cDNA cloning and characterization of lamina-associated polypeptide 1C (LAP1C), an integral protein of the inner nuclear membrane

Quantitative proteome analysis of LAP1-deficient human fibroblasts: A pilot approach for predicting the signaling pathways deregulated in LAP1-associated diseases

Lamina-associated polypeptide 1 (LAP1), a ubiquitously expressed nuclear envelope protein, appears to be essential for the maintenance of cell homeostasis. Although rare, mutations in the human LAP1-encoding TOR1AIP1 gene cause severe diseases and can culminate in the premature death of affected individuals. Despite there is increasing evidence of the pathogenicity of TOR1AIP1 mutations, the current knowledge on LAP1's physiological roles in humans is limited; hence, investigation is required to elucidate the critical functions of this protein, which can be achieved by uncovering the molecular consequences of LAP1 depletion, a topic that remains largely unexplored. In this work, the proteome of patient-derived LAP1-deficient fibroblasts carrying a pathological TOR1AIP1 mutation (LAP1 E482A) was quantitatively analyzed to identify global changes in protein abundance levels relatively to control fibroblasts. An in silico functional enrichment analysis of the mass spectrometry-identified differentially expressed proteins was also performed, along with additional in vitro functional assays, to unveil the biological processes that are potentially dysfunctional in LAP1 E482A fibroblasts. Collectively, our findings suggest that LAP1 deficiency may induce significant alterations in various cellular activities, including DNA repair, messenger RNA degradation/translation, proteostasis and glutathione metabolism/antioxidant response. This study sheds light on possible new functions of human LAP1 and could set the basis for subsequent in-depth mechanistic investigations. Moreover, by identifying deregulated signaling pathways in LAP1-deficient cells, our work may offer valuable molecular targets for future disease-modifying therapies for TOR1AIP1-associated nuclear envelopathies.

Analysis of the GFP-labelled β-dystroglycan interactome in HEK-293 transfected cells reveals novel intracellular networks

Dystroglycan (DG) is a cell adhesion complex that is widely expressed in tissues. It is composed by two subunits, α-DG, a highly glycosylated protein that interacts with several extracellular matrix proteins, and transmembrane β-DG whose, cytodomain binds to the actin cytoskeleton. Glycosylation of α-DG is crucial for functioning as a receptor for its multiple extracellular binding partners. Perturbation of α-DG glycosylation is the central event in the pathogenesis of severe pathologies such as muscular dystrophy and cancer. β-DG acts as a scaffold for several cytoskeletal and nuclear proteins and very little is known about the fine regulation of some of these intracellular interactions and how they are perturbed in diseases. To start filling this gap by identifying uncharacterized intracellular networks preferentially associated with β-DG, HEK-293 cells were transiently transfected with a plasmid carrying the β-DG subunit with GFP fused at its C-terminus. With this strategy, we aimed at forcing β-DG to occupy multiple intracellular locations instead of sitting tightly at its canonical plasma membrane milieu, where it is commonly found in association with α-DG. Immunoprecipitation by anti-GFP antibodies followed by shotgun proteomic analysis led to the identification of an interactome formed by 313 exclusive protein matches for β-DG binding. A series of already known β-DG interactors have been found, including ezrin and emerin, whilst significant new matches, which include potential novel β-DG interactors and their related networks, were identified in diverse subcellular compartments, such as cytoskeleton, endoplasmic reticulum/Golgi, mitochondria, nuclear membrane and the nucleus itself. Of particular interest amongst the novel identified matches, Lamina-Associated Polypeptide-1B (LAP1B), an inner nuclear membrane protein, whose mutations are known to cause nuclear envelopathies characterized by muscular dystrophy, was found to interact with β-DG in HEK-293 cells. This evidence was confirmed by immunoprecipitation, Western blotting and immunofluorescence experiments. We also found by immunofluorescence experiments that LAP1B looses its nuclear envelope localization in C2C12 DG-knock-out cells, suggesting that LAP1B requires β-DG for a proper nuclear localization. These results expand the role of β-DG as a nuclear scaffolding protein and provide novel evidence of a possible link between dystroglycanopathies and nuclear envelopathies displaying with muscular dystrophy.

Deep mining of the Sequence Read Archive reveals major genetic innovations in coronaviruses and other nidoviruses of aquatic vertebrates

Virus discovery by genomics and metagenomics empowered studies of viromes, facilitated characterization of pathogen epidemiology, and redefined our understanding of the natural genetic diversity of viruses with profound functional and structural implications. Here we employed a data-driven virus discovery approach that directly queries unprocessed sequencing data in a highly parallelized way and involves a targeted viral genome assembly strategy in a wide range of sequence similarity. By screening more than 269,000 datasets of numerous authors from the Sequence Read Archive and using two metrics that quantitatively assess assembly quality, we discovered 40 nidoviruses from six virus families whose members infect vertebrate hosts. They form 13 and 32 putative viral subfamilies and genera, respectively, and include 11 coronaviruses with bisegmented genomes from fishes and amphibians, a giant 36.1 kilobase coronavirus genome with a duplicated spike glycoprotein (S) gene, 11 tobaniviruses and 17 additional corona-, arteri-, cremega-, nanhypo- and nangoshaviruses. Genome segmentation emerged in a single evolutionary event in the monophyletic lineage encompassing the subfamily Pitovirinae. We recovered the bisegmented genome sequences of two coronaviruses from RNA samples of 69 infected fishes and validated the presence of poly(A) tails at both segments using 3'RACE PCR and subsequent Sanger sequencing. We report a genetic linkage between accessory and structural proteins whose phylogenetic relationships and evolutionary distances are incongruent with the phylogeny of replicase proteins. We rationalize these observations in a model of inter-family S recombination involving at least five ancestral corona- and tobaniviruses of aquatic hosts. In support of this model, we describe an individual fish co-infected with members from the families Coronaviridae and Tobaniviridae. Our results expand the scale of the known extraordinary evolutionary plasticity in nidoviral genome architecture and call for revisiting fundamentals of genome expression, virus particle biology, host range and ecology of vertebrate nidoviruses.

TOR1AIP1-Associated Nuclear Envelopathies

Human TOR1AIP1 encodes LAP1, a nuclear envelope protein expressed in most human tissues, which has been linked to various biological processes and human diseases. The clinical spectrum of diseases related to mutations in TOR1AIP1 is broad, including muscular dystrophy, congenital myasthenic syndrome, cardiomyopathy, and multisystemic disease with or without progeroid features. Although rare, these recessively inherited disorders often lead to early death or considerable functional impairment. Developing a better understanding of the roles of LAP1 and mutant TOR1AIP1-associated phenotypes is paramount to allow therapeutic development. To facilitate further studies, this review provides an overview of the known interactions of LAP1 and summarizes the evidence for the function of this protein in human health. We then review the mutations in the TOR1AIP1 gene and the clinical and pathological characteristics of subjects with these mutations. Lastly, we discuss challenges to be addressed in the future.

