Laminopathies: multiple disorders arising from defects in nuclear architecture

Lamin A/C promotes DNA base excision repair

The A-type lamins (lamin A/C), encoded by the LMNA gene, are important structural components of the nuclear lamina. LMNA mutations lead to degenerative disorders known as laminopathies, including the premature aging disease Hutchinson-Gilford progeria syndrome. In addition, altered lamin A/C expression is found in various cancers. Reports indicate that lamin A/C plays a role in DNA double strand break repair, but a role in DNA base excision repair (BER) has not been described. We provide evidence for reduced BER efficiency in lamin A/C-depleted cells (Lmna null MEFs and lamin A/C-knockdown U2OS). The mechanism involves impairment of the APE1 and POLβ BER activities, partly effectuated by associated reduction in poly-ADP-ribose chain formation. Also, Lmna null MEFs displayed reduced expression of several core BER enzymes (PARP1, LIG3 and POLβ). Absence of Lmna led to accumulation of 8-oxoguanine (8-oxoG) lesions, and to an increased frequency of substitution mutations induced by chronic oxidative stress including GC>TA transversions (a fingerprint of 8-oxoG:A mismatches). Collectively, our results provide novel insights into the functional interplay between the nuclear lamina and cellular defenses against oxidative DNA damage, with implications for cancer and aging.

Molecular evolution of SUN-domain containing proteins in diverse plant species and their expression profiling in response to developmental and perturbation stimuli

SUN (Sad1/UNC-84) domain-containing proteins are highly conserved throughout evolution. They are localized to the inner membrane of the nuclear envelope and are involved in nuclear migration and nucleoskeleton formation. In the present study, a genome-wide investigation was performed in three dicotyledonous (Arabidopsis thaliana, Glycine max and Medicago truncatula) and three monocotyledonous (Oryza sativa, Zea mays and Sorghum bicolor) plants. A total of 56 SUN proteins encoded by 30 genes were identified. Based on their length, transmembrane topology, conserved domains and phylogenetic relationships, they could be divided into two previously defined groups- Cter-SUN and mid-SUN proteins. Expression of these genes was analyzed in different developmental stages, tissues and various unfavorable conditions such as salinity, drought, and hormonal treatment. Analyses indicated that the expression of SUN1/2 transcripts are ubiquitous; that of SUN3/4 are development/tissue regulated, and SUN5 are inflorescence stage-specific. This study provides an initial framework for the characterization and functional validation of the plant SUN family.

E3 ubiquitin ligase RNF123 targets lamin B1 and lamin-binding proteins

Lamins are key nuclear proteins which are important for maintaining nuclear structure and function. Mutations in lamins cause a spectrum of genetic diseases termed as laminopathies. RING finger containing E3 ubiquitin ligase, RNF123, is transcriptionally upregulated in cells expressing rod domain lamin A mutations. However, the functional relevance of RNF123 in laminopathic cells is not clear. Using a mass spectrometry-based approach, we identified lamins and lamin-binding proteins retinoblastoma protein (pRb), lamina-associated polypeptide 2α (LAP2α), and emerin as RNF123-interacting proteins. We determined that RNF123 mediated the ubiquitination of these proteins and caused the proteasomal degradation of pRb, LAP2α, and lamin B1. Furthermore, these proteins were also targeted for proteasomal degradation in cells expressing lamin A rod domain mutants G232E, Q294P, and R386K. Overexpression of RNF123 resulted in delayed transit through the S-phase which was alleviated by coexpression of pRb or LAP2α. Our findings imply that RNF123-mediated ubiquitination of lamin-binding proteins may contribute to disease-causing mechanisms in laminopathies by depletion of key nuclear proteins and defects in cell cycle kinetics.

