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

LMNA-related muscular dystrophy involving myoblast proliferation and apoptosis through the FOXO1/GADD45A pathway

LMNA-related muscular dystrophy is a major disease phenotype causing mortality and morbidity in laminopathies, but its pathogenesis is still unclear. To explore the molecular pathogenesis, a knock-in mouse harbouring the Lmna-W520R mutation was modelled. Morphological and motor functional analyses showed that homozygous mutant mice revealed severe muscular atrophy, profound motor dysfunction, and shortened lifespan, while heterozygotes showed a variant arrangement of muscle bundles and mildly reduced motor capacity. Mechanistically, the FOXO1/GADD45A pathway involving muscle atrophy processes was found to be altered in vitro and in vivo assays. The expression levels of FOXO1 and its downstream regulatory molecule GADD45A significantly increased in atrophic muscle tissue. The elevated expression of FOXO1 was associated with decreased H3K27me3 in its gene promotor region. Overexpression of GADD45A induced apoptosis and cell cycle arrest of myoblasts in vitro, and it could be partially restored by the FOXO1 inhibitor AS1842856, which also slowed the muscle atrophy process with improved motor function and prolonged survival time of homozygous mutant mice in vivo. Notably, the inhibitor also partly rescued the apoptosis and cell cycle arrest of hiPSC-derived myoblasts harbouring the LMNA-W520R mutation. Together, these data suggest that the activation of the FOXO1/GADD45A pathway contributes to the pathogenesis of LMNA-related muscle atrophy, and it might serve as a potential therapeutic target for laminopathies.

Skeletal Muscle Satellite Cells Co-Opt the Tenogenic Gene Scleraxis to Instruct Regeneration

Skeletal muscles connect bones and tendons for locomotion and posture. Understanding the regenerative processes of muscle, bone and tendon is of importance to basic research and clinical applications. Despite their interconnections, distinct transcription factors have been reported to orchestrate each tissue's developmental and regenerative processes. Here we show that Scx expression is not detectable in adult muscle stem cells (also known as satellite cells, SCs) during quiescence. Scx expression begins in activated SCs and continues throughout regenerative myogenesis after injury. By SC-specific Scx gene inactivation (ScxcKO), we show that Scx function is required for SC expansion/renewal and robust new myofiber formation after injury. We combined single-cell RNA-sequencing and CUT&RUN to identify direct Scx target genes during muscle regeneration. These target genes help explain the muscle regeneration defects of ScxcKO, and are not overlapping with Scx -target genes identified in tendon development. Together with a recent finding of a subpopulation of Scx -expressing connective tissue fibroblasts with myogenic potential during early embryogenesis, we propose that regenerative and developmental myogenesis co-opt the Scx gene via different mechanisms.

Dysfunctional polycomb transcriptional repression contributes to lamin A/C-dependent muscular dystrophy

Lamin A is a component of the inner nuclear membrane that, together with epigenetic factors, organizes the genome in higher order structures required for transcriptional control. Mutations in the lamin A/C gene cause several diseases belonging to the class of laminopathies, including muscular dystrophies. Nevertheless, molecular mechanisms involved in the pathogenesis of lamin A-dependent dystrophies are still largely unknown. The polycomb group (PcG) of proteins are epigenetic repressors and lamin A interactors, primarily involved in the maintenance of cell identity. Using a murine model of Emery-Dreifuss muscular dystrophy (EDMD), we show here that lamin A loss deregulated PcG positioning in muscle satellite stem cells, leading to derepression of non-muscle-specific genes and p16INK4a, a senescence driver encoded in the Cdkn2a locus. This aberrant transcriptional program caused impairment in self-renewal, loss of cell identity, and premature exhaustion of the quiescent satellite cell pool. Genetic ablation of the Cdkn2a locus restored muscle stem cell properties in lamin A/C-null dystrophic mice. Our findings establish a direct link between lamin A and PcG epigenetic silencing and indicate that lamin A-dependent muscular dystrophy can be ascribed to intrinsic epigenetic dysfunctions of muscle stem cells.

