/3 signaling. Using human lung biopsies, we observed that these nuclear and cytosolic interactions are also present in tissue and found that fibrotic tissues exhibit locally diminished and cytoplasmically shifted LEMD3-SMAD2/3 interactions, as noted in vitro Our work reveals novel LEMD3 biology and stiffness-dependent regulation of TGFβ by LEMD3, providing a novel target to antagonize pathological TGFβ signaling.

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.

Combined approach for finding susceptibility genes in DISH/chondrocalcinosis families: whole-genome-wide linkage and IBS/IBD studies

Twelve families with exuberant and early-onset calcium pyrophosphate dehydrate chondrocalcinosis (CC) and diffuse idiopathic skeletal hyperostosis (DISH), hereafter designated DISH/CC, were identified in Terceira Island, the Azores, Portugal. Ninety-two (92) individuals from these families were selected for whole-genome-wide linkage analysis. An identity-by-descent (IBD) analysis was performed in 10 individuals from 5 of the investigated pedigrees. The chromosome area with the maximal logarithm of the odds score (1.32; P=0.007) was not identified using the IBD/identity-by-state (IBS) analysis; therefore, it was not investigated further. From the IBD/IBS analysis, two candidate genes, LEMD3 and RSPO4, were identified and sequenced. Nine genetic variants were identified in the RSPO4 gene; one regulatory variant (rs146447064) was significantly more frequent in control individuals than in DISH/CC patients (P=0.03). Four variants were identified in LEMD3, and the rs201930700 variant was further investigated using segregation analysis. None of the genetic variants in RSPO4 or LEMD3 segregated within the studied families. Therefore, although a major genetic effect was shown to determine DISH/CC occurrence within these families, the specific genetic variants involved were not identified.

The Role of the Nuclear Envelope Protein MAN1 in Mesenchymal Stem Cell Differentiation

Mutations in MAN1, a protein of the nuclear envelope, cause bone phenotypes characterized by hyperostosis. The mechanism of this pro-osteogenic phenotype remains unknown. We increased and decreased MAN1 expression in mesenchymal stem cells (MSC) upon which standard osteogenic and adipogenic differentiation were performed. MAN1 knockdown increased osteogenesis and mineralization. In contrast, osteogenesis remained stable upon MAN1 overexpression. Regarding a mechanism, we found that low levels of MAN1 facilitated the nuclear accumulation of regulatory smads and smads-related complexes, with a concurrently high expression of nuclear β-Catenin. In addition, we found adipogenesis to be decreased in both conditions, although predominantly affected by MAN1 overexpression. Finally, lamin A, a protein of the nuclear envelope that regulates MSC differentiation, was unaffected by changes in MAN1. In conclusion, our studies demonstrated that lower levels of MAN1 in differentiating MSC are associated with higher osteogenesis and lower adipogenesis. High levels of MAN1 only affected adipogenesis. These effects could have an important role in the understanding of the role of the proteins of the nuclear envelope in bone formation. J. Cell. Biochem. 118: 4425-4435, 2017. © 2017 Wiley Periodicals, Inc.

The Soft- and Hard-Heartedness of Cardiac Fibroblasts: Mechanotransduction Signaling Pathways in Fibrosis of the Heart

Cardiac fibrosis, the excessive accumulation of extracellular matrix (ECM), remains an unresolved problem in most forms of heart disease. In order to be successful in preventing, attenuating or reversing cardiac fibrosis, it is essential to understand the processes leading to ECM production and accumulation. Cardiac fibroblasts are the main producers of cardiac ECM, and harbor great phenotypic plasticity. They are activated by the disease-associated changes in mechanical properties of the heart, including stretch and increased tissue stiffness. Despite much remaining unknown, an interesting body of evidence exists on how mechanical forces are translated into transcriptional responses important for determination of fibroblast phenotype and production of ECM constituents. Such mechanotransduction can occur at multiple cellular locations including the plasma membrane, cytoskeleton and nucleus. Moreover, the ECM functions as a reservoir of pro-fibrotic signaling molecules that can be released upon mechanical stress. We here review the current status of knowledge of mechanotransduction signaling pathways in cardiac fibroblasts that culminate in pro-fibrotic gene expression.

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.