Association of lamin A/C with muscle gene-specific promoters in myoblasts

The A-type and B-type lamins form a filamentous meshwork underneath the inner nuclear membrane called the nuclear lamina, which is an important component of nuclear architecture in metazoan cells. The lamina interacts with large, mostly repressive chromatin domains at the nuclear periphery. In addition, genome–lamina interactions also involve dynamic association of lamin A/C with gene promoters in adipocytes. Mutations in the human lamin A gene cause a spectrum of hereditary diseases called the laminopathies which affect muscle, cardiac and adipose tissues. Since most mutations in lamin A/C affect skeletal muscle, we investigated lamin–chromatin interactions at promoters of muscle specific genes in both muscle and non-muscle cell lines by ChIP-qPCR. We observed that lamin A/C was specifically associated with promoter regions of muscle genes in myoblasts but not in fibroblasts. Lamin A/C dissociated from the promoter regions of the differentiation specific MyoD, myogenin and muscle creatine kinase genes when myoblasts were induced to differentiate. In the promoter regions of the myogenin and MyoD genes, the binding of lamin A/C in myoblasts inversely correlated with the active histone mark, H3K4me3. Lamin A/C binding on muscle genes was reduced and differentiation potential was enhanced on treatment of myoblasts with a histone deacetylase inhibitor. These findings suggest a role for lamina–chromatin interactions in muscle differentiation and have important implications for the pathological mechanisms of striated muscle associated laminopathies.

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Lamina–chromatin interactions are important for nuclear processes.

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We show lamin A/C binding to promoters of muscle genes in myoblasts.

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Lamin A/C binding is reduced upon myoblast differentiation.

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Lamin A/C binding inversely correlates with active histone marks on muscle genes.

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Our findings suggest that lamin A/C binding to promoters is cell-type specific.

Mechanical regulation of mesenchymal stem cell differentiation

Biophysical cues play a key role in directing the lineage commitment of mesenchymal stem cells or multipotent stromal cells (MSCs), but the mechanotransductive mechanisms at play are still not fully understood. This review article first describes the roles of both substrate mechanics (e.g. stiffness and topography) and extrinsic mechanical cues (e.g. fluid flow, compression, hydrostatic pressure, tension) on the differentiation of MSCs. A specific focus is placed on the role of such factors in regulating the osteogenic, chondrogenic, myogenic and adipogenic differentiation of MSCs. Next, the article focuses on the cellular components, specifically integrins, ion channels, focal adhesions and the cytoskeleton, hypothesized to be involved in MSC mechanotransduction. This review aims to illustrate the strides that have been made in elucidating how MSCs sense and respond to their mechanical environment, and also to identify areas where further research is needed.

LMNA-associated cardiocutaneous progeria: an inherited autosomal dominant premature aging syndrome with late onset

Hutchinson-Gilford Progeria Syndrome (HGPS) is a premature aging disorder caused by mutations in LMNA, which encodes the nuclear scaffold proteins lamin A and C. In HGPS and related progerias, processing of prelamin A is blocked at a critical step mediated by the zinc metalloprotease ZMPSTE24. LMNA-linked progerias can be grouped into two classes: (1) the processing-deficient, early onset "typical" progerias (e.g., HGPS), and (2) the processing-proficient "atypical" progeria syndromes (APS) that are later in onset. Here we describe a previously unrecognized progeria syndrome with prominent cutaneous and cardiovascular manifestations belonging to the second class. We suggest the name LMNA-associated cardiocutaneous progeria syndrome (LCPS) for this disorder. Affected patients are normal at birth but undergo progressive cutaneous changes in childhood and die in middle age of cardiovascular complications, including accelerated atherosclerosis, calcific valve disease, and cardiomyopathy. In addition, the proband demonstrated cancer susceptibility, a phenotype rarely described for LMNA-based progeria disorders. The LMNA mutation that caused LCPS in this family is a heterozygous c.899A>G (p.D300G) mutation predicted to alter the coiled-coil domain of lamin A/C. In skin fibroblasts isolated from the proband, the processing and levels of lamin A and C are normal. However, nuclear morphology is aberrant and rescued by treatment with farnesyltransferase inhibitors, as is also the case for HGPS and other laminopathies. Our findings advance knowledge of human LMNA progeria syndromes, and raise the possibility that typical and atypical progerias may converge upon a common mechanism to cause premature aging disease.

Lamin A/C haploinsufficiency modulates the differentiation potential of mouse embryonic stem cells

Background

Lamins are structural proteins that are the major determinants of nuclear architecture and play important roles in various nuclear functions including gene regulation and cell differentiation. Mutations in the human lamin A gene cause a spectrum of genetic diseases that affect specific tissues. Most available mouse models for laminopathies recapitulate disease symptoms for muscle diseases and progerias. However, loss of human lamin A/C also has highly deleterious effects on fetal development. Hence it is important to understand the impact of lamin A/C expression levels on embryonic differentiation pathways.