Direct reprogramming of human smooth muscle and vascular endothelial cells reveals defects associated with aging and Hutchinson-Gilford progeria syndrome

Vascular dysfunctions are a common feature of multiple age-related diseases. However, modeling healthy and pathological aging of the human vasculature represents an unresolved experimental challenge. Here, we generated induced vascular endothelial cells (iVECs) and smooth muscle cells (iSMCs) by direct reprogramming of healthy human fibroblasts from donors of different ages and Hutchinson-Gilford Progeria Syndrome (HGPS) patients. iVECs induced from old donors revealed upregulation of GSTM1 and PALD1, genes linked to oxidative stress, inflammation and endothelial junction stability, as vascular aging markers. A functional assay performed on PALD1 KD VECs demonstrated a recovery in vascular permeability. We found that iSMCs from HGPS donors overexpressed bone morphogenetic protein (BMP)−4, which plays a key role in both vascular calcification and endothelial barrier damage observed in HGPS. Strikingly, BMP4 concentrations are higher in serum from HGPS vs. age-matched mice. Furthermore, targeting BMP4 with blocking antibody recovered the functionality of the vascular barrier in vitro, hence representing a potential future therapeutic strategy to limit cardiovascular dysfunction in HGPS. These results show that iVECs and iSMCs retain disease-related signatures, allowing modeling of vascular aging and HGPS in vitro.

PCAF Involvement in Lamin A/C-HDAC2 Interplay during the Early Phase of Muscle Differentiation

Lamin A/C has been implicated in the epigenetic regulation of muscle gene expression through dynamic interaction with chromatin domains and epigenetic enzymes. We previously showed that lamin A/C interacts with histone deacetylase 2 (HDAC2). In this study, we deepened the relevance and regulation of lamin A/C-HDAC2 interaction in human muscle cells. We present evidence that HDAC2 binding to lamina A/C is related to HDAC2 acetylation on lysine 75 and expression of p300-CBP associated factor (PCAF), an acetyltransferase known to acetylate HDAC2. Our findings show that lamin A and farnesylated prelamin A promote PCAF recruitment to the nuclear lamina and lamin A/C binding in human myoblasts committed to myogenic differentiation, while protein interaction is decreased in differentiating myotubes. Interestingly, PCAF translocation to the nuclear envelope, as well as lamin A/C-PCAF interaction, are reduced by transient expression of lamin A mutated forms causing Emery Dreifuss muscular dystrophy. Consistent with this observation, lamin A/C interaction with both PCAF and HDAC2 is significantly reduced in Emery-Dreifuss muscular dystrophy myoblasts. Overall, these results support the view that, by recruiting PCAF and HDAC2 in a molecular platform, lamin A/C might contribute to regulate their epigenetic activity required in the early phase of muscle differentiation.

Knockdown of LAP2α inhibits osteogenic differentiation of human adipose-derived stem cells by activating NF-κB

BACKGROUND

Lamina-associated polypeptide 2α (LAP2α) is a nucleoplasmic protein that has been involved in the regulation of the cell cycle, gene transcription, and adult stem cell function. LAP2α down-regulation is linked to age-related osteoporosis and bone deformities; however, the underlying mechanisms remain obscure. The present study aimed to elucidate the function of LAP2α in the osteogenic differentiation of human adipose-derived stem cells (hASCs), which are attractive sources for bone tissue engineering.

METHODS

The expression of LAP2α during the osteogenic differentiation of hASCs was detected firstly. A loss of function investigation was then carried out to characterize the function of LAP2α in osteogenic differentiation of hASCs both in vitro and in vivo. Moreover, RNA-sequences, western blotting, and confocal analyses were performed to clarify the molecular mechanism of LAP2α-regulated osteogenesis.

RESULTS

We found that LAP2α expression was upregulated upon osteogenic induction. Both in vitro and in vivo experiments indicated that LAP2α knockdown resulted in impaired osteogenic differentiation of hASCs. Mechanistically, we revealed that LAP2α deficiency activated nuclear factor kappa B (NF-κB) signaling by controlling the cytoplasmic-nuclear translocation of p65.

CONCLUSIONS

Collectively, our findings revealed that LAP2α functions as an essential regulator for osteogenesis of hASCs by modulating NF-κB signaling, thus providing novel insights for mesenchymal stem cell-mediated bone tissue engineering.