Hepatocytes Deficient in Nuclear Envelope Protein Lamina-associated Polypeptide 1 are an Ideal Mammalian System to Study Intranuclear Lipid Droplets

Lipid droplets (LDs) are generally considered to be synthesized in the ER and utilized in the cytoplasm. However, LDs have been observed inside nuclei in some cells, although recent research on nuclear LDs has focused on cultured cell lines. To better understand nuclear LDs that occur in vivo, here we examined LDs in primary hepatocytes from mice following depletion of the nuclear envelope protein lamina-associated polypeptide 1 (LAP1). Microscopic image analysis showed that LAP1-depleted hepatocytes contain frequent nuclear LDs, which differ from cytoplasmic LDs in their associated proteins. We found type 1 nucleoplasmic reticula, which are invaginations of the inner nuclear membrane, are often associated with nuclear LDs in these hepatocytes. Furthermore, in vivo depletion of the nuclear envelope proteins lamin A and C from mouse hepatocytes led to severely abnormal nuclear morphology, but significantly fewer nuclear LDs than were observed upon depletion of LAP1. In addition, we show both high-fat diet feeding and fasting of mice increased cytoplasmic lipids in LAP1-depleted hepatocytes but reduced nuclear LDs, demonstrating a relationship of LD formation with nutritional state. Finally, depletion of microsomal triglyceride transfer protein did not change the frequency of nuclear LDs in LAP1-depleted hepatocytes, suggesting that it is not required for the biogenesis of nuclear LDs in these cells. Together, these data show that LAP1-depleted hepatocytes represent an ideal mammalian system to investigate the biogenesis of nuclear LDs and their partitioning between the nucleus and cytoplasm in response to changes in nutritional state and cellular metabolism in vivo.

Molecular Pathology of Laminopathies

The nuclear envelope is composed of the nuclear membranes, nuclear lamina, and nuclear pore complexes. Laminopathies are diseases caused by mutations in genes encoding protein components of the lamina and these other nuclear envelope substructures. Mutations in the single gene encoding lamin A and C, which are expressed in most differentiated somatic cells, cause diseases affecting striated muscle, adipose tissue, peripheral nerve, and multiple systems with features of accelerated aging. Mutations in genes encoding other nuclear envelope proteins also cause an array of diseases that selectively affect different tissues or organs. In some instances, the molecular and cellular consequences of laminopathy-causing mutations are known. However, even when these are understood, mechanisms explaining specific tissue or organ pathology remain enigmatic. Current mechanistic hypotheses focus on how alterations in the nuclear envelope may affect gene expression, including via the regulation of signaling pathways, or cellular mechanics, including responses to mechanical stress.

Selective loss of a LAP1 isoform causes a muscle-specific nuclear envelopathy

The nuclear envelope (NE) separates the nucleus from the cytoplasm in all eukaryotic cells. A disruption of the NE structure compromises normal gene regulation and leads to severe human disorders collectively classified as nuclear envelopathies and affecting skeletal muscle, heart, brain, skin, and bones. The ubiquitous NE component LAP1B is encoded by TOR1AIP1, and the use of an alternative start codon gives rise to the shorter LAP1C isoform. TOR1AIP1 mutations have been identified in patients with diverging clinical presentations such as muscular dystrophy, progressive dystonia with cerebellar atrophy, and a severe multi-systemic disorder, but the correlation between the mutational effect and the clinical spectrum remains to be determined. Here, we describe a novel TOR1AIP1 patient manifesting childhood-onset muscle weakness and contractures, and we provide clinical, histological, ultrastructural, and genetic data. We demonstrate that the identified TOR1AIP1 frameshift mutation leads to the selective loss of the LAP1B isoform, while the expression of LAP1C was preserved. Through comparative review of all previously reported TOR1AIP1 cases, we delineate a genotype/phenotype correlation and conclude that LAP1B-specific mutations cause a progressive skeletal muscle phenotype, while mutations involving a loss of both LAP1B and LAP1C isoforms induce a syndromic disorder affecting skeletal muscle, brain, eyes, ear, skin, and bones.

Nuclear Accumulation of LAP1:TRF2 Complex during DNA Damage Response Uncovers a Novel Role for LAP1

Lamina-associated polypeptide 1 (LAP1) is a nuclear envelope (NE) protein whose function remains poorly characterized. In a recent LAP1 protein interactome study, a putative regulatory role in the DNA damage response (DDR) has emerged and telomeric repeat-binding factor 2 (TRF2), a protein intimately associated with this signaling pathway, was among the list of LAP1 interactors. To gain insights into LAP1's physiological properties, the interaction with TRF2 in human cells exposed to DNA-damaging agents was investigated. The direct LAP1:TRF2 binding was validated in vitro by blot overlay and in vivo by co-immunoprecipitation after hydrogen peroxide and bleomycin treatments. The regulation of this protein interaction by LAP1 phosphorylation was demonstrated by co-immunoprecipitation and mass spectrometry following okadaic acid exposure. The involvement of LAP1 and TRF2 in the DDR was confirmed by their increased nuclear protein levels after bleomycin treatment, evaluated by immunoblotting, as well as by their co-localization with DDR factors at the NE and within the nucleoplasm, assessed by immunocytochemistry. Effectively, we showed that the LAP1:TRF2 complex is established during a cellular response against DNA damage. This work proposes a novel functional role for LAP1 in the DDR, revealing a potential biological mechanism that may be disrupted in LAP1-associated pathologies.

Postnatal development of mice with combined genetic depletions of lamin A/C, emerin and lamina-associated polypeptide 1

Mutations in LMNA encoding lamin A/C and EMD encoding emerin cause cardiomyopathy and muscular dystrophy. Lmna null mice develop these disorders and have a lifespan of 7-8 weeks. Emd null mice show no overt pathology and have normal skeletal muscle but with regeneration defects. We generated mice with germline deletions of both Lmna and Emd to determine the effects of combined loss of the encoded proteins. Mice without lamin A/C and emerin are born at the expected Mendelian ratio, are grossly normal at birth but have shorter lifespans than those lacking only lamin A/C. However, there are no major differences between these mice with regards to left ventricular function, heart ultrastructure or electrocardiographic parameters except for slower heart rates in the mice lacking both lamin A/C and emerin. Skeletal muscle is similarly affected in both of these mice. Lmna+/- mice also lacking emerin live to at least 1 year and have no significant differences in growth, heart or skeletal muscle compared to Lmna+/- mice. Deletion of the mouse gene encoding lamina-associated protein 1 leads to prenatal death; however, mice with heterozygous deletion of this gene lacking both lamin A/C and emerin are born at the expected Mendelian ratio but had a shorter lifespan than those only lacking lamin A/C and emerin. These results show that mice with combined deficiencies of three interacting nuclear envelope proteins have normal embryonic development and that early postnatal defects are primarily driven by loss of lamin A/C or lamina-associated polypeptide 1 rather than emerin.