Methodology and Principal Findings

We have investigated the differentiation potential of mouse embryonic stem cells containing reduced levels of lamin A/C by detailed lineage analysis of embryoid bodies derived from these cells by in vitro culture. We initially carried out a targeted disruption of one allele of the mouse lamin A/C gene (Lmna). Undifferentiated wild-type and Lmna^+/− embryonic stem cells showed similar expression of pluripotency markers and cell cycle profiles. Upon spontaneous differentiation into embryoid bodies, markers for visceral endoderm such as α-fetoprotein were highly upregulated in haploinsufficient cells. However, neuronal markers such as β-III tubulin and nestin were downregulated. Furthermore, we observed a reduction in the commitment of Lmna^+/− cells into the myogenic lineage, but no discernible effects on cardiac, adipocyte or osteocyte lineages. In the next series of experiments, we derived embryonic stem cell clones expressing lamin A/C short hairpin RNA and examined their differentiation potential. These cells expressed pluripotency markers and, upon differentiation, the expression of lineage-specific markers was altered as observed with Lmna^+/− embryonic stem cells.

Conclusions

We have observed significant effects on embryonic stem cell differentiation to visceral endoderm, neuronal and myogenic lineages upon depletion of lamin A/C. Hence our results implicate lamin A/C level as an important determinant of lineage-specific differentiation during embryonic development.

WITHDRAWN: Protein sumoylation and human diseases

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Oxidative stress induces an ATM-independent senescence pathway through p38 MAPK-mediated lamin B1 accumulation

We report crosstalk between three senescence-inducing conditions, DNA damage response (DDR) defects, oxidative stress (OS) and nuclear shape alterations. The recessive autosomal genetic disorder Ataxia telangiectasia (A-T) is associated with DDR defects, endogenous OS and premature ageing. Here, we find frequent nuclear shape alterations in A-T cells, as well as accumulation of the key nuclear architecture component lamin B1. Lamin B1 overexpression is sufficient to induce nuclear shape alterations and senescence in wild-type cells, and normalizing lamin B1 levels in A-T cells reciprocally reduces both nuclear shape alterations and senescence. We further show that OS increases lamin B1 levels through p38 Mitogen Activated Protein kinase activation. Lamin B1 accumulation and nuclear shape alterations also occur during stress-induced senescence and oncogene-induced senescence (OIS), two canonical senescence situations. These data reveal lamin B1 as a general molecular mediator that controls OS-induced senescence, independent of established Ataxia Telangiectasia Mutated (ATM) roles in OIS.

SUMOylation in carcinogenesis

SUMOylation is a post-translational modification characterized by covalent and reversible binding of small ubiquitin-like modifier (SUMO) to a target protein. In mammals, four different isoforms, termed SUMO-1, -2, -3 and -4 have been identified so far. SUMO proteins are critically involved in the modulation of nuclear organization and cell viability. Their expression is significantly increased in processes associated with carcinogenesis such as cell growth, differentiation, senescence, oxidative stress and apoptosis. Little is known about the role of SUMOylation in cancer development. Therefore the present review focuses on possible implications of SUMOylation in carcinogenesis highlighting its impact as an important regulatory cell cycle protein. Moreover, novel opportunities for therapeutic approaches are discussed. The differential expression levels, the target protein preferences and the function of the SUMO pathway in different cancer subtypes raises unexpected issues questioning our understanding of the implication of SUMO in carcinogenesis.

RefilinB (FAM101B) targets filamin A to organize perinuclear actin networks and regulates nuclear shape

The intracellular localization and shape of the nucleus plays a central role in cellular and developmental processes. In fibroblasts, nuclear movement and shape are controlled by a specific perinuclear actin network made of contractile actin filament bundles called transmembrane actin-associated nuclear (TAN) lines that form a structure called the actin cap. The identification of regulatory proteins associated with this specific actin cytoskeletal dynamic is a priority for understanding actin-based changes in nuclear shape and position in normal and pathological situations. Here, we first identify a unique family of actin regulators, the refilin proteins (RefilinA and RefilinB), that stabilize specifically perinuclear actin filament bundles. We next identify the actin-binding filamin A (FLNA) protein as the downstream effector of refilins. Refilins act as molecular switches to convert FLNA from an actin branching protein into one that bundles. In NIH 3T3 fibroblasts, the RefilinB/FLNA complex organizes the perinuclear actin filament bundles forming the actin cap. Finally, we demonstrate that in epithelial normal murine mammary gland (NmuMG) cells, the RefilinB/FLNA complex controls formation of a new perinuclear actin network that accompanies nuclear shape changes during the epithelial-mesenchymal transition (EMT). Our studies open perspectives for further functional analyses of this unique actin-based network and shed light on FLNA function during development and in human syndromes associated with FLNA mutations.