Photopolymerizable Hydrogel-Encapsulated Fibromodulin-Reprogrammed Cells for Muscle Regeneration

A central challenge in tissue engineering is obtaining a suitable cell type with a capable delivery vehicle to replace or repair damaged or diseased tissues with tissue mimics. Notably, for skeletal muscle tissue engineering, given the inadequate availability and regenerative capability of endogenous myogenic progenitor cells as well as the tumorigenic risks presented by the currently available pluri- and multipotent stem cells, seeking a safe regenerative cell source is urgently demanded. To conquer this problem, we previously established a novel reprogramming technology that can generate multipotent cells from dermal fibroblasts using a single protein, fibromodulin (FMOD). The yield FMOD-reprogrammed (FReP) cells exhibit exceeding myogenic capability without tumorigenic risk, making them a promising and safe cell source for skeletal muscle establishment. In addition to using the optimal cell for implantation, it is equally essential to maintain cellular localization and retention in the recipient tissue environment for critical-sized muscle tissue establishment. In this study, we demonstrate that the photopolymerizable methacrylated glycol chitosan (MeGC)/type I collagen (ColI)-hydrogel provides a desirable microenvironment for encapsulated FReP cell survival, spreading, extension, and formation of myotubes in the hydrogel three-dimensionally in vitro, without undesired osteogenic, chondrogenic, or tenogenic differentiation. Furthermore, gene profiling revealed a paired box 7 (PAX7) → myogenic factor 5 (MYF5) → myogenic determination 1 (MYOD1) → myogenin (MYOG) → myosin cassette elevation in the encapsulated FReP cells during myogenic differentiation, which is similar to that of the predominant driver of endogenous skeletal muscle regeneration, satellite cells. These findings constitute the evidence that the FReP cell-MeGC/ColI-hydrogel construct is a promising tissue engineering mimic for skeletal muscle generation in vitro, and thus possesses the extraordinary potential for further in vivo validation.

Role of RNA Binding Proteins with prion-like domains in muscle and neuromuscular diseases

A number of neuromuscular and muscular diseases, including amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA) and several myopathies, are associated to mutations in related RNA-binding proteins (RBPs), including TDP-43, FUS, MATR3 or hnRNPA1/B2. These proteins harbor similar modular primary sequence with RNA binding motifs and low complexity domains, that enables them to phase separate and create liquid microdomains. These RBPs have been shown to critically regulate multiple events of RNA lifecycle, including transcriptional events, splicing and RNA trafficking and sequestration. Here, we review the roles of these disease-related RBPs in muscle and motor neurons, and how their dysfunction in these cell types might contribute to disease.

Mechanotransduction, nuclear architecture and epigenetics in Emery Dreifuss Muscular Dystrophy: tous pour un, un pour tous

The alteration of the several roles that Lamin A/C plays in the mammalian cell leads to a broad spectrum of pathologies that – all together – are named laminopathies. Among those, the Emery Dreifuss Muscular Dystrophy (EDMD) is of particular interest as, despite the several known mutations of Lamin A/C, the genotype–phenotype correlation still remains poorly understood; this suggests that the epigenetic background of patients might play an important role during the time course of the disease. Historically, both a mechanical role of Lamin A/C and a regulative one have been suggested as the driving force of laminopathies; however, those two hypotheses are not mutually exclusive. Recent scientific evidence shows that Lamin A/C sustains the correct gene expression at the epigenetic level thanks to the Lamina Associated Domains (LADs) reorganization and the crosstalk with the Polycomb Group of Proteins (PcG). Furthermore, the PcG-dependent histone mark H3K27me3 increases under mechanical stress, finally pointing out the link between the mechano-properties of the nuclear lamina and epigenetics. Here, we summarize the emerging mechanisms that could explain the high variability seen in Emery Dreifuss muscular dystrophy.

Up-regulation of Toll-like receptors 7 and 9 and its potential implications in the pathogenic mechanisms of LMNA-related myopathies

Laminopathies are a heterogeneous group of diseases, caused by mutations in lamin A/C proteins. The most common laminopathy (LMNA-related myopathies, LMNA-RM) affects skeletal and cardiac muscles; muscle histopathology is variable, ranging from mild unspecific changes to dystrophic features, sometimes with inflammatory evidence. Whether the genetic defect might activate innate immune components, leading to chronic inflammation, myofiber necrosis and fibrosis, is still unknown. By qPCR, a significant up-regulation of Toll-like receptor (TLR) 7 and 9 transcripts was found in LMNA-RM compared to other myopathic and non-myopathic muscles. A marked TLR7/9 staining was observed on LMNA-RM blood vessels and muscle fibers and, when present, on infiltrating cells, mainly macrophages, scattered in the tissue or localized close to degenerated muscle fibers and connective tissue. Our results recognize innate immunity as a player in LMNA-RM pathogenesis. Modulation of TLR7/9 signaling pathways and decrease of macrophage-mediated inflammation might be potential therapeutic strategies in LMNA-RM management.