Combined loss of LAP1B and LAP1C results in an early onset multisystemic nuclear envelopathy

Nuclear envelopathies comprise a heterogeneous group of diseases caused by mutations in genes encoding nuclear envelope proteins. Mutations affecting lamina-associated polypeptide 1 (LAP1) result in two discrete phenotypes of muscular dystrophy and progressive dystonia with cerebellar atrophy. We report 7 patients presenting at birth with severe progressive neurological impairment, bilateral cataract, growth retardation and early lethality. All the patients are homozygous for a nonsense mutation in the TOR1AIP1 gene resulting in the loss of both protein isoforms LAP1B and LAP1C. Patient-derived fibroblasts exhibit changes in nuclear envelope morphology and large nuclear-spanning channels containing trapped cytoplasmic organelles. Decreased and inefficient cellular motility is also observed in these fibroblasts. Our study describes the complete absence of both major human LAP1 isoforms, underscoring their crucial role in early development and organogenesis. LAP1-associated defects may thus comprise a broad clinical spectrum depending on the availability of both isoforms in the nuclear envelope throughout life.

Mechanisms of allelic and clinical heterogeneity of lamin A/C phenotypes

Mutations in the lamin A/C (LMNA) gene cause a broad range of clinical syndromes that show tissue-restricted abnormalities of post mitotic tissues, such as muscle, nerve, heart, and adipose tissue. Mutations in other nuclear envelope proteins cause clinically overlapping disorders. The majority of mutations are dominant single amino acid changes (toxic protein produced by the single mutant gene), and patients are heterozygous with both normal and abnormal proteins. Experimental support has been provided for different models of cellular pathogenesis in nuclear envelope diseases, including changes in heterochromatin formation at the nuclear membrane (epigenomics), changes in the timing of steps during terminal differentiation of cells, and structural abnormalities of the nuclear membrane. These models are not mutually exclusive and may be important in different cells at different times of development. Recent experiments using fusion proteins of normal and mutant lamin A/C proteins fused to a bacterial adenine methyltransferase (DamID) provided compelling evidence of mutation-specific perturbation of epigenomic imprinting during terminal differentiation. These gain-of-function properties include lineage-specific ineffective genomic silencing during exit from the cell cycle (heterochromatinization), as well as promiscuous initiation of silencing at incorrect places in the genome. To date, these findings have been limited to a few muscular dystrophy and lipodystrophy LMNA mutations but seem shared with a distinct nuclear envelope disease, emerin-deficient muscular dystrophy. The dominant-negative structural model and gain-of-function epigenomic models for distinct LMNA mutations are not mutually exclusive, and it is likely that both models contribute to aspects of the many complex clinical phenotypes observed.

Descriptive Analysis of LAP1 Distribution and That of Associated Proteins throughout Spermatogenesis

Spermatogenesis comprises highly complex differentiation processes. Nuclear envelope (NE) proteins have been associated with these processes, including lamins, lamina-associated polypeptide (LAP) 2 and the lamin B-receptor. LAP1 is an important NE protein whose function has not been fully elucidated, but several binding partners allow predicting putative LAP1 functions. To date, LAP1 had not been associated with spermatogenesis. In this study, LAP1 expression and cellular/subcellular localization during spermatogenesis in human and mouse testes is established for the first time. The fact that LAP1 is expressed during nuclear elongation in spermiogenesis and is located at the spermatids' centriolar pole is singularly important. LAP1 binds to members of the protein phosphatase 1 (PP1) family. Similar localization of LAP1 and PP1γ2, a testis-specific PP1 isoform, suggests a shared function for both proteins during spermiogenesis. Furthermore, this study suggests an involvement of LAP1 in manchette development and chromatin regulation possibly via interaction with acetylated α-tubulin and lamins, respectively. Taken together, the present results indicate that, by moving to the posterior pole in spermatids, LAP1 can contribute to the achievement of non-random, sperm-specific chromatin distribution, as well as modulate cellular remodeling during spermiogenesis. In addition, LAP1 seems to be associated with dynamic microtubule changes related to manchette formation and flagella development.

Torsins Are Essential Regulators of Cellular Lipid Metabolism

Torsins are developmentally essential AAA+ proteins, and mutation of human torsinA causes the neurological disease DYT1 dystonia. They localize in the ER membranes, but their cellular function remains unclear. We now show that dTorsin is required in Drosophila adipose tissue, where it suppresses triglyceride levels, promotes cell growth, and elevates membrane lipid content. We also see that human torsinA at the inner nuclear membrane is associated with membrane expansion and elevated cellular lipid content. Furthermore, the key lipid metabolizing enzyme, lipin, is mislocalized in dTorsin-KO cells, and dTorsin increases levels of the lipin substrate, phosphatidate, and reduces the product, diacylglycerol. Finally, genetic suppression of dLipin rescues dTorsin-KO defects, including adipose cell size, animal growth, and survival. These findings identify that torsins are essential regulators of cellular lipid metabolism and implicate disturbed lipid biology in childhood-onset DYT1 dystonia.

Evolutionary diversification of the BetaM interactome acquired through co-option of the ATP1B4 gene in placental mammals

ATP1B4 genes represent a rare instance of orthologous vertebrate gene co-option that radically changed properties of the encoded BetaM proteins, which function as Na,K-ATPase subunits in lower vertebrates and birds. Eutherian BetaM has lost its ancestral function and became a muscle-specific resident of the inner nuclear membrane. Our earlier work implicated BetaM in regulation of gene expression through direct interaction with the transcriptional co-regulator SKIP. To gain insight into evolution of BetaM interactome we performed expanded screening of eutherian and avian cDNA libraries using yeast-two-hybrid and split-ubiquitin systems. The inventory of identified BetaM interactors includes lamina-associated protein LAP-1, myocyte nuclear envelope protein Syne1, BetaM itself, heme oxidases HMOX1 and HMOX2; transcription factor LZIP/CREB3, ERGIC3, PHF3, reticulocalbin-3, and β-sarcoglycan. No new interactions were found for chicken BetaM and human Na,K-ATPase β1, β2 and β3 isoforms, indicating the uniqueness of eutherian BetaM interactome. Analysis of truncated forms of BetaM indicates that residues 72-98 adjacent to the membrane in nucleoplasmic domain are important for the interaction with SKIP. These findings demonstrate that evolutionary alterations in structural and functional properties of eutherian BetaM proteins are associated with the increase in its interactome complexity.