Nucleo-cytoplasmic shuttling of the endonuclease ankyrin repeats and LEM domain-containing protein 1 (Ankle1) is mediated by canonical nuclear export- and nuclear import signals

BACKGROUND

Ankyrin repeats and LEM domain containing protein 1 (Ankle1) belongs to the LEM protein family, whose members share a chromatin-interacting LEM motif. Unlike most other LEM proteins, Ankle1 is not an integral protein of the inner nuclear membrane but shuttles between the nucleus and the cytoplasm. It contains a GIY-YIG-type nuclease domain, but its function is unknown. The mammalian genome encodes only one other GIY-YIG domain protein, termed Slx1. Slx1 has been described as a resolvase that processes Holliday junctions during homologous recombination-mediated DNA double strand break repair. Resolvase activity is regulated in a spatial and temporal manner during the cell cycle. We hypothesized that Ankle1 may have a similar function and its nucleo-cytoplasmic shuttling may contribute to the regulation of Ankle1 activity. Hence, we aimed at identifying the domains mediating Ankle1 shuttling and investigating whether cellular localization is affected during DNA damage response.

RESULTS

Sequence analysis predicts the presence of two canonical nuclear import and export signals in Ankle1. Immunofluorescence microscopy of cells expressing wild-type and various mutated Ankle1-fusion proteins revealed a C-terminally located classical monopartite nuclear localization signal and a centrally located CRM1-dependent nuclear export signal that mediate nucleo-cytoplasmic shuttling of Ankle1. These sequences are also functional in heterologous proteins. The predominant localization of Ankle1 in the cytoplasm, however, does not change upon induction of several DNA damage response pathways throughout the cell cycle.

CONCLUSIONS

We identified the domains mediating nuclear import and export of Ankle1. Ankle1's cellular localization was not affected following DNA damage.

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.

Src1 is a Protein of the Inner Nuclear Membrane Interacting with the Dictyostelium Lamin NE81

The nuclear envelope (NE) consists of the outer and inner nuclear membrane (INM), whereby the latter is bound to the nuclear lamina. Src1 is a Dictyostelium homologue of the helix-extension-helix family of proteins, which also includes the human lamin-binding protein MAN1. Both endogenous Src1 and GFP-Src1 are localized to the NE during the entire cell cycle. Immuno-electron microscopy and light microscopy after differential detergent treatment indicated that Src1 resides in the INM. FRAP experiments with GFP-Src1 cells suggested that at least a fraction of the protein could be stably engaged in forming the nuclear lamina together with the Dictyostelium lamin NE81. Both a BioID proximity assay and mis-localization of soluble, truncated mRFP-Src1 at cytosolic clusters consisting of an intentionally mis-localized mutant of GFP-NE81 confirmed an interaction of Src1 and NE81. Expression GFP-Src1^1–646, a fragment C-terminally truncated after the first transmembrane domain, disrupted interaction of nuclear membranes with the nuclear lamina, as cells formed protrusions of the NE that were dependent on cytoskeletal pulling forces. Protrusions were dependent on intact microtubules but not actin filaments. Our results indicate that Src1 is required for integrity of the NE and highlight Dictyostelium as a promising model for the evolution of nuclear architecture.

Nuclear Egress of Herpesviruses: The Prototypic Vesicular Nucleocytoplasmic Transport

Herpesvirus particles mature in two different cellular compartments. While capsid assembly and packaging of the genomic linear double-stranded DNA occur in the nucleus, virion formation takes place in the cytoplasm by the addition of numerous tegument proteins as well as acquisition of the viral envelope by budding into cellular vesicles derived from the trans-Golgi network containing virally encoded glycoproteins. To gain access to the final maturation compartment, herpesvirus nucleocapsids have to cross a formidable barrier, the nuclear envelope (NE). Since the ca. 120 nm diameter capsids are unable to traverse via nuclear pores, herpesviruses employ a vesicular transport through both leaflets of the NE. This process involves proteins which support local dissolution of the nuclear lamina to allow access of capsids to the inner nuclear membrane (INM), drive vesicle formation from the INM and mediate inclusion of the capsid as well as scission of the capsid-containing vesicle (also designated as "primary virion"). Fusion of the vesicle membrane (i.e., the "primary envelope") with the outer nuclear membrane subsequently results in release of the nucleocapsid into the cytoplasm for continuing virion morphogenesis. While this process has long been thought to be unique for herpesviruses, a similar pathway for nuclear egress of macromolecular complexes has recently been observed in Drosophila. Thus, herpesviruses may have coopted a hitherto unrecognized cellular mechanism of vesicle-mediated nucleocytoplasmic transport. This could have far reaching consequences for our understanding of cellular functions as again unraveled by the study of viruses.

