Annexin-A6 in Membrane Repair of Human Skeletal Muscle Cell: A Role in the Cap Subdomain

Defects in membrane repair contribute to the development of some muscular dystrophies, highlighting the importance to decipher the membrane repair mechanisms in human skeletal muscle. In murine myofibers, the formation of a cap subdomain composed notably by annexins (Anx) is critical for membrane repair. We applied membrane damage by laser ablation to human skeletal muscle cells and assessed the behavior of annexin-A6 (AnxA6) tagged with GFP by correlative light and electron microscopy (CLEM). We show that AnxA6 was recruited to the site of membrane injury within a few seconds after membrane injury. In addition, we show that the deficiency in AnxA6 compromises human sarcolemma repair, demonstrating the crucial role played by AnxA6 in this process. An AnxA6-containing cap-subdomain was formed in damaged human myotubes in about one minute. Through transmission electron microscopy (TEM), we observed that extension of the sarcolemma occurred during membrane resealing, which participated in forming a dense lipid structure in order to plug the hole. By properties of membrane folding and curvature, AnxA6 helped in the formation of this tight structure. The compaction of intracellular membranes-which are used for membrane resealing and engulfed in extensions of the sarcolemma-may also facilitate elimination of the excess of lipid and protein material once cell membrane has been repaired. These data reinforce the role played by AnxA6 and the cap subdomain in membrane repair of skeletal muscle cells.

The Autophagy Regulatory Molecule CSRP3 Interacts with LC3 and Protects Against Muscular Dystrophy

CSRP3/MLP (cysteine-rich protein 3/muscle Lim protein), a member of the cysteine-rich protein family, is a muscle-specific LIM-only factor specifically expressed in skeletal muscle. CSRP3 is critical in maintaining the structure and function of normal muscle. To investigate the mechanism of disease in CSRP3 myopathy, we performed siRNA-mediated CSRP3 knockdown in chicken primary myoblasts. CSRP3 silencing resulted in the down-regulation of the expression of myogenic genes and the up-regulation of atrophy-related gene expressions. We found that CSRP3 interacted with LC3 protein to promote the formation of autophagosomes during autophagy. CSRP3-silencing impaired myoblast autophagy, as evidenced by inhibited autophagy-related ATG5 and ATG7 mRNA expression levels, and inhibited LC3II and Beclin-1 protein accumulation. In addition, impaired autophagy in CSRP3-silenced cells resulted in increased sensitivity to apoptosis cell death. CSRP3-silenced cells also showed increased caspase-3 and caspase-9 cleavage. Moreover, apoptosis induced by CSRP3 silencing was alleviated after autophagy activation. Together, these results indicate that CSRP3 promotes the correct formation of autophagosomes through its interaction with LC3 protein, which has an important role in skeletal muscle remodeling and maintenance.

Laminin-Coated Poly(Methyl Methacrylate) (PMMA) Nanofiber Scaffold Facilitates the Enrichment of Skeletal Muscle Myoblast Population

Myoblasts, the contractile cells of skeletal muscle, have been invaluable for fundamental studies of muscle development and clinical applications for muscle loss. A major limitation to the myoblast-based therapeutic approach is contamination with non-contractile fibroblasts