Lamina Associated Polypeptide 1 (LAP1) Interactome and Its Functional Features

Lamina-associated polypeptide 1 (LAP1) is a type II transmembrane protein of the inner nuclear membrane encoded by the human gene TOR1AIP1. LAP1 is involved in maintaining the nuclear envelope structure and appears be involved in the positioning of lamins and chromatin. To date, LAP1’s precise function has not been fully elucidated but analysis of its interacting proteins will permit unraveling putative associations to specific cellular pathways and cellular processes. By assessing public databases it was possible to identify the LAP1 interactome, and this was curated. In total, 41 interactions were identified. Several functionally relevant proteins, such as TRF2, TERF2IP, RIF1, ATM, MAD2L1 and MAD2L1BP were identified and these support the putative functions proposed for LAP1. Furthermore, by making use of the Ingenuity Pathways Analysis tool and submitting the LAP1 interactors, the top two canonical pathways were “Telomerase signalling” and “Telomere Extension by Telomerase” and the top functions “Cell Morphology”, “Cellular Assembly and Organization” and “DNA Replication, Recombination, and Repair”. Once again, putative LAP1 functions are reinforced but novel functions are emerging.

Access of torsinA to the inner nuclear membrane is activity dependent and regulated in the endoplasmic reticulum

TorsinA (also known as torsin-1A) is a membrane-embedded AAA+ ATPase that has an important role in the nuclear envelope lumen. However, most torsinA is localized in the peripheral endoplasmic reticulum (ER) lumen where it has a slow mobility that is incompatible with free equilibration between ER subdomains. We now find that nuclear-envelope-localized torsinA is present on the inner nuclear membrane (INM) and ask how torsinA reaches this subdomain. The ER system contains two transmembrane proteins, LAP1 and LULL1 (also known as TOR1AIP1 and TOR1AIP2, respectively), that reversibly co-assemble with and activate torsinA. Whereas LAP1 localizes on the INM, we show that LULL1 is in the peripheral ER and does not enter the INM. Paradoxically, interaction between torsinA and LULL1 in the ER targets torsinA to the INM. Native gel electrophoresis reveals torsinA oligomeric complexes that are destabilized by LULL1. Mutations in torsinA or LULL1 that inhibit ATPase activity reduce the access of torsinA to the INM. Furthermore, although LULL1 binds torsinA in the ER lumen, its effect on torsinA localization requires cytosolic-domain-mediated oligomerization. These data suggest that LULL1 oligomerizes to engage and transiently disassemble torsinA oligomers, and is thereby positioned to transduce cytoplasmic signals to the INM through torsinA.

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.

Identification of a novel human LAP1 isoform that is regulated by protein phosphorylation

Lamina associated polypeptide 1 (LAP1) is an integral protein of the inner nuclear membrane that is ubiquitously expressed. LAP1 binds to lamins and chromatin, probably contributing to the maintenance of the nuclear envelope architecture. Moreover, LAP1 also interacts with torsinA and emerin, proteins involved in DYT1 dystonia and X-linked Emery-Dreifuss muscular dystrophy disorder, respectively. Given its relevance to human pathological conditions, it is important to better understand the functional diversity of LAP1 proteins. In rat, the LAP1 gene (TOR1AIP1) undergoes alternative splicing to originate three LAP1 isoforms (LAP1A, B and C). However, it remains unclear if the same occurs with the human TOR1AIP1 gene, since only the LAP1B isoform had thus far been identified in human cells. In silico analysis suggested that, across different species, potential new LAP1 isoforms could be generated by alternative splicing. Using shRNA to induce LAP1 knockdown and HPLC-mass spectrometry analysis the presence of two isoforms in human cells was described and validated: LAP1B and LAP1C; the latter is putatively N-terminal truncated. LAP1B and LAP1C expression profiles appear to be dependent on the specific tissues analyzed and in cultured cells LAP1C was the major isoform detected. Moreover, LAP1B and LAP1C expression increased during neuronal maturation, suggesting that LAP1 is relevant in this process. Both isoforms were found to be post-translationally modified by phosphorylation and methionine oxidation and two LAP1B/LAP1C residues were shown to be dephosphorylated by PP1. This study permitted the identification of the novel human LAP1C isoform and partially unraveled the molecular basis of LAP1 regulation.

LAP1 is a crucial protein for the maintenance of the nuclear envelope structure and cell cycle progression

Cell division in eukaryotes requires the disassembly of the nuclear envelope (NE) at the beginning of mitosis and its reassembly at the end of mitosis. These processes are complex and involve coordinated steps where NE proteins have a crucial role. Lamina-associated polypeptide 1 (LAP1) is an inner nuclear membrane protein that has been associated with cell cycle events. In support of this role, LAP1 has been implicated in the regulation of the NE reassembly and assembly of the mitotic spindle during mitosis. In this study, we demonstrated that LAP1 intracellular levels vary during the cell cycle in SH-SY5Y cells, and that LAP1 is highly phosphorylated during mitosis. It is also clear that LAP1 co-localized with acetylated α-tubulin in the mitotic spindle and with γ-tubulin in centrosomes (main microtubule organizing center) in mitotic cells. Moreover, LAP1 knockdown resulted in decreased number of mitotic cells and decreased levels of acetylated α-tubulin (marker of microtubules stability) and lamin B1. Additionally, it was possible to determine that LAP1 is important for centrosome positioning near the NE. These findings place LAP1 at a key position to participate in the maintenance of the NE structure and progression of the cell cycle.

Depletion of lamina-associated polypeptide 1 from cardiomyocytes causes cardiac dysfunction in mice

We previously showed that striated muscle-selective depletion of lamina-associated polypeptide 1 (LAP1), an integral inner nuclear membrane protein, leads to profound muscular dystrophy with premature death in mice. As LAP1 is also depleted in hearts of these mice, we examined their cardiac phenotype. Striated muscle-selective LAP1 knockout mice display ventricular systolic dysfunction with abnormal induction of genes encoding cardiomyopathy related proteins. To eliminate possible confounding effects due to skeletal muscle pathology, we generated a new mouse line in which LAP1 is deleted in a cardiomyocyte-selective manner. These mice had no skeletal muscle pathology and appeared overtly normal at 20 weeks of age. However, cardiac echocardiography revealed that they developed left ventricular systolic dysfunction and cardiac gene expression analysis revealed abnormal induction of cardiomyopathy-related genes. Our results demonstrate that LAP1 expression in cardiomyocytes is required for normal left ventricular function, consistent with a report of cardiomyopathy in a human subject with mutation in the gene encoding LAP1.