Abbreviations: DMD, Duchenne muscular dystrophy; EDMD2, Emery-Dreifuss muscular dystrophy type 2; FSHD, facio-scapulo-humeral muscular dystrophy; LGMD1B, limb-girdle muscular dystrophy type 1B; LMNA-CMD, LMNA-related congenital muscular dystrophy; LMNA-RM, LMNA-related myopathies; sIBM, sporadic inclusion body myositis; TLR, Toll-like receptor

A Muscle Hybrid Promoter as a Novel Tool for Gene Therapy

Gene therapy is a promising strategy to cure rare diseases. The lack of regulatory sequences ensuring specific and robust expression in skeletal and cardiac muscle is a substantial limitation of gene therapy efficiency targeting the muscle tissue. Here we describe a novel muscle hybrid (MH) promoter that is highly active in both skeletal and cardiac muscle cells. It has an easily exchangeable modular structure, including an intronic module that highly enhances the expression of the gene driven by it. In cultured myoblasts, myotubes, and cardiomyocytes, the MH promoter gives relatively stable expression as well as higher activity and protein levels than the standard CMV and desmin gene promoters or the previously developed synthetic or CKM-based promoters. Combined with AAV2/9, the MH promoter also provides a high in vivo expression level in skeletal muscle and the heart after both intramuscular and systemic delivery. It is much more efficient than the desmin-encoding gene promoter, and it maintains the same specificity. This novel promoter has potential for gene therapy in muscle cells. It can provide stable transgene expression, ensuring high levels of therapeutic protein, and limited side effects because of its specificity. This constitutes an improvement in the efficiency of genetic disease therapy.

Emerin Is Required for Proper Nucleus Reassembly after Mitosis: Implications for New Pathogenetic Mechanisms for Laminopathies Detected in EDMD1 Patients

Emerin is an essential LEM (LAP2, Emerin, MAN1) domain protein in metazoans and an integral membrane protein associated with inner and outer nuclear membranes. Mutations in the human EMD gene coding for emerin result in the rare genetic disorder: Emery–Dreifuss muscular dystrophy type 1 (EDMD1). This disease belongs to a broader group called laminopathies—a heterogeneous group of rare genetic disorders affecting tissues of mesodermal origin. EDMD1 phenotype is characterized by progressive muscle wasting, contractures of the elbow and Achilles tendons, and cardiac conduction defects. Emerin is involved in many cellular and intranuclear processes through interactions with several partners: lamins; barrier-to-autointegration factor (BAF), β-catenin, actin, and tubulin. Our study demonstrates the presence of the emerin fraction which associates with mitotic spindle microtubules and centrosomes during mitosis and colocalizes during early mitosis with lamin A/C, BAF, and membranes at the mitotic spindle. Transfection studies with cells expressing EGFP-emerin protein demonstrate that the emerin fusion protein fraction also localizes to centrosomes and mitotic spindle microtubules during mitosis. Transient expression of emerin deletion mutants revealed that the resulting phenotypes vary and are mutant dependent. The most frequent phenotypes include aberrant nuclear shape, tubulin network mislocalization, aberrant mitosis, and mislocalization of centrosomes. Emerin deletion mutants demonstrated different chromatin binding capacities in an in vitro nuclear assembly assay and chromatin-binding properties correlated with the strength of phenotypic alteration in transfected cells. Aberrant tubulin staining and microtubule network phenotype appearance depended on the presence of the tubulin binding region in the expressed deletion mutants. We believe that the association with tubulin might help to “deliver” emerin and associated membranes to decondensing chromatin. Preliminary analyses of cells from Polish patients with EDMD1 revealed that for several mutations thought to be null for emerin protein, a truncated emerin protein was present. We infer that the EDMD1 phenotype may be strengthened by the toxicity of truncated emerin expressed in patients with certain nonsense mutations in EMD.