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.

Purification and Structural Analysis of LEM-Domain Proteins

LAP2-emerin-MAN1 (LEM)-domain proteins are modular proteins characterized by the presence of a conserved motif of about 50 residues. Most LEM-domain proteins localize at the inner nuclear membrane, but some are also found in the endoplasmic reticulum or nuclear interior. Their architecture has been analyzed by predicting the limits of their globular domains, determining the 3D structure of these domains and in a few cases calculating the 3D structure of specific domains bound to biological targets. The LEM domain adopts an α-helical fold also found in SAP and HeH domains of prokaryotes and unicellular eukaryotes. The LEM domain binds to BAF (barrier-to-autointegration factor; BANF1), which interacts with DNA and tethers chromatin to the nuclear envelope. LAP2 isoforms also share an N-terminal LEM-like domain, which binds DNA. The structure and function of other globular domains that distinguish LEM-domain proteins from each other have been characterized, including the C-terminal dimerization domain of LAP2α and C-terminal WH and UHM domains of MAN1. LEM-domain proteins also have large intrinsically disordered regions that are involved in intra- and intermolecular interactions and are highly regulated by posttranslational modifications in vivo.

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.

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.

Nuclear envelope protein MAN1 regulates clock through BMAL1

Circadian clocks serve as internal pacemakers that influence many basic homeostatic processes; consequently, the expression and function of their components are tightly regulated by intricate networks of feedback loops that fine-tune circadian processes. Our knowledge of these components and pathways is far from exhaustive. In recent decades, the nuclear envelope has emerged as a global gene regulatory machine, although its role in circadian regulation has not been explored. We report that transcription of the core clock component BMAL1 is positively modulated by the inner nuclear membrane protein MAN1, which directly binds the BMAL1 promoter and enhances its transcription. Our results establish a novel connection between the nuclear periphery and circadian rhythmicity, therefore bridging two global regulatory systems that modulate all aspects of bodily functions.

Evolvement of LEM proteins as chromatin tethers at the nuclear periphery

The nuclear envelope in eukaryotic cells has important roles in chromatin organization. The inner nuclear membrane contains over 60 transmembrane proteins. LEM [LAP2 (lamina-associated polypeptide 2)/emerin/MAN1] domain-containing proteins of the inner nuclear membrane are involved in tethering chromatin to the nuclear envelope and affect gene expression. They contain a common structural, bihelical motif, the so-called LEM domain, which mediates binding to a conserved chromatin protein, BAF (barrier to autointegration factor). Interestingly, this domain is highly related to other bihelical motifs, termed HeH (helix-extension-helix) and SAP {SAF (scaffold attachment factor)/acinus/PIAS [protein inhibitor of activated STAT (signal transducer and activator of transcription)]} motifs, which are directly linked to DNA. In the present paper, we summarize evidence that the LEM motif evolved from the HeH and SAP domains concomitantly with BAF. In addition, we discuss the potential evolution of HeH/SAP and LEM domain-containing proteins and their role in chromatin tethering and gene regulation from unicellular eukaryotes to mammals.

Fission yeast Lem2 and Man1 perform fundamental functions of the animal cell nuclear lamina

In animal cells the nuclear lamina, which consists of lamins and lamin-associated proteins, serves several functions: it provides a structural scaffold for the nuclear envelope and tethers proteins and heterochromatin to the nuclear periphery. In yeast, proteins and large heterochromatic domains including telomeres are also peripherally localized, but there is no evidence that yeast have lamins or a fibrous nuclear envelope scaffold. Nonetheless, we found that the Lem2 and Man1 proteins of the fission yeast Schizosaccharomyces pombe, evolutionarily distant relatives of the Lap2/Emerin/Man1 (LEM) sub-family of animal cell lamin-associated proteins, perform fundamental functions of the animal cell lamina. These integral inner nuclear membrane localized proteins, with nuclear localized DNA binding Helix-Extension-Helix (HEH) domains, impact nuclear envelope structure and integrity, are essential for the enrichment of telomeres at the nuclear periphery and by means of their HEH domains anchor chromatin, most likely transcriptionally repressed heterochromatin, to the nuclear periphery. These data indicate that the core functions of the nuclear lamina are conserved between fungi and animal cells and can be performed in fission yeast, without lamins or other intermediate filament proteins.