Lamina-associated polypeptide 1: protein interactions and tissue-selective functions

Mutations in genes encoding widely expressed nuclear envelope proteins often lead to diseases that manifest in specific tissues. Lamina-associated polypeptide 1 (LAP1) is an integral protein of the inner nuclear membrane that is expressed in most cells and tissues. Within the nuclear envelope, LAP1 interacts physically with lamins, torsinA and emerin, suggesting it may serve as a key node for transducing signals across the inner nuclear membrane. Indeed, recent in vivo studies in genetically modified mice strongly support functional links between LAP1 and both torsinA (in neurons) and emerin (in muscle). These studies suggest that tissue-selective diseases caused by mutations in genes encoding nuclear envelope proteins may result, at least in part, from the selective disruption of discrete nuclear envelope protein complexes.

The nuclear envelope protein, LAP1B, is a novel protein phosphatase 1 substrate

Protein phosphatase 1 (PP1) binding proteins are quintessential regulators, determining substrate specificity and defining subcellular localization and activity of the latter. Here, we describe a novel PP1 binding protein, the nuclear membrane protein lamina associated polypeptide 1B (LAP1B), which interacts with the DYT1 dystonia protein torsinA. The PP1 binding domain in LAP1B was here identified as the REVRF motif at amino acids 55-59. The LAP1B:PP1 complex can be immunoprecipitated from cells in culture and rat cortex and the complex was further validated by yeast co-transformations and blot overlay assays. PP1, which is enriched in the nucleus, binds to the N-terminal nuclear domain of LAP1B, as shown by immunocolocalization and domain specific binding studies. PP1 dephosphorylates LAP1B, confirming the physiological relevance of this interaction. These findings place PP1 at a key position to participate in the pathogenesis of DYT1 dystonia and related nuclear envelope-based diseases.

The nuclear envelope localization of DYT1 dystonia torsinA-ΔE requires the SUN1 LINC complex component

Background

DYT1 dystonia is an autosomal dominant neurological condition caused by a mutation that removes a single glutamic acid residue (ΔE) from the torsinA (torA) AAA+ protein. TorA appears to possess a nuclear envelope (NE) localized activity that requires Lamina-Associated-Polypeptide 1 (LAP1), which is an inner nuclear membrane localized torA-binding partner. Although hypoactive, the DYT1 dystonia torA-ΔE isoform often concentrates in the NE, suggesting that torA-ΔE also interacts with an NE-localized binding partner.

Results

We confirm that NE-localized torA-ΔE does not co-immunoprecipitate with LAP1, and find that torA-ΔE continues to concentrate in the NE of cells that lack LAP1. Instead, we find that variability in torA-ΔE localization correlates with the presence of the SUN-domain and Nesprin proteins that assemble into the LINC complex. We also find that siRNA depletion of SUN1, but not other LINC complex components, removes torA-ΔE from the NE. In contrast, the LAP1-dependent NE-accumulation of an ATP-locked torA mutant is unaffected by loss of LINC complex proteins. This SUN1 dependent torA-ΔE localization requires the torA membrane association domain, as well as a putative substrate-interaction residue, Y147, neither of which are required for torA interaction with LAP1. We also find that mutation of these motifs, or depletion of SUN1, decreases the amount of torA-WT that colocalizes with NE markers, indicating that each also underlies a normal NE-localized torA binding interaction.

Conclusions

These data suggest that the disease causing ΔE mutation promotes an association between torA and SUN1 that is distinct to the interaction between LAP1 and ATP-bound torA. This evidence for two NE-localized binding partners suggests that torA may act on multiple substrates and/or possesses regulatory co-factor partners. In addition, finding that the DYT1 mutation causes abnormal association with SUN1 implicates LINC complex dysfunction in DYT1 dystonia pathogenesis, and suggests a gain-of-function activity contributes to this dominantly inherited disease.

System analysis shows distinct mechanisms and common principles of nuclear envelope protein dynamics

The ER–inner nuclear membrane trafficking of 15 integral membrane proteins followed by FRAP shows distinct ATP- and Ran-dependent translocation mechanisms.

The nuclear envelope contains >100 transmembrane proteins that continuously exchange with the endoplasmic reticulum and move within the nuclear membranes. To better understand the organization and dynamics of this system, we compared the trafficking of 15 integral nuclear envelope proteins using FRAP. A surprising 30-fold range of mobilities was observed. The dynamic behavior of several of these proteins was also analyzed after depletion of ATP and/or Ran, two functions implicated in endoplasmic reticulum–inner nuclear membrane translocation. This revealed that ATP- and Ran-dependent translocation mechanisms are distinct and not used by all inner nuclear membrane proteins. The Ran-dependent mechanism requires the phenylalanine-glycine (FG)-nucleoporin Nup35, which is consistent with use of the nuclear pore complex peripheral channels. Intriguingly, the addition of FGs to membrane proteins reduces FRAP recovery times, and this also depends on Nup35. Modeling of three proteins that were unaffected by either ATP or Ran depletion indicates that the wide range in mobilities could be explained by differences in binding affinities in the inner nuclear membrane.

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.

Interaction of torsinA with its major binding partners is impaired by the dystonia-associated DeltaGAG deletion

Early onset (DYT1) torsion dystonia is a dominantly inherited movement disorder associated with a three-base pair (DeltaGAG) deletion that removes a glutamic acid residue from the protein torsinA. TorsinA is an essential AAA(+) (ATPases associated with a variety of cellular activities) ATPase found in the endoplasmic reticulum and nuclear envelope of higher eukaryotes, but what it does and how changes caused by the DeltaGAG deletion lead to dystonia are not known. Here, we asked how the DYT1 mutation affects association of torsinA with interacting proteins. Using immunoprecipitation and mass spectrometry, we first established that the related transmembrane proteins LULL1 and LAP1 are prominent binding partners for torsinA in U2OS cells. Comparative analysis demonstrates that these two proteins are targeted to the endoplasmic reticulum or nuclear envelope by their divergent N-terminal domains. Binding of torsinA to their C-terminal lumenal domains is stabilized when residues in any one of three motifs implicated in ATP hydrolysis (Walker B, sensor 1, and sensor 2) are mutated. Importantly, the DeltaGAG deletion does not stabilize this binding. Indeed, deleting the DeltaGAG encoded glutamic acid residue from any of the three ATP hydrolysis mutants destabilizes their association with LULL1 and LAP1C, suggesting a possible basis for loss of torsinA function. Impaired interaction of torsinA with LULL1 and/or LAP1 may thus contribute to the development of dystonia.