Hutchinson-Gilford Progeria Syndrome-Current Status and Prospects for Gene Therapy Treatment

Hutchinson-Gilford progeria syndrome (HGPS) is one of the most severe disorders among laminopathies—a heterogeneous group of genetic diseases with a molecular background based on mutations in the LMNA gene and genes coding for interacting proteins. HGPS is characterized by the presence of aging-associated symptoms, including lack of subcutaneous fat, alopecia, swollen veins, growth retardation, age spots, joint contractures, osteoporosis, cardiovascular pathology, and death due to heart attacks and strokes in childhood. LMNA codes for two major, alternatively spliced transcripts, give rise to lamin A and lamin C proteins. Mutations in the LMNA gene alone, depending on the nature and location, may result in the expression of abnormal protein or loss of protein expression and cause at least 11 disease phenotypes, differing in severity and affected tissue. LMNA gene-related HGPS is caused by a single mutation in the LMNA gene in exon 11. The mutation c.1824C > T results in activation of the cryptic donor splice site, which leads to the synthesis of progerin protein lacking 50 amino acids. The accumulation of progerin is the reason for appearance of the phenotype. In this review, we discuss current knowledge on the molecular mechanisms underlying the development of HGPS and provide a critical analysis of current research trends in this field. We also discuss the mouse models available so far, the current status of treatment of the disease, and future prospects for the development of efficient therapies, including gene therapy for HGPS.

Skeletal Muscle Dystrophy mutant of lamin A alters the structure and dynamics of the Ig fold domain

Mutations in the different domains of A-type lamin proteins cause a diverse plethora of diseases collectively termed as laminopathies which can affect multiple organs. Ig fold is one such domain of lamin A which is implicated in numerous nuclear interactions wherein the mutations lead to different laminopathies. W514R is one such mutation in the Ig fold which leads to severe phenotypes in Skeletal Muscle Dystrophy (SMD) which is a class of laminopathies. In this report, we elucidated gross alterations in structure and dynamics at the level of individual amino acids. These studies indicate altered conformational features of residues in the close vicinity of W514. Imaging of mammalian cells transfected with the mutant have shown distinct perturbation of the nuclear meshwork with concomitant alteration in nuclear interactions as a result of increased oligomerization of Ig W514R. Hence, this novel approach of amalgamating theoretical and experimental procedures to predict the severity of a mutant in the context of laminopathies could be extended for numerous lamin A mutants.

Prioritization of Variants Detected by Next Generation Sequencing According to the Mutation Tolerance and Mutational Architecture of the Corresponding Genes

The biggest challenge geneticists face when applying next-generation sequencing technology to the diagnosis of rare diseases is determining which rare variants, from the dozens or hundreds detected, are potentially implicated in the patient’s phenotype. Thus, variant prioritization is an essential step in the process of rare disease diagnosis. In addition to conducting the usual in-silico analyses to predict variant pathogenicity (based on nucleotide/amino-acid conservation and the differences between the physicochemical features of the amino-acid change), three important concepts should be borne in mind. The first is the “mutation tolerance” of the genes in which variants are located. This describes the susceptibility of a given gene to any functional mutation and depends on the strength of purifying selection acting against it. The second is the “mutational architecture” of each gene. This describes the type and location of mutations previously identified in the gene, and their association with different phenotypes or degrees of severity. The third is the mode of inheritance (inherited vs. de novo) of the variants detected. Here, we discuss the importance of each of these concepts for variant prioritization in the diagnosis of rare diseases. Using real data, we show how genes, rather than variants, can be prioritized by calculating a gene-specific mutation tolerance score. We also illustrate the influence of mutational architecture on variant prioritization using five paradigmatic examples. Finally, we discuss the importance of familial variant analysis as final step in variant prioritization.