The nuclear envelope as a chromatin organizer

In the past 15 years our perception of nuclear envelope function has evolved perhaps nearly as much as the nuclear envelope itself evolved in the last 3 billion years. Historically viewed as little more than a diffusion barrier between the cytoplasm and the nucleoplasm, the nuclear envelope is now known to have roles in the cell cycle, cytoskeletal stability and cell migration, genome architecture, epigenetics, regulation of transcription, splicing, and DNA replication. Here we will review both what is known and what is speculated about the role of the nuclear envelope in genome organization, particularly with respect to the positioning and repositioning of genes and chromosomes within the nucleus during differentiation.

Lumenal interactions in nuclear pore complex assembly and stability

A mechanism of nuclear pore complex assembly into intact nuclear envelopes remains elusive. We explore roles of conserved inner nuclear membrane proteins, Heh1p and Heh2p, in this process. The data support the existence of a lumenal bridge between Heh1p and the nucleoporin Pom152p, which might facilitate early nuclear pore complex assembly events.

Nuclear pore complexes (NPCs) provide a gateway for the selective transport of macromolecules across the nuclear envelope (NE). Although we have a solid understanding of NPC composition and structure, we do not have a clear grasp of the mechanism of NPC assembly. Here, we demonstrate specific defects in nucleoporin distribution in strains lacking Heh1p and Heh2p—two conserved members of the LEM (Lap2, emerin, MAN1) family of integral inner nuclear membrane proteins. These effects on nucleoporin localization are likely of functional importance as we have defined specific genetic interaction networks between HEH1 and HEH2, and genes encoding nucleoporins in the membrane, inner, and outer ring complexes of the NPC. Interestingly, expression of a domain of Heh1p that resides in the NE lumen is sufficient to suppress both the nucleoporin mislocalization and growth defects in heh1Δpom34Δ strains. We further demonstrate a specific physical interaction between the Heh1p lumenal domain and the massive cadherin-like lumenal domain of the membrane nucleoporin Pom152p. These findings support a role for Heh1p in the assembly or stability of the NPC, potentially through the formation of a lumenal bridge with Pom152p.

Structural analysis of the Smad2-MAN1 interaction that regulates transforming growth factor-β signaling at the inner nuclear membrane

MAN1, an integral protein of the inner nuclear membrane, influences transforming growth factor-β (TGF-β) signaling by directly interacting with R-Smads. Heterozygous loss of function mutations in the gene encoding MAN1 cause sclerosing bone dysplasias and an increased level of TGF-β signaling in cells. As a first step in elucidating the mechanism of TGF-β pathway regulation by MAN1, we characterized the structure of the MAN1 C-terminal region that binds Smad2. Using nuclear magnetic resonance spectroscopy, we observed that this region is comprised of a winged helix domain, a structurally heterogeneous linker, a U2AF homology motif (UHM) domain, and a disordered C-terminus. From nuclear magnetic resonance and small-angle X-ray scattering data, we calculated a family of models for this MAN1 region. Our data indicate that the linker plays the role of an intramolecular UHM ligand motif (ULM) interacting with the UHM domain. We mapped the Smad2 binding site onto the MAN1 structure by combining GST pull-down, fluorescence, and yeast two-hybrid approaches. The linker region, the UHM domain, and the C-terminus are necessary for Smad2 binding with a micromolar affinity. Moreover, the intramolecular interaction between the linker and the UHM domain is critical for Smad2 binding. On the basis of the structural heterogeneity and binding properties of the linker, we suggest that it can interact with other UHM domains, thus regulating the MAN1-Smad2 interaction.