Evolution of Na,K-ATPase beta m-subunit into a coregulator of transcription in placental mammals

Change in gene functions (gene cooption) is one of the key mechanisms of molecular evolution. Genes can acquire new functions via alteration in properties of encoded proteins and/or via changes in temporal or spatial regulation of expression. Here we demonstrate radical changes in the functions of orthologous ATP1B4 genes during evolution of vertebrates. Expression of ATP1B4 genes is brain-specific in teleost fishes, whereas it is predominantly muscle-specific in tetrapods. The encoded beta m-proteins in fish, amphibian, and avian species are beta-subunits of Na,K-ATPase located in the plasma membrane. In placental mammals beta m-proteins lost their ancestral functions, accumulate in nuclear membrane of perinatal myocytes, and associate with transcriptional coregulator Ski-interacting protein (SKIP). Through interaction with SKIP, eutherian beta m acquired new functions as exemplified by regulation of TGF-beta-responsive reporters and by augmentation of mRNA levels of Smad7, an inhibitor of TGF-beta signaling. Thus, orthologous vertebrate ATP1B4 genes represent an instance of gene cooption that created fundamental changes in the functional properties of the encoded proteins.

Mechanotransduction from the ECM to the genome: Are the pieces now in place?

A multitude of biochemical signaling processes have been characterized that affect gene expression and cellular activity. However, living cells often need to integrate biochemical signals with mechanical information from their microenvironment as they respond. In fact, the signals received by shape alone can dictate cell fate. This mechanotrasduction of information is powerful, eliciting proliferation, differentiation, or apoptosis in a manner dependent upon the extent of physical deformation. The cells internal "prestressed" structure and its "hardwired" interaction with the extra-cellular matrix (ECM) appear to confer this ability to filter biochemical signals and decide between divergent cell functions influenced by the nature of signals from the mechanical environment. In some instances mechanical signaling through the tissue microenvironment has been shown to be dominant over genomic defects, imparting a normal phenotype on cells that otherwise have transforming genetic lesions. This mechanical control of phenotype is postulated to have a central role in embryogenesis, tissue physiology as well as the pathology of a wide variety of diseases, including cancer. We will briefly review studies showing physical continuity between the external cellular microenvironment and the interior of the cell nucleus. Newly characterized structures, termed nuclear envelope lamina spanning complexes (NELSC), and their interactions will be described as part of a model for mechanical transduction of extracellular cues from the ECM to the genome.

A-type lamin networks in light of laminopathic diseases

Lamins are major structural components of the lamina providing mechanical support for the nuclear envelope in vertebrates. A subgroup of lamins, the A-type lamins, are only expressed in differentiated cells and serve important functions both at the nuclear envelope and in the nucleoplasm in higher order chromatin organization and gene regulation. Mutations in A-type lamins cause a variety of diseases from muscular dystrophy and lipodystrophy to systemic diseases such as premature ageing syndromes. The molecular basis of these diseases is still unknown. Here we summarize known interactions of A-type lamins with components of the nuclear envelope and the nucleoplasm and discuss their potential involvement in the etiology and molecular mechanisms of the diseases. Lamin binding partners involve chromatin proteins potentially involved in higher order chromatin organization, transcriptional regulators controlling gene expression during cell cycle progression, differentiation and senescence, and several enzymes involved in a multitude of functions.

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.

Loss of the dystonia-associated protein torsinA selectively disrupts the neuronal nuclear envelope

An enigmatic feature of many genetic diseases is that mutations in widely expressed genes cause tissue-specific illness. One example is DYT1 dystonia, a neurodevelopmental disease caused by an in-frame deletion (Deltagag) in the gene encoding torsinA. Here we show that neurons from both torsinA null (Tor1a(-/-)) and homozygous disease mutant "knockin" mice (Tor1a(Deltagag/Deltagag)) contain severely abnormal nuclear membranes, although non-neuronal cell types appear normal. These membrane abnormalities develop in postmigratory embryonic neurons and subsequently worsen with further neuronal maturation, a finding evocative of the developmental dependence of DYT1 dystonia. These observations demonstrate that neurons have a unique requirement for nuclear envelope localized torsinA function and suggest that loss of this activity is a key molecular event in the pathogenesis of DYT1 dystonia.

Proteome analysis of a rat liver nuclear insoluble protein fraction and localization of a novel protein, ISP36, to compartments in the interchromatin space

A rat liver nuclear insoluble protein fraction was analyzed to investigate candidate proteins participating in nuclear architecture formation. Proteins were subjected to two-dimensional separation by reversed-phase HPLC in 60% formic acid and SDS/PAGE. The method produced good resolution of insoluble proteins. One hundred and thirty-eight proteins were separated, and 28 of these were identified. The identified proteins included one novel protein, seven known nuclear proteins and 12 known nuclear matrix proteins. The novel 36 kDa protein was further investigated for its subnuclear localization. The human ortholog of the protein was expressed in Escherichia coli and antibodies were raised against the recombinant protein. Exclusive localization of the protein to the nuclear insoluble protein fraction was confirmed by cell fractionation followed by immunoblotting. Immunostaining of mouse C3H cells suggested that the 36 kDa protein was a constituent of an insoluble macromolecular complex spread throughout the interchromatin space of the nucleus. The protein was designated 'interchromatin space protein of 36 kDa', ISP36.

The AAA+ protein torsinA interacts with a conserved domain present in LAP1 and a novel ER protein

A glutamic acid deletion (ΔE) in the AAA+ protein torsinA causes DYT1 dystonia. Although the majority of torsinA resides within the endoplasmic reticulum (ER), torsinA binds a substrate in the lumen of the nuclear envelope (NE), and the ΔE mutation enhances this interaction. Using a novel cell-based screen, we identify lamina-associated polypeptide 1 (LAP1) as a torsinA-interacting protein. LAP1 may be a torsinA substrate, as expression of the isolated lumenal domain of LAP1 inhibits the NE localization of “substrate trap” EQ-torsinA and EQ-torsinA coimmunoprecipitates with LAP1 to a greater extent than wild-type torsinA. Furthermore, we identify a novel transmembrane protein, lumenal domain like LAP1 (LULL1), which also appears to interact with torsinA. Interestingly, LULL1 resides in the main ER. Consequently, torsinA interacts directly or indirectly with a novel class of transmembrane proteins that are localized in different subdomains of the ER system, either or both of which may play a role in the pathogenesis of DYT1 dystonia.

