Modulation of cytoskeleton in cardiomyopathy caused by mutations in LMNA gene.

Dilated cardiomyopathy caused by mutations in LMNA, encoding A-type lamins (i.e., LMNA cardiomyopathy), is characterized by a left ventricle enlargement and ultimately results in poor cardiac contractility associated with conduction defects. Despite current strategies to aggressively manage the symptoms, the disorder remains a common cause of sudden death and heart failure with decreased ejection fraction. Patient care includes cardioverter defibrillator implantation but the last therapeutic option remains cardiac transplantation. A-type lamins are intermediate filaments and are the main components of the nuclear lamina, a meshwork underlying the inner nuclear membrane, which plays an essential role in both maintaining the nuclear structure and organizing the cytoskeletal structures within the cell. Cytoskeletal proteins function as scaffold to resist external mechanical stress. An increasing amount of evidence demonstrates that LMNA mutations can lead to disturbances in several structural and cytoskeletal components of the cell such as microtubules, actin cytoskeleton, and intermediate filaments. Collectively, this review focuses on the significance of these cytoskeletal modulators and emphasizes their potential therapeutic role in LMNA cardiomyopathy. Indeed, molecular tuning of cytoskeletal dynamics has been successfully used in preclinical models and provides adequate grounds for a therapeutic approach for patients with LMNA cardiomyopathy.

Redundant and Specific Roles of A-Type Lamins and Lamin B Receptor in Herpes Simplex Virus 1 Infection.

We investigated whether A-type lamins (lamin A/C) and lamin B receptor (LBR) are redundant during herpes simplex virus 1 (HSV-1) infection in HeLa cells expressing lamin A/C and LBR. Lamin A/C and LBR double knockout (KO) in HSV-1-infected HeLa cells significantly impaired expressions of HSV-1 early and late genes, maturation of replication compartments, marginalization of host chromatin to the nuclear periphery, enlargement of host cell nuclei, and viral DNA replication. Phenotypes of HSV-1-infected HeLa cells were restored by the ectopic expression of lamin A/C or LBR in lamin A/C and LBR double KO cells. Of note, lamin A/C single KO, but not LBR single KO, promoted the aberrant accumulation of virus particles outside the inner nuclear membrane (INM) and viral replication, as well as decreasing the frequency of virus particles inside the INM without affecting viral gene expression and DNA replication, time-spatial organization of replication compartments and host chromatin, and nuclear enlargement. These results indicated that lamin A/C and LBR had redundant and specific roles during HSV-1 infection. Thus, lamin A/C and LBR redundantly regulated the dynamics of the nuclear architecture, including the time-spatial organization of replication compartments and host chromatin, as well as promoting nuclear enlargement for efficient HSV-1 gene expression and DNA replication. In contrast, lamin A/C inhibited HSV-1 nuclear export through the INM during viral nuclear egress, which is a unique property of lamin A/C. IMPORTANCE This study demonstrated that lamin A/C and LBR had redundant functions associated with HSV-1 gene expression and DNA replication by regulating the dynamics of the nuclear architecture during HSV-1 infection. This is the first report to demonstrate the redundant roles of lamin A/C and LBR as well as the involvement of LBR in the regulation of these viral and cellular features in HSV-1-infected cells. These findings provide evidence for the specific property of lamin A/C to inhibit HSV-1 nuclear egress, which has long been considered but without direct proof.

Transient nuclear lamin A/C accretion aids in recovery from vapor nanobubble-induced permeabilisation of the plasma membrane.

Vapor nanobubble (VNB) photoporation is a physical method for intracellular delivery that has gained significant interest in the past decade. It has successfully been used to introduce molecular cargo of diverse nature into different cell types with high throughput and minimal cytotoxicity. For translational purposes, it is important to understand whether and how photoporation affects cell homeostasis. To obtain a comprehensive view on the transcriptional rewiring that takes place after VNB photoporation, we performed a longitudinal shotgun RNA-sequencing experiment. Six hours after photoporation, we found a marked upregulation of LMNA transcripts as well as their protein products, the A-type lamins. At the same time point, we observed a significant increase in several heterochromatin marks, suggesting a global stiffening of the nucleus. These molecular features vanished 24 h after photoporation. Since VNB-induced chromatin condensation was prolonged in LMNA knockout cells, A-type lamins may be required for restoring the nucleus to its original state. Selective depletion of A-type lamins reduced cell viability after VNB photoporation, while pharmacological stimulation of LMNA transcription increased the percentage of successfully transfected cells that survived after photoporation. Therefore, our results suggest that cells respond to VNB photoporation by temporary upregulation of A-type lamins to facilitate their recovery.