SUMO and its role in human diseases

The covalent attachment of small ubiquition-like modifier (SUMO) polypeptides, or sumoylation, is an important regulator of the functional properties of many proteins. Among these are many proteins implicated in human diseases including cancer and Huntington's, Alzheimer's, and Parkinson's diseases, as well as spinocerebellar ataxia 1 and amyotrophic lateral sclerosis. The results of two more recent studies identify two additional human disease-associated proteins that are sumoylated, amyloid precursor protein (APP), and lamin A. APP sumoylation modulates Aβ peptide levels, suggesting a potential role in Alzheimer's disease, and decreased lamin A sumoylation due to mutations near its SUMO site has been implicated in causing some forms of familial dilated cardiomyopathy.

Structure and expression of the maize (Zea mays L.) SUN-domain protein gene family: evidence for the existence of two divergent classes of SUN proteins in plants

BACKGROUND

The nuclear envelope that separates the contents of the nucleus from the cytoplasm provides a surface for chromatin attachment and organization of the cortical nucleoplasm. Proteins associated with it have been well characterized in many eukaryotes but not in plants. SUN (Sad1p/Unc-84) domain proteins reside in the inner nuclear membrane and function with other proteins to form a physical link between the nucleoskeleton and the cytoskeleton. These bridges transfer forces across the nuclear envelope and are increasingly recognized to play roles in nuclear positioning, nuclear migration, cell cycle-dependent breakdown and reformation of the nuclear envelope, telomere-led nuclear reorganization during meiosis, and karyogamy.

RESULTS

We found and characterized a family of maize SUN-domain proteins, starting with a screen of maize genomic sequence data. We characterized five different maize ZmSUN genes (ZmSUN1-5), which fell into two classes (probably of ancient origin, as they are also found in other monocots, eudicots, and even mosses). The first (ZmSUN1, 2), here designated canonical C-terminal SUN-domain (CCSD), includes structural homologs of the animal and fungal SUN-domain protein genes. The second (ZmSUN3, 4, 5), here designated plant-prevalent mid-SUN 3 transmembrane (PM3), includes a novel but conserved structural variant SUN-domain protein gene class. Mircroarray-based expression analyses revealed an intriguing pollen-preferred expression for ZmSUN5 mRNA but low-level expression (50-200 parts per ten million) in multiple tissues for all the others. Cloning and characterization of a full-length cDNA for a PM3-type maize gene, ZmSUN4, is described. Peptide antibodies to ZmSUN3, 4 were used in western-blot and cell-staining assays to show that they are expressed and show concentrated staining at the nuclear periphery.

CONCLUSIONS

The maize genome encodes and expresses at least five different SUN-domain proteins, of which the PM3 subfamily may represent a novel class of proteins with possible new and intriguing roles within the plant nuclear envelope. Expression levels for ZmSUN1-4 are consistent with basic cellular functions, whereas ZmSUN5 expression levels indicate a role in pollen. Models for possible topological arrangements of the CCSD-type and PM3-type SUN-domain proteins are presented.

Consensus statement on standard of care for congenital muscular dystrophies

Congenital muscular dystrophies are a group of rare neuromuscular disorders with a wide spectrum of clinical phenotypes. Recent advances in understanding the molecular pathogenesis of congenital muscular dystrophy have enabled better diagnosis. However, medical care for patients with congenital muscular dystrophy remains very diverse. Advances in many areas of medical technology have not been adopted in clinical practice. The International Standard of Care Committee for Congenital Muscular Dystrophy was established to identify current care issues, review literature for evidence-based practice, and achieve consensus on care recommendations in 7 areas: diagnosis, neurology, pulmonology, orthopedics/rehabilitation, gastroenterology/ nutrition/speech/oral care, cardiology, and palliative care. To achieve consensus on the care recommendations, 2 separate online surveys were conducted to poll opinions from experts in the field and from congenital muscular dystrophy families. The final consensus was achieved in a 3-day workshop conducted in Brussels, Belgium, in November 2009. This consensus statement describes the care recommendations from this committee.