Autoimmune cytopenias associated with autoantibodies to nuclear envelope polypeptides

A subset of anti-nuclear autoantibodies (ANA) are directed against nuclear envelope (NE) polypeptides and display by indirect immunofluorescence (IIF) a ring-like fluorescent pattern. We report herein 19 patients with autoimmune cytopenias associated with antibodies (Abs) to NE polypeptides. Anti-NE specificity was determined by immunoblot, using NE preparations and purified lamina fractions. Eleven sera reacted with lamin B(1), and two reacted with both lamin B(1) and an unidentified 150-kDa protein (p150). One serum reacted with only p150. Four sera reacted with lamins A and C, and one reacted with and an unidentified 52-kDa NE polypeptide (p52). Autoimmune cytopenias included hemolytic anemia (7 cases), thrombocytopenia (13 cases), and neutropenia (6 cases). Five patients had 2 (3 cases) or 3 (2 cases) different cytopenias. Antiphospholipid antibodies (APLA) were detected in 14 patients, 2 of whom experienced thromboembolic events. A liver disorder was present in 7 patients. Systemic lupus erythematosus and lupus-like syndrome were diagnosed in 11 and 2 patients, respectively. Cytopenias responded to steroids alone (13 patients), or together with intravenous immunoglobulins (2 patients), or cyclophosphamide (2 patients). Two patients did not require treatment. Our results suggest that anti-NE Abs need to be sought for in patients with peripheral cytopenias, particularly when they are associated with APLA and/or liver disorders. Their detection strongly suggests an autoimmune process. Such cytopenias are often manifestations of a lupus or lupus-like disease and are responsive to steroids.

A Phosphorylation Cluster in the Chromatin-binding Region Regulates Chromosome Association of LAP2α

LAP2alpha is a LEM family protein associated with nucleoplasmic A-type lamins and chromatin in interphase. Like lamins and other lamina proteins LAP2alpha is cytoplasmic in metaphase, but it associates with chromosomes prior to nuclear envelope formation in late anaphase to telophase. In vitro phosphorylation analysis and mass spectrometry identified a cluster of at least three mitotic cyclin-dependent kinase 1 phosphorylation sites in the C-terminal chromatin-binding region of LAP2alpha as well as four additional potential sites in the cluster, some of which were targeted alternatively in LAP2alpha mutated at the major sites. LAP2alpha mutants containing serine --> alanine mutations at all seven sites revealed a clear phenotype. Mutated LAP2alpha remained associated with chromosomes throughout mitosis, but the dissociation of lamins into the cytoplasm and nuclear envelope disassembly were not affected. These data demonstrate the in vivo significance of mitotic phosphorylation for the dynamic behavior of LAP2alpha in the cell cycle and show that, unlike the interaction with lamins, the chromatin association of LAP2alpha is regulated by multiple mitosis-specific phosphorylation at sites clustered within a defined region in the C terminus of the protein.

The nuclear lamina and its functions in the nucleus

The nuclear lamina is a structure near the inner nuclear membrane and the peripheral chromatin. It is composed of lamins, which are also present in the nuclear interior, and lamin-associated proteins. The increasing number of proteins that interact with lamins and the compound interactions between these proteins and chromatin-associated proteins make the nuclear lamina a highly complex but also a very exciting structure. The nuclear lamina is an essential component of metazoan cells. It is involved in most nuclear activities including DNA replication, RNA transcription, nuclear and chromatin organization, cell cycle regulation, cell development and differentiation, nuclear migration, and apoptosis. Specific mutations in nuclear lamina genes cause a wide range of heritable human diseases. These diseases include Emery-Dreifuss muscular dystrophy, limb girdle muscular dystrophy, dilated cardiomyopathy (DCM) with conduction system disease, familial partial lipodystrophy (FPLD), autosomal recessive axonal neuropathy (Charcot-Marie-Tooth disorder type 2, CMT2), mandibuloacral dysplasia (MAD), Hutchison Gilford Progeria syndrome (HGS), Greenberg Skeletal Dysplasia, and Pelger-Huet anomaly (PHA). Genetic analyses in Caenorhabditis elegans, Drosophila, and mice show new insights into the functions of the nuclear lamina, and recent structural analyses have begun to unravel the molecular structure and assembly of lamins and their associated proteins.

Proteins that bind A-type lamins: integrating isolated clues

What do such diverse molecules as DNA, actin, retinoblastoma protein and protein kinase Cα all have in common? They and additional partners bind `A-type' lamins, which form stable filaments in animal cell nuclei. Mutations in A-type lamins cause a bewildering range of tissue-specific diseases, termed `laminopathies', including Emery-Dreifuss muscular dystrophy and the devastating Hutchinson-Gilford progeria syndrome, which mimics premature aging. Considered individually and collectively, partners for A-type lamins form four loose groups: architectural partners, chromatin partners, gene-regulatory partners and signaling partners. We describe 16 partners in detail, summarize their binding sites in A-type lamins, and sketch portraits of ternary complexes and functional pathways that might depend on lamins in vivo. On the basis of our limited current knowledge, we propose lamin-associated complexes with multiple components relevant to nuclear structure (e.g. emerin, nesprin 1α, actin) or signaling and gene regulation (e.g. LAP2α, retinoblastoma, E2F-DP heterodimers, genes) as `food for thought'. Testing these ideas will deepen our understanding of nuclear function and human disease.

Life at the edge: the nuclear envelope and human disease

A group of human diseases, known as 'laminopathies', are associated with defects in proteins of the nuclear envelope. Most laminopathy mutations have been mapped to the A-type lamin gene, which is expressed in most adult cell types. So, why should different mutations in a near-ubiquitously expressed gene be associated with various discrete tissue-restricted diseases? Attempts to resolve this paradox are uncovering new molecular interactions #151; both inside the nucleus and at its periphery -- which indicate that the nuclear envelope has functions that go beyond mere housekeeping.

Molecular cloning of one isotype of human lamina-associated polypeptide 1s and a topological analysis using its deletion mutants

LAP1s (lamina-associated polypeptide 1s) are type 2 integral membrane proteins with a single membrane-spanning region of the inner nuclear membrane. We report here on the cloning of the full-length cDNA of human LAP1B (huLAP1B) that encodes 584 amino acids. The sequence homology between the predicted rat LAP1B and huLAP1B was found to be 73.6%. A topological analysis was carried out by transiently expressing N-terminal GFP fused deletion mutants of huLAP1B in cells. The transmembrane (TM) domain (aa 346-368) is required for the localization of the nuclear and endoplasmic reticulum membrane and that the TM domain and the C-terminal half of the nucleoplasmic domain (aa 190-331) are sufficient for the proper localization of LAP1B. In contrast, the well-conserved lumenal domain of the nuclear membrane is not required for its topological function. Biochemical analysis showed that huLAP1B is retained within the nucleus via interactions of the nucleoplasmic portion with nuclear components.