Spindle associated membrane protein 1 (Samp1) is required for the differentiation of muscle cells

Muscles are developed and regenerated in a differentiation process called myogenesis, which involves components of the nuclear envelope. We have investigated Samp1 (Spindle Associated Membrane Protein 1), a transmembrane nuclear envelope protein, which interacts with emerin and lamin A, both of which are linked to Emery-Dreifuss muscular dystrophy (EDMD). We found that the levels of Samp1 increased seven-fold during differentiation of mouse C2C12 muscle progenitor cells. To test if Samp1 could have a role in myogenesis we developed stable C2C12 knockdown cell lines expressing short hairpin RNA targeting Samp1 expression. The Samp1 depleted C2C12 cells displayed normal mobility and normal distribution of emerin and lamin A. However, Samp1 depletion increased ERK signaling and completely blocked differentiation of C2C12 cells, which failed to express myogenic marker proteins and failed to form myotubes. The block in myogenesis in Samp1 depleted cells was completely rescued by ectopic expression of RNAi resistant human Samp1, showing that Samp1 is required for muscle differentiation.

A Novel Lamin A Mutant Responsible for Congenital Muscular Dystrophy Causes Distinct Abnormalities of the Cell Nucleus

A-type lamins, the intermediate filament proteins participating in nuclear structure and function, are encoded by LMNA. LMNA mutations can lead to laminopathies such as lipodystrophies, premature aging syndromes (progeria) and muscular dystrophies. Here, we identified a novel heterozygous LMNA p.R388P de novo mutation in a patient with a non-previously described severe phenotype comprising congenital muscular dystrophy (L-CMD) and lipodystrophy. In culture, the patient’s skin fibroblasts entered prematurely into senescence, and some nuclei showed a lamina honeycomb pattern. C2C12 myoblasts were transfected with a construct carrying the patient’s mutation; R388P-lamin A (LA) predominantly accumulated within the nucleoplasm and was depleted at the nuclear periphery, altering the anchorage of the inner nuclear membrane protein emerin and the nucleoplasmic protein LAP2-alpha. The mutant LA triggered a frequent and severe nuclear dysmorphy that occurred independently of prelamin A processing, as well as increased histone H3K9 acetylation. Nuclear dysmorphy was not significantly improved when transfected cells were treated with drugs disrupting microtubules or actin filaments or modifying the global histone acetylation pattern. Therefore, releasing any force exerted at the nuclear envelope by the cytoskeleton or chromatin did not rescue nuclear shape, in contrast to what was previously shown in Hutchinson-Gilford progeria due to other LMNA mutations. Our results point to the specific cytotoxic effect of the R388P-lamin A mutant, which is clinically related to a rare and severe multisystemic laminopathy phenotype.

The effect of the lamin A and its mutants on nuclear structure, cell proliferation, protein stability, and mobility in embryonic cells

LMNA gene encodes for nuclear intermediate filament proteins lamin A/C. Mutations in this gene lead to a spectrum of genetic disorders, collectively referred to as laminopathies. Lamin A/C are widely expressed in most differentiated somatic cells but not in early embryos and some undifferentiated cells. To investigate the role of lamin A/C in cell phenotype maintenance and differentiation, which could be a determinant of the pathogenesis of laminopathies, we examined the role played by exogenous lamin A and its mutants in differentiated cell lines (HeLa, NHDF) and less-differentiated HEK 293 cells. We introduced exogenous wild-type and mutated (H222P, L263P, E358K D446V, and ∆50) lamin A into different cell types and analyzed proteins’ impact on proliferation, protein mobility, and endogenous nuclear envelope protein distribution. The mutants give rise to a broad spectrum of nuclear phenotypes and relocate lamin C. The mutations ∆50 and D446V enhance proliferation in comparison to wild-type lamin A and control cells, but no changes in exogenous protein mobility measured by FRAP were observed. Interestingly, although transcripts for lamins A and C are at similar level in HEK 293 cells, only lamin C protein is detected in western blots. Also, exogenous lamin A and its mutants, when expressed in HEK 293 cells underwent posttranscriptional processing. Overall, our results provide new insight into the maintenance of lamin A in less-differentiated cells. Embryonic cells are very sensitive to lamin A imbalance, and its upregulation disturbs lamin C, which may influence gene expression and many regulatory pathways.