Diseases of the nuclear envelope

In the past decade, a wide range of fascinating monogenic diseases have been linked to mutations in the LMNA gene, which encodes the A-type nuclear lamins, intermediate filament proteins of the nuclear envelope. These diseases include dilated cardiomyopathy with variable muscular dystrophy, Dunnigan-type familial partial lipodystrophy, a Charcot-Marie-Tooth type 2 disease, mandibuloacral dysplasia, and Hutchinson-Gilford progeria syndrome. Several diseases are also caused by mutations in genes encoding B-type lamins and proteins that associate with the nuclear lamina. Studies of these so-called laminopathies or nuclear envelopathies, some of which phenocopy common human disorders, are providing clues about functions of the nuclear envelope and insights into disease pathogenesis and human aging.

Lamin-binding Proteins

A- and B-type lamins are the major intermediate filaments of the nucleus. Lamins engage in a plethora of stable and transient interactions, near the inner nuclear membrane and throughout the nucleus. Lamin-binding proteins serve an amazingly diverse range of functions. Numerous inner-membrane proteins help anchor lamin filaments to the nuclear envelope, serving as part of the nuclear "lamina" network that is essential for nuclear architecture and integrity. Certain lamin-binding proteins of the inner membrane bind partners in the outer membrane and mechanically link lamins to the cytoskeleton. Inside the nucleus, lamin-binding proteins appear to serve as the "adaptors" by which the lamina organizes chromatin, influences gene expression and epigenetic regulation, and modulates signaling pathways. Transient interactions of lamins with key components of the transcription and replication machinery may provide an additional level of regulation or support to these essential events.

Overlapping functions of nuclear envelope proteins NET25 (Lem2) and emerin in regulation of extracellular signal-regulated kinase signaling in myoblast differentiation

Mutations in certain nuclear envelope (NE) proteins cause muscular dystrophies and other disorders, but the disease mechanisms remain unclear. The nuclear envelope transmembrane protein NET25 (Lem2) is a truncated paralog of MAN1, an NE component linked to bone disorders. NET25 and MAN1 share an approximately 40-residue LEM homology domain with emerin, the protein mutated in X-linked Emery-Dreifuss muscular dystrophy. However, roles for NET25 and MAN1 in myogenesis have not yet been described. Using RNA interference in C2C12 myoblasts, we show for the first time that both NET25 and MAN1 are required for myogenic differentiation. NET25 depletion causes hyperactivation of extracellular signal-regulated kinase 1/2 at the onset of differentiation, and pharmacological inhibition of this transient overactivation rescues myogenesis. In contrast, pharmacological inhibition of both mitogen-activated protein kinase and transforming growth factor beta signaling is required to rescue differentiation after MAN1 depletion. Ectopic expression of silencing-resistant NET25 rescues myogenesis after depletion of emerin but not after MAN1 silencing. Thus, NET25 and emerin have at least partially overlapping functions during myogenic differentiation, which are distinct from those of MAN1. Our work supports the hypothesis that deregulation of cell signaling contributes to NE-linked disorders and suggests that mutations in NET25 and MAN1 may cause muscle diseases.

Analysis of activin/TGFB-signaling modulators within the normal and dysfunctional adult human testis reveals evidence of altered signaling capacity in a subset of seminomas

Activin is a pleiotropic growth factor belonging to the transforming growth factor-beta (TGFB) superfamily of signaling molecules. Regulated activin signaling is known to influence several steps in rodent male gamete differentiation. TGFB ligand isoforms, TGFB1-B3, also influence germ cell survival in the rodent testis at the onset of spermatogenesis and around the time of puberty. Given the importance of regulated activin and TGFB signaling in testis development and function, we sought to investigate the cellular production sites of activin/TGFB-signaling modulators in normal and dysfunctional adult human testes samples. Signaling transducers phosphorylated SMAD2/3, and signaling modulators SMAD6, MAN-1, inhibin alpha (INHA), and beta-glycan were detected in Bouins fixed, paraffin-embedded adult human testis sections using immunohistochemistry. Additional samples examined were from testicular cancer patients and from normal men subjected to gonadotropin suppression with androgen-based contraceptives. Our findings identify distinct differences between normal and gonadotropin-deprived human testis in the expression and cellular localization of activin/TGFB-signaling modulators. The presence of a nuclear phosphorylated SMAD2/3 signal in all analyzed seminoma specimens indicated active activin/TGFB signaling. Moreover, a subset of seminoma specimens exhibited selective enhanced expression of beta-glycan (4 out of 28 seminoma tumors), INHA (6 out of 28), and MAN-1 (6 out of 28), highlighting potential functional differences between individual tumors in their capacity to regulate activin/TGFB signaling. Within the heterogenous nonseminomas, expression of signaling modulators was variable and reflected the degree of somatic differentiation. Thus, synthesis of activin and TGFB-signaling modulators may be affected by spermatogenic disruption and altered hormone levels in the testis.