The role of the nuclear envelope in Emery-Dreifuss muscular dystrophy

The X-linked form of Emery-Dreifuss muscular dystrophy (X-EDMD) is caused by absence, or greatly reduced amounts, of the inner nuclear-membrane protein, emerin. The autosomal dominant form (AD-EDMD) is caused by missense mutations in lamins A and C, two components of the nuclear lamina that interact directly with emerin. Lamin A/C mutations also cause one form of dilated cardiomyopathy (CMD1A) and one form of limb-girdle muscular dystrophy (LGMD1B), both of which have clinical features in common with EDMD, as well as a rare, unrelated form of lipodystrophy (FPLD). Evidence is now emerging that defective assembly of the nuclear lamina is a feature of all these diseases, although not necessarily the direct cause. Why only heart and skeletal muscle, and possibly connective tissue, are affected in EDMD and why expression of the disease is so extremely variable between individuals remains to be explained.

The reproductive importance of P-type ATPases

P-type ATPases are integral membrane proteins that use the free energy of ATP hydrolysis to generate transmembrane electrochemical ion gradients to support a variety of cellular processes. They have eight signature motifs, eight or ten transmembrane domains, highly conserved phosphorylation and ATP-binding sites, and similar hydropathic profiles. This review summarizes recent insights in the relationship of P-type ATPases to successful reproduction, and the hormone dependence of some family members. Because protein topology is central to understanding the pump action of this family of enzymes, this review also describes the dramatic change in the primary structure of one family member that may mediate transcription in the uterus.

The nuclear envelope in muscular dystrophy and cardiovascular diseases

Considerable interest has been focused on the nuclear envelope in recent years following the realization that several human diseases are linked to defects in genes encoding nuclear envelope specific proteins, most notably A-type lamins and emerin. These disorders, described as laminopathies or nuclear envelopathies, include both X-linked and autosomal dominant forms of Emery-Dreifuss muscular dystrophy, dilated cardiomyopathy with conduction system defects, limb girdle muscular dystrophy 1B with atrioventricular conduction disturbances, and Dunnigan-type familial partial lipodystrophy. Certain of these diseases are associated with nuclear structural abnormalities that can be seen in a variety of cells and tissues. These observations clearly demonstrate that A-type lamins in particular play a central role, not only in the maintenance of nuclear envelope integrity but also in the large-scale organization of nuclear architecture. What is not obvious, however, is why defects in nuclear envelope proteins that are found in most adult cell types should give rise to pathologies associated predominantly with skeletal and cardiac muscle and adipocytes. The recognition of these various disorders now raises the novel possibility that the nuclear envelope may have functions that go beyond housekeeping and which impact upon cell-type specific nuclear processes.

Nuclear envelope dynamics

The nuclear envelope (NE) provides a semi permeable barrier between the nucleus and cytoplasm and plays a central role in the regulation of macromolecular trafficking between these two compartments. In addition to this transport function, the NE is a key determinant of interphase nuclear architecture. Defects in NE proteins such as A-type lamins and the inner nuclear membrane protein, emerin, result in several human diseases that include cardiac and skeletal myopathies as well as lipodystrophy. Certain disease-linked A-type lamin defects cause profound changes in nuclear organization such as loss of peripheral heterochromatin and redistribution of other nuclear envelope components. While clearly essential in maintenance of nuclear integrity, the NE is a highly dynamic organelle. In interphase it is constantly remodeled to accommodate nuclear growth. During mitosis it must be completely dispersed so that the condensed chromosomes may gain access to the mitotic spindle. Upon completion of mitosis, dispersed NE components are reutilized in the assembly of nuclei within each daughter cell. These complex NE rearrangements are under precise temporal and spatial control and involve interactions with microtubules, chromatin, and a variety of cell-cycle regulatory molecules.Key words: nuclear envelope, lamin, nuclear pore complex, nuclear membranes, mitosis.

Nuclear membrane protein LAP2β mediates transcriptional repression alone and together with its binding partner GCL (germ-cell-less)

LAP2β is an integral membrane protein of the nuclear envelope involved in chromatin and nuclear architecture. Using the yeast two-hybrid system, we have cloned a novel LAP2β-binding protein, mGCL, which contains a BTB/POZ domain and is the mouse homologue of the Drosophila germ-cell-less (GCL) protein. In Drosophila embryos, GCL was shown to be essential for germ cell formation and was localized to the nuclear envelope. Here, we show that, in mammalian cells, GCL is co-localized with LAP2β to the nuclear envelope. Nuclear fractionation studies reveal that mGCL acts as a nuclear matrix component and not as an integral protein of the nuclear envelope. Recently, mGCL was found to interact with the DP3α component of the E2F transcription factor. This interaction reduced the transcriptional activity of the E2F-DP heterodimer, probably by anchoring the complex to the nuclear envelope. We demonstrate here that LAP2β is also capable of reducing the transcriptional activity of the E2F-DP complex and that it is more potent than mGCL in doing so. Co-expression of both LAP2β and mGCL with the E2F-DP complex resulted in a reduced transcriptional activity equal to that exerted by the pRb protein.

Lamina-associated polypeptide 2alpha binds intranuclear A-type lamins

The nucleoskeletal protein lamina-associated polypeptide 2(α) (LAP2*) contains a large, unique C terminus and differs significantly from its alternatively spliced, mostly membrane-integrated isoforms, such as LAP2beta. Unlike lamin B-binding LAP2beta, LAP2alpha was found by confocal immunofluorescence microscopy to colocalize preferentially with A-type lamins in the newly formed nuclei assembled after mitosis. While only a subfraction of lamins A and C (lamin A/C) was associated with the predominantly nuclear LAP2alpha in telophase, the majority of lamin A/C colocalized with LAP2alpha in G(1)-phase nuclei. Furthermore, selective disruption of A-type lamin structures by overexpression of lamin mutants in HeLa cells caused a redistribution of LAP2alpha. Coimmunoprecipitation experiments revealed that a fraction of lamin A/C formed a stable, SDS-resistant complex with LAP2alpha in interphase cells and in postmetaphase cell extracts. Blot overlay binding studies revealed a direct binding of LAP2alpha to exclusively A-type lamins and located the interaction domains to the C-terminal 78 amino acids of LAP2alpha and to residues 319–566 in lamin A/C, which include the C terminus of the rod and the entire tail common to lamin A/C. These findings suggest that LAP2alpha and A-type lamins cooperate in the organization of internal nuclear structures.