Lmo7 is dispensable for skeletal muscle and cardiac function

Emery-Dreifuss muscular dystrophy (EDMD) is a degenerative disease primarily affecting skeletal muscles in early childhood as well as cardiac muscle at later stages. EDMD is caused by a number of mutations in genes encoding proteins associated with the nuclear envelope (e.g., Emerin, Lamin A/C, and Nesprin). Recently, a novel protein, Lim-domain only 7 (lmo7) has been reported to play a role in the molecular pathogenesis of EDMD. Prior in vitro and in vivo studies suggested the intriguing possibility that Lmo7 plays a role in skeletal or cardiac muscle pathophysiology. To further understand the in vivo role of Lmo7 in striated muscles, we generated a novel Lmo7-null (lmo7(-/-)) mouse line. Using this mouse line, we examined skeletal and cardiac muscle physiology, as well as the role of Lmo7 in a model of muscular dystrophy and regeneration using the dystrophin-deficient mdx mouse model. Our results demonstrated that lmo7(-/-) mice had no abnormalities in skeletal muscle morphology, physiological function, or regeneration. Cardiac function was also unaffected. Moreover, we found that ablation of lmo7 in mdx mice had no effect on the observed myopathy and muscular regeneration exhibited by mdx mice. Molecular analyses also showed no changes in dystrophin complex factors, MAPK pathway components, and Emerin levels in lmo7 knockout mice. Taken together, we conclude that Lmo7 is dispensable for skeletal muscle and cardiac physiology and pathophysiology.

Altered nuclear structure in myotonic dystrophy type 1-derived fibroblasts

Myotonic dystrophy type 1 (DM1) is a multisystem genetic disorder caused by a triplet nucleotide repeat expansion in the 3' untranslated region of the Dystrophia Myotonica-Protein Kinase (DMPK) gene. DMPK gene transcripts containing CUG expanded repeats accumulate in nuclear foci and ultimately cause altered splicing/gene expression of numerous secondary genes. The study of primary cell cultures derived from patients with DM1 has allowed the identification and further characterization of molecular mechanisms underlying the pathology in the natural context of the disease. In this study we show for the first time impaired nuclear structure in fibroblasts of DM1 patients. DM1-derived fibroblasts exhibited altered localization of the nuclear envelope (NE) proteins emerin and lamins A/C and B1 with concomitant increased size and altered shape of nuclei. Abnormal NE organization is more common in DM1 fibroblasts containing abundant nuclear foci, implying expression of the expanded RNA as determinant of nuclear defects. That transient expression of the DMPK 3' UTR containing 960 CTG but not with the 3' UTR lacking CTG repeats is sufficient to generate NE disruption in normal fibroblasts confirms the direct impact of mutant RNA on NE architecture. We also evidence nucleoli distortion in DM1 fibroblasts by immunostaining of the nucleolar protein fibrillarin, implying a broader effect of the mutant RNA on nuclear structure. In summary, these findings reveal that NE disruption, a hallmark of laminopathy disorders, is a novel characteristic of DM1.

Cytoskeleton and Adhesion in Myogenesis

The function of muscle is to contract, which means to exert force on a substrate. The adaptations required for skeletal muscle differentiation, from a prototypic cell, involve specialization of housekeeping cytoskeletal contracting and supporting systems into crystalline arrays of proteins. Here I discuss the changes that all three cytoskeletal systems (microfilaments, intermediate filaments, and microtubules) undergo through myogenesis. I also discuss their interaction, through the membrane, to extracellular matrix and to other cells, where force will be exerted during contraction. The three cytoskeletal systems are necessary for the muscle cell and must exert complementary roles in the cell. Muscle is a responsive system, where structure and function are integrated: the structural adaptations it undergoes depend on force production. In this way, the muscle cytoskeleton is a portrait of its physiology. I review the cytoskeletal proteins and structures involved in muscle function and focus particularly on their role in myogenesis, the process by which this incredible muscle machine is made. Although the focus is on skeletal muscle, some of the discussion is applicable to cardiac and smooth muscle.

Advances in basic and clinical research in laminopathies

Lamins (LMNA) are the main proteins of the nuclear lamina considered to be the ancestors of all intermediate filament proteins. They form complex protein assemblies with integral proteins of the inner nuclear membrane, transcriptional regulators, histones and chromatin modifiers. During recent years, interest in lamins has greatly increased due to the identification of many distinct heritable human disorders associated with lamin mutations. These disorders, collectively termed laminopathies, range from muscular dystrophies to premature aging. They may affect muscle, fat, bone, nerve and skin tissues. The workshop was addressed to understand lamin organization and its roles in nuclear processes, mutations in lamins affecting cell and tissues functions, the biology of the nucleus and laminopathic disease mechanisms, all aspects important for designing future therapies.