Cell cycle-dependent phosphorylation of MAN1

The LEM (LAP2beta, Emerin, and MAN1) proteins are essential for nuclear membrane targeting to chromatin via an association with barrier-to-autointegration factor (BAF). Herein, we focused on the mitotic phosphorylation of MAN1 and its biological role. MAN1 was phosphorylated in a cell cycle-dependent manner in the Xenopus egg cell-free system, and the mitotic phosphorylation at the N-terminal region of MAN1 suppressed the binding of MAN1 to BAF. Titansphere column chromatography followed by MS/MS sequencing identified at least three M-phase-specific phosphorylation sites, Thr-209, Ser-351, and Ser-402, and one cell cycle-independent phosphorylation site, Ser-463. An in vitro BAF binding assay involving mutants S402A and S402E suggested that the phosphorylation of Ser-402 was important for regulation of the binding of MAN1 to BAF.

The inner nuclear membrane protein Src1 associates with subtelomeric genes and alters their regulated gene expression

Inner nuclear membrane proteins containing a LEM (LAP2, emerin, and MAN1) domain participate in different processes, including chromatin organization, gene expression, and nuclear envelope biogenesis. In this study, we identify a robust genetic interaction between transcription export (TREX) factors and yeast Src1, an integral inner nuclear membrane protein that is homologous to vertebrate LEM2. DNA macroarray analysis revealed that the expression of the phosphate-regulated genes PHO11, PHO12, and PHO84 is up-regulated in src1Δ cells. Notably, these PHO genes are located in subtelomeric regions of chromatin and exhibit a perinuclear location in vivo. Src1 spans the nuclear membrane twice and exposes its N and C domains with putative DNA-binding motifs to the nucleoplasm. Genome-wide chromatin immunoprecipitation–on-chip analyses indicated that Src1 is highly enriched at telomeres and subtelomeric regions of the yeast chromosomes. Our data show that the inner nuclear membrane protein Src1 functions at the interface between subtelomeric gene expression and TREX-dependent messenger RNA export through the nuclear pore complexes.

Tissue-specific defects are caused by loss of the Drosophila MAN1 LEM domain protein

The nuclear lamina represents a protein network required for nuclear structure and function. One family of lamina proteins is defined by an approximately 40-aa LAP2, Emerin, and MAN1 (LEM) domain (LEM-D) that binds the nonspecific DNA-binding protein, barrier-to-autointegration factor (BAF). Through interactions with BAF, LEM-D proteins serve as a bridge between chromosomes and the nuclear envelope. Mutations in genes encoding LEM-D proteins cause human laminopathies that are associated with tissue-restricted pathologies. Drosophila has five genes that encode proteins with LEM homology. Using yeast two-hybrid analyses, we demonstrate that four encode proteins that bind Drosophila (d)BAF. In addition to dBAF, dMAN1 associates with lamins, the LEM-D protein Bocksbeutel, and the receptor-regulated Smads, demonstrating parallel protein interactions with vertebrate homologs. P-element mobilization was used to generate null dMAN1 alleles. These mutants showed decreased viability, with surviving adults displaying male sterility, decreased female fertility, wing patterning and positioning defects, flightlessness, and locomotion difficulties that became more severe with age. Increased phospho-Smad staining in dMAN1 mutant wing discs is consistent with a role in transforming growth factor (TGF)-beta/bone morphogenic protein (BMP) signaling. The tissue-specific, age-enhanced dMAN1 mutant phenotypes are reminiscent of human laminopathies, suggesting that studies in Drosophila will provide insights into lamina dysfunction associated with disease.

"Laminopathies": a wide spectrum of human diseases

Mutations in genes encoding the intermediate filament nuclear lamins and associated proteins cause a wide spectrum of diseases sometimes called "laminopathies." Diseases caused by mutations in LMNA encoding A-type lamins include autosomal dominant Emery-Dreifuss muscular dystrophy and related myopathies, Dunnigan-type familial partial lipodystrophy, Charcot-Marie-Tooth disease type 2B1 and developmental and accelerated aging disorders. Duplication in LMNB1 encoding lamin B1 causes autosomal dominant leukodystrophy and mutations in LMNB2 encoding lamin B2 are associated with acquired partial lipodystrophy. Disorders caused by mutations in genes encoding lamin-associated integral inner nuclear membrane proteins include X-linked Emery-Dreifuss muscular dystrophy, sclerosing bone dysplasias, HEM/Greenberg skeletal dysplasia and Pelger-Huet anomaly. While mutations and clinical phenotypes of "laminopathies" have been carefully described, data explaining pathogenic mechanisms are only emerging. Future investigations will likely identify new "laminopathies" and a combination of basic and clinical research will lead to a better understanding of pathophysiology and the development of therapies.

Proteins that associate with lamins: many faces, many functions

Lamin-associated polypeptides (LAPs) comprise inner nuclear membrane proteins tightly associated with the peripheral lamin scaffold as well as proteins forming stable complexes with lamins in the nucleoplasm. The involvement of LAPs in a wide range of human diseases may be linked to an equally bewildering range of their functions, including sterol reduction, histone modification, transcriptional repression, and Smad- and beta-catenin signaling. Many LAPs are likely to be at the center of large multi-protein complexes, components of which may dictate their functions, and a few LAPs have defined enzymatic activities. Here we discuss the definition of LAPs, review their many binding partners, elaborate their functions in nuclear architecture, chromatin organization, gene expression and signaling, and describe what is currently known about their links to human disease.

What MAN1 does to the Smads

The inner nuclear membrane protein MAN1 has been identified as an important factor in transforming growth factor beta/bone morphogenic protein (TGFbeta/BMP) signaling. Loss of MAN1 results in three autosomal dominant diseases in humans; all three characterized by increased bone density. Xenopus embryos lacking MAN1 develop severe morphological defects. Both in humans and in Xenopus embryos the defects originate from deregulation of TGFbeta/BMP signaling. Several independent studies have shown that MAN1 is antagonizing TGFbeta/BMP signaling through binding to regulatory Smads. Here, recent progress in understanding MAN1 functions is summarized and a model for MAN1-dependent regulation of TGFbeta/BMP signaling is proposed.

Nuclear envelope transmembrane proteins (NETs) that are up-regulated during myogenesis

BACKGROUND

The nuclear lamina is a protein meshwork lining the inner nuclear membrane, which contains a polymer of nuclear lamins associated with transmembrane proteins of the inner nuclear membrane. The lamina is involved in nuclear structure, gene expression, and association of the cytoplasmic cytoskeleton with the nucleus. We previously identified a group of 67 novel putative nuclear envelope transmembrane proteins (NETs) in a large-scale proteomics analysis. Because mutations in lamina proteins have been linked to several human diseases affecting skeletal muscle, we examined NET expression during differentiation of C2C12 myoblasts. Our goal was to identify new nuclear envelope and lamina components whose expression is coordinated with muscle differentiation.

RESULTS

Using transcriptional microarray analysis, we found that expression of 6 of the NETs significantly increases during myoblast differentiation. We confirmed these results using quantitative RT-PCR, and furthermore, found that all 6 NETs are expressed at high levels in adult mouse skeletal muscle relative to 9 other tissues examined. Using epitope-tagged cDNAs, we determined that the 5 NETs we could analyze (NETs 9, 25, 32, 37 and 39) all target to the nuclear envelope in C2C12 cells. Furthermore, the 3 NETs that we could analyze by immunoblotting were highly enriched in nuclear envelopes relative to microsomal membranes purified from mouse liver. Database searches showed that 4 of the 6 up-regulated NETs contain regions of homology to proteins previously linked to signaling.

CONCLUSION

This work identified 6 NETs that are predicted to have important functions in muscle development and/or maintenance from their expression patterns during myoblast differentiation and in mouse tissues. We confirmed that 5 of these NETs are authentic nuclear envelope proteins. Four members of this group have potential signaling functions at the NE, based on their sequence homologies.