Muscular laminopathies: role of prelamin A in early steps of muscle differentiation

Lamin A is a nuclear envelope constituent involved in a group of human disorders, collectively referred to as laminopathies, which include Emery-Dreifuss muscular dystrophy. Because increasing evidence suggests a role of lamin A precursor in nuclear functions, we investigated the processing of prelamin A along muscle differentiation. Both protein levels and cellular localization of prelamin A appears to be modulated during C2C12 mouse myoblasts activation. Similar changes also occur in the expression of two lamin A-binding proteins: emerin and LAP2α. Furthermore prelamin A forms a complex with LAP2α in differentiating myoblasts. Prelamin A accumulation in cycling myoblasts by expressing unprocessable mutants affects LAP2α and PCNA amount and increases caveolin 3 mRNA and protein levels, whilst accumulation of prelamin A in differentiated muscle cells following treatment with a farnesyl transferase inhibitor inhibits caveolin 3 expression. These data provide evidence for a critical role of lamin A precursor in the early steps of muscle cell differentiation. In fact the post-translational processing of prelamin A affects caveolin 3 expression and influences the myoblast differentiation process. Thus, altered lamin A processing could affect myoblast differentiation and/or muscle regeneration and might contribute to the myopathic phenotype.

Adipose tissue lamin A/C messenger RNA expression in women

Mutations in the lamin A/C gene (LMNA) cause lipodystrophy. However, little data are available on lamin A/C expression in various fat depots in women. We recruited 34 women scheduled for gynecologic surgery. Blood samples were collected on the morning of surgery to obtain a detailed lipid profile. Radiological examinations were performed to measure total body fat mass and abdominal fat accumulation. Fat samples were taken from the subcutaneous (SC) fat depot and from the greater omentum (OM) during the surgical procedure. Whole adipose tissue samples were used for total messenger RNA (mRNA) extraction and real-time polymerase chain reaction quantification of the LMNA transcript. No association was observed between lamin A/C mRNA expression, either in SC or OM fat tissue, and adiposity measures. Women with low SC lamin A/C expression, identified on the basis of the median value of SC lamin A/C mRNA expression, had a significantly altered lipid profile including lower levels of high-density lipoprotein (HDL) cholesterol and HDL(2) cholesterol and reduced HDL(2) cholesterol to HDL(3) cholesterol ratio (P < .05 for all). These women were also characterized by higher cholesterol to HDL cholesterol, low-density lipoprotein-triglycerides, very low-density lipoprotein-apolipoprotein B, and low-density lipoprotein cholesterol to HDL cholesterol (P < .05 for all). Low SC lamin A/C mRNA expression levels were also associated with significantly increased lipolysis in isolated fat cells from this fat depot. Specifically, the response to lipolytic agent isoproterenol was significantly increased at doses ranging from 10(-5) to 10(-10) mol/L (P < .05). A similar trend was observed in OM fat cells but did not reach significance. In conclusion, low lamin A/C expression in SC adipose tissue is associated with significant alterations in the lipid profile and increased fat cell lipolysis, independent of the level of total or abdominal adiposity.

Mechanobiology and the microcirculation: cellular, nuclear and fluid mechanics

Endothelial cells are stimulated by shear stress throughout the vasculature and respond with changes in gene expression and by morphological reorganization. Mechanical sensors of the cell are varied and include cell surface sensors that activate intracellular chemical signaling pathways. Here, possible mechanical sensors of the cell including reorganization of the cytoskeleton and the nucleus are discussed in relation to shear flow. A mutation in the nuclear structural protein lamin A, related to Hutchinson-Gilford progeria syndrome, is reviewed specifically as the mutation results in altered nuclear structure and stiffer nuclei; animal models also suggest significantly altered vascular structure. Nuclear and cellular deformation of endothelial cells in response to shear stress provides partial understanding of possible mechanical regulation in the microcirculation. Increasing sophistication of fluid flow simulations inside the vessel is also an emerging area relevant to the microcirculation as visualization in situ is difficult. This integrated approach to study--including medicine, molecular and cell biology, biophysics and engineering--provides a unique understanding of multi-scale interactions in the microcirculation.