Proliferation of progeria cells is enhanced by lamina-associated polypeptide 2α (LAP2α) through expression of extracellular matrix proteins.

Lamina-associated polypeptide 2α (LAP2α) localizes throughout the nucleoplasm and interacts with the fraction of lamins A/C that is not associated with the peripheral nuclear lamina. The LAP2α-lamin A/C complex negatively affects cell proliferation. Lamins A/C are encoded by LMNA, a single heterozygous mutation of which causes Hutchinson-Gilford progeria syndrome (HGPS). This mutation generates the lamin A variant progerin, which we show here leads to loss of LAP2α and nucleoplasmic lamins A/C, impaired proliferation, and down-regulation of extracellular matrix components. Surprisingly, contrary to wild-type cells, ectopic expression of LAP2α in cells expressing progerin restores proliferation and extracellular matrix expression but not the levels of nucleoplasmic lamins A/C. We conclude that, in addition to its cell cycle-inhibiting function with lamins A/C, LAP2α can also regulate extracellular matrix components independently of lamins A/C, which may help explain the proliferation-promoting function of LAP2α in cells expressing progerin.

Nuclear envelope-related lipodystrophies.

Several alterations in nuclear envelope proteins building up the lamina meshwork beneath the inner nuclear membrane (mutations in lamins A/C, alterations of prelamin-A maturation, lamin B mutations or deregulation) have been shown to be responsible for or associated to human lipodystrophic syndromes. Lipodystrophic syndromes are rare and heterogeneous diseases, either genetic or acquired, characterized by generalized or partial fat atrophy associated with metabolic complications comprising insulin-resistant diabetes, dyslipidemia, and non-alcoholic fatty liver disease. Recent advances in the molecular genetics of different types of lipodystrophies generally pointed to primary adipocyte alterations leading to impaired adipogenesis and/or deregulation of the adipocyte lipid droplet. However, the precise mechanisms linking nuclear envelope abnormalities to lipodystrophies remain largely unknown. The phenotype of nuclear envelope-linked lipodystrophies ranges from the typical familial partial lipodystrophy of the Dunnigan type (FPLD2), due to heterozygous substitutions of the 482nd arginine of lamins A/C, to complex diseases that can combine lipodystrophy, metabolic complications, muscular or cardiac alterations and/or signs of accelerated aging. In this review we present the clinical, tissular and cellular characteristics of the nuclear envelope-linked lipodystrophies, as well as their hypothetical pathophysiological mechanisms.

A-type lamins involvement in transport and implications in cancer?

Nuclear lamins and transport are intrinsically linked, but their relationship is yet to be fully unraveled. A multitude of complex, coupled interactions between lamins and nucleoporins (Nups), which mediate active transport into and out of the nucleus, combined with well documented dysregulation of lamins in many cancers, suggests that lamins and nuclear transport may play a pivotal role in carcinogenesis and the preservation of cancer. Changes of function related to lamin/Nup activity can principally lead to DNA damage, further increasing the genetic diversity within a tumor, which could lead to the reduction the effectiveness of antineoplastic treatments. This review discusses and synthesizes different connections of lamins to nuclear transport and offers a number of outlook questions, the answers to which could reveal a new perspective on the connection of lamins to molecular transport of cancer therapeutics, in addition to their established role in nuclear mechanics.

Effect of genetic background on the cardiac phenotype in a mouse model of Emery-Dreifuss muscular dystrophy.

A-type lamins gene (LMNA) mutations cause an autosomal dominant inherited form of Emery-Dreifuss muscular dystrophy (EDMD). EDMD is characterized by slowly progressive muscle weakness and wasting and dilated cardiomyopathy, often leading to heart failure-related disability. EDMD is highly penetrant with poor prognosis and there is currently no specific therapy available. Clinical variability ranges from early onset with severe presentation in childhood to late onset with slow progression in adulthood. Genetic background is a well-known factor that significantly affects phenotype in several mouse models of human diseases. This phenotypic variability is attributed, at least in part, to genetic modifiers that regulate the disease process. To characterize the phenotype of A-type lamins mutation on different genetic background, we created and phenotyped C57BL/6JRj-Lmna H222P/H222P mice (C57 Lmna p.H222P) and compared them with the 129S2/SvPasCrl-Lmna H222P/H222P mice (129 Lmna p.H222P). These mouse strains were compared with their respective control strains at multiple time points between 3 and 10 months of age. Both contractile and electrical cardiac muscle functions, as well as survival were characterized. We found that 129 Lmna p.H222P mice showed significantly reduced body weight and reduced cardiac function earlier than in the C57 Lmna p.H222P mice. We also revealed that only 129 Lmna p.H222P mice developed heart arrhythmias. The 129 Lmna p.H222P model with an earlier onset and more pronounced cardiac phenotype may be more useful for evaluating therapies that target cardiac muscle function, and heart arrhythmias.