Lamin A rod domain mutants target heterochromatin protein 1alpha and beta for proteasomal degradation by activation of F-box protein, FBXW10

BACKGROUND

Lamins are major structural proteins of the nucleus and contribute to the organization of various nuclear functions. Mutations in the human lamin A gene cause a number of highly degenerative diseases, collectively termed as laminopathies. Cells expressing lamin mutations exhibit abnormal nuclear morphology and altered heterochromatin organization; however, the mechanisms responsible for these defects are not well understood.

METHODOLOGY AND PRINCIPAL FINDINGS

The lamin A rod domain mutants G232E, Q294P and R386K are either diffusely distributed or form large aggregates in the nucleoplasm, resulting in aberrant nuclear morphology in various cell types. We examined the effects of these lamin mutants on the distribution of heterochromatin protein 1 (HP1) isoforms. HeLa cells expressing these mutants showed a heterogeneous pattern of HP1alpha and beta depletion but without altering HP1gamma levels. Changes in HP1alpha and beta were not observed in cells expressing wild-type lamin A or mutant R482L, which assembled normally at the nuclear rim. Treatment with proteasomal inhibitors led to restoration of levels of HP1 isoforms and also resulted in stable association of lamin mutants with the nuclear periphery, rim localization of the inner nuclear membrane lamin-binding protein emerin and partial improvement of nuclear morphology. A comparison of the stability of HP1 isoforms indicated that HP1alpha and beta displayed increased turnover and higher basal levels of ubiquitination than HP1gamma. Transcript analysis of components of the ubiquitination pathway showed that a specific F-box protein, FBXW10 was induced several-fold in cells expressing lamin mutants. Importantly, ectopic expression of FBXW10 in HeLa cells led to depletion of HP1alpha and beta without alteration of HP1gamma levels.

CONCLUSIONS

Mislocalized lamins can induce ubiquitin-mediated proteasomal degradation of certain HP1 isoforms by activation of FBXW10, a member of the F-box family of proteins that is involved in E3 ubiquitin ligase activity.

Manipulation and isolation of single cells and nuclei

The heterogeneous behavior of cells within a cell population makes measurements at the multicellular level insensitive to changes in single cells. Single-cell and single-nucleus analyses are therefore important to address this deficiency which will aid in the understanding of fundamental biology at both the cellular and subcellular levels. Recent technological advancements have enabled the development of new methodologies capable of handling these new challenges. This review highlights various techniques used in single-cell and single-nucleus manipulation and isolation. In particular, the applications related to microfluidics, electrical, optical, and physical methods will be discussed. Ultimately, it is hoped that these techniques will enable fundamental tests to be conducted on single cells and nuclei. One important potential outcome is that this will contribute not only towards detection and isolation of diseased cells but also more accurate diagnosis and prognosis of human diseases.

Mechanisms of muscle degeneration, regeneration, and repair in the muscular dystrophies

To withstand the rigors of contraction, muscle fibers have specialized protein complexes that buffer against mechanical stress and a multifaceted repair system that is rapidly activated after injury. Genetic studies first identified the mechanosensory signaling network that connects the structural elements of muscle and, more recently, have identified repair elements of muscle. Defects in the genes encoding the components of these systems lead to muscular dystrophy, a family of genetic disorders characterized by progressive muscle wasting. Although the age of onset, affected muscles, and severity vary considerably, all muscular dystrophies are characterized by muscle necrosis that overtakes the regenerative capacity of muscle. The resulting replacement of muscle by fatty and fibrous tissue leaves muscle increasingly weak and nonfunctional. This review discusses the cellular mechanisms that are primarily and secondarily disrupted in muscular dystrophy, focusing on membrane degeneration, muscle regeneration, and the repair of muscle.

A-type lamins and signaling: the PI 3-kinase/Akt pathway moves forward

Lamin A/C is a nuclear lamina constituent mutated in a number of human inherited disorders collectively referred to as laminopathies. The occurrence and significance of lamin A/C interplay with signaling molecules is an old question, suggested by pioneer studies performed in vitro. However, this relevant question has remained substantially unanswered, until data obtained in cellular and organismal models of laminopathies have indicated two main aspects of lamin A function. The first aspect is that lamins establish functional interactions with different protein platforms, the second aspect is that lamin A/C activity and altered function may elicit different effects in different cells and tissue types and even in different districts of the same tissue. Both these observations strongly suggest that signaling mechanisms targeting lamin A/C or its binding partners may regulate such a plastic behavior. A number of very recent data show involvement of kinases, as Akt and Erk, or phosphatases, as PP1 and PP2, in lamin A-linked cellular mechanisms. Moreover, altered activation of signaling in laminopathies and rescue of the pathological phenotype in animal models by inhibitors of signaling pathways, strongly suggest that signaling effectors related to lamin A/C may be implicated in the pathogenesis of laminopathies and may represent targets of therapeutic intervention. In face of such an open perspective of basic and applied research, we review current evidence of lamin A/C interplay with signaling molecules, with particular emphasis on the lamin A-Akt interaction and on the biological significance of their relationship.

Autosomal-dominant distal myopathy associated with a recurrent missense mutation in the gene encoding the nuclear matrix protein, matrin 3

Distal myopathies represent a heterogeneous group of inherited skeletal muscle disorders. One type of adult-onset, progressive autosomal-dominant distal myopathy, frequently associated with dysphagia and dysphonia (vocal cord and pharyngeal weakness with distal myopathy [VCPDM]), has been mapped to chromosome 5q31 in a North American pedigree. Here, we report the identification of a second large VCPDM family of Bulgarian descent and fine mapping of the critical interval. Sequencing of positional candidate genes revealed precisely the same nonconservative S85C missense mutation affecting an interspecies conserved residue in the MATR3 gene in both families. MATR3 is expressed in skeletal muscle and encodes matrin 3, a component of the nuclear matrix, which is a proteinaceous network that extends throughout the nucleus. Different disease related haplotype signatures in the two families provided evidence that two independent mutational events at the same position in MATR3 cause VCPDM. Our data establish proof of principle that the nuclear matrix is crucial for normal skeletal muscle structure and function and put VCPDM on the growing list of monogenic disorders associated with the nuclear proteome.

Sumoylation and human disease pathogenesis

Covalent modification by SUMO polypeptides, or sumoylation, is an important regulator of the functional properties of many proteins. Among these are several proteins implicated in human diseases including cancer, Huntington's, Alzheimer's, and Parkinson's diseases, as well as spinocerebellar ataxia 1 and amyotrophic lateral sclerosis. Recent reports reveal two new examples of human disease-associated proteins that are SUMO modified: amyloid precursor protein and lamin A. These findings point to a function for sumoylation in modulating amyloid-beta peptide levels, indicating a potential role in Alzheimer's disease, and for decreased lamin A sumoylation as a causative factor in familial dilated cardiomyopathy.

Deficiency of the E3 ubiquitin ligase TRIM32 in mice leads to a myopathy with a neurogenic component

Limb-girdle muscular dystrophy type 2H (LGMD2H) and sarcotubular myopathy are hereditary skeletal muscle disorders caused by mutations in TRIM32. We previously identified TRIM32 as an E3 ubiquitin ligase that binds to myosin and ubiquitinates actin. To date four TRIM32 mutations have been linked to LGMD2H, all of which occur in the C-terminal NHL domains. Unexpectedly, a fifth mutation in the B-box of TRIM32 causes a completely different, multisystemic disorder, Bardet-Biedl syndrome type 11. It is not understood how allelic mutations in TRIM32 can create such diverse phenotypic outcomes. To generate a tool for elucidating the complex in vivo functions of TRIM32, we created the first murine Trim32 knock-out model (T32KO). Histological analysis of T32KO skeletal muscles revealed mild myopathic changes. Electron microscopy showed areas with Z-line streaming and a dilated sarcotubular system with vacuoles -- the latter being a prominent feature of sarcotubular myopathy. Therefore, our model replicates phenotypes of LGMD2H and sarcotubular myopathy. The level of Trim32 expression in normal mouse brain exceeds that observed in skeletal muscle by more than 100 times, as we demonstrated by real-time PCR. Intriguingly, analysis of T32KO neural tissue revealed a decreased concentration of neurofilaments and a reduction in myelinated motoraxon diameters. The axonal changes suggest a shift toward a slower motor unit type. Not surprisingly, T32KO soleus muscle expressed an elevated type I slow myosin isotype with a concomitant reduction in the type II fast myosin. These data suggest that muscular dystrophy due to TRIM32 mutations involves both neurogenic and myogenic characteristics.