The Pathogenesis and Therapies of Striated Muscle Laminopathies.

Emery-Dreifuss muscular dystrophy (EDMD) is a genetic condition characterized by early contractures, skeletal muscle weakness, and cardiomyopathy. During the last 20 years, various genetic approaches led to the identification of causal genes of EDMD and related disorders, all encoding nuclear envelope proteins. By their respective localization either at the inner nuclear membrane or the outer nuclear membrane, these proteins interact with each other and establish a connection between the nucleus and the cytoskeleton. Beside this physical link, these proteins are also involved in mechanotransduction, responding to environmental cues, such as increased tension of the cytoskeleton, by the activation or repression of specific sets of genes. This ability of cells to adapt to environmental conditions is altered in EDMD. Increased knowledge on the pathophysiology of EDMD has led to the development of drug or gene therapies that have been tested on mouse models. This review proposed an overview of the functions played by the different proteins involved in EDMD and related disorders and the current therapeutic approaches tested so far.

Nurturing the genome: A-type lamins preserve genomic stability.

A-type lamins provide a scaffold for tethering chromatin and protein complexes regulating nuclear structure and function. Interest in lamins increased after mutations in the LMNA gene were found to be associated with a variety of human disorders termed laminopathies. These include muscular dystrophy, cardiomyopathy, lipodystrophy, peripheral neuropathy and premature aging syndromes such as progeria. In addition, altered expression of A-type lamins is emerging as a contributing factor to tumorigenesis. How different alterations in a gene that is ubiquitously expressed can cause such an array of systemic as well as tissue specific diseases remains an enigma. Several lines of evidence indicate that mutant forms of A-type lamins impact on genome function and integrity. A current model suggests that genomic instability plays a major part in the pathophysiology of some lamin-related diseases. However, this model remains to be fully investigated. Here we discuss recent studies revealing novel functions for A-type lamins in the maintenance of telomeres and in the DNA damage response (DDR) pathway. These findings have shed some light onto the putative molecular mechanisms by which alterations in A-type lamins induce genomic instability and contribute to disease.

Dynamic properties of meiosis-specific lamin C2 and its impact on nuclear envelope integrity.

A hallmark of meiosis is the precise pairing and the stable physical connection (synapsis) of the homologous chromosomes. These processes are essential prerequisite for their proper segregation. Pairing of the homologs during meiotic prophase I critically depends on characteristic movements of chromosomes. These movements, in turn, require attachment of meiotic telomeres to the nuclear envelope and their subsequent dynamic repositioning. Dynamic repositioning of meiotic telomeres goes along with profound structural reorganization of the nuclear envelope. The short A-type lamin C2 is thought to play a critical role in this process due to its specific expression during meiotic prophase I and the unique localization surrounding telomere attachments. Consistent with this notion, here we provide compelling evidence that meiosis-specific lamin C2 features a significantly increased mobility compared to somatic lamins as revealed by photobleaching techniques. We show that this property can be clearly ascribed to the lack of the N-terminal head and the significantly shorter α-helical coil domain. Moreover, expression of lamin C2 in somatic cells induces nuclear deformations and alters the distribution of the endogenous nuclear envelope proteins lamin B1, LAP2, SUN1 and SUN2. Together, our data define lamin C2 as a "natural lamin deletion mutant" that confers unique properties to the nuclear envelope which would be essential for dynamic telomere repositioning during meiotic prophase I.

D4Z4 as a prototype of CTCF and lamins-dependent insulator in human cells.

Using cellular models that mimic the organizations of the subtelomeric 4q35 locus found in patients affected with Facio-Scapulo-Humeral Dystrophy (FSHD) and in healthy individuals, we recently investigated the biological function of the D4Z4 macrosatellite in this subtelomeric context.We demonstrated that D4Z4 acts as a CTCF and A-type lamins dependent insulator element exhibiting both enhancer- blocking and barrier activities, and displaces a telomere towards the nuclear periphery. This peripheral positioning activity lies within a short sequence that interacts with CTCF and A-type lamins. Depletion in either of these two proteins suppresses these perinuclear activities, revealing the existence of a subtelomeric sequence that is sufficient to position an adjacent telomere to the nuclear periphery. We discuss here the biological implications of these results in the light of our current knowledge in related fields and the potential implication of other CTCF and A-type lamins insulators in the light of human pathologies.