Sumoylation regulates lamin A function and is lost in lamin A mutants associated with familial cardiomyopathies

Lamin A mutations cause many diseases, including cardiomyopathies and Progeria Syndrome. The covalent attachment of small ubiquitin-like modifier (SUMO) polypeptides regulates the function of many proteins. Until now, no examples of human disease-causing mutations that occur within a sumoylation consensus sequence and alter sumoylation were known. We show that lamin A is sumoylated at lysine 201 and that two lamin A mutants associated with familial dilated cardiomyopathy, E203G and E203K, exhibit decreased sumoylation. E203 occupies the conserved +2 position in the sumoylation consensus ΨKXE. Lamin A mutants E203G, E203K, and K201R all exhibit a similar aberrant subcellular localization and are associated with increased cell death. Fibroblasts from an individual with the E203K lamin A mutation also exhibit decreased lamin A sumoylation and increased cell death. These results suggest that SUMO modification is important for normal lamin A function and implicate an involvement for altered sumoylation in the E203G/E203K lamin A cardiomyopathies.

Nuclear shape, mechanics, and mechanotransduction

In eukaryotic cells, the nucleus contains the genome and is the site of transcriptional regulation. The nucleus is the largest and stiffest organelle and is exposed to mechanical forces transmitted through the cytoskeleton from outside the cell and from force generation within the cell. Here, we discuss the effect of intra- and extracellular forces on nuclear shape and structure and how these force-induced changes could be implicated in nuclear mechanotransduction, ie, force-induced changes in cell signaling and gene transcription. We review mechanical studies of the nucleus and nuclear structural proteins, such as lamins. Dramatic changes in nuclear shape, organization, and stiffness are seen in cells where lamin proteins are mutated or absent, as in genetically engineered mice, RNA interference studies, or human disease. We examine the different mechanical pathways from the force-responsive cytoskeleton to the nucleus. We also highlight studies that link changes in nuclear shape with cell function during developmental, physiological, and pathological modifications. Together, these studies suggest that the nucleus itself may play an important role in the response of the cell to force.

The human granulocyte nucleus: Unusual nuclear envelope and heterochromatin composition

The human blood granulocyte (neutrophil) is adapted to find and destroy infectious agents. The nucleus of the human neutrophil has a segmented appearance, consisting of a linear or branched array of three or four lobes. Adequate levels of lamin B receptor (LBR) are necessary for differentiation of the lobulated nucleus. The levels of other components of the nuclear envelope may also be important for nuclear shape determination. In the present study, immunostaining and immunoblotting procedures explored the levels of various components of the nuclear envelope and heterochromatin, comparing freshly isolated human neutrophils with granulocytic forms of HL-60 cells, a tissue culture model system. In comparison to granulocytic HL-60 cells, blood neutrophil nuclear envelopes contain low-to-negligible amounts of LBR, lamins A/C, B1 and B2, LAP2beta and emerin. Surprisingly, a "mitotic" chromosome marker, H3(S10)phos, is elevated in neutrophil nuclei, compared to granulocytic HL-60 cells. Furthermore, neutrophil nuclei appear to be more fragile to methanol fixation, than observed with granulocytic HL-60 cells. Thus, the human neutrophil nucleus appears to be highly specialized, possessing a paucity of nuclear envelope-stabilizing proteins. In consequence, the neutrophil nucleus appears to be very malleable, supporting rapid migration through tight tissue spaces.

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.

Identification of cyclin D3 as a new interaction partner of lamin A/C

Lamin A/C is a major component of the nuclear lamina. An intact nuclear lamina has been proposed to be necessary for muscle differentiation. Cyclin D3 is known to be upregulated in differentiated muscle cells and to form insoluble complexes with cell-cycle regulatory factors in these cells. We have examined the possibility of direct binding interactions between lamin A/C and cyclin D3 by in vitro binding assays and co-immunoprecipitation studies with muscle cells. Our results indicate that cyclin D3 binds specifically to amino acid residues 383-474 of lamin A/C and associates with lamin A/C in muscle cells. The identification of cyclin D3 as a novel binding partner of lamin A/C has important implications for a role for lamin A/C in muscle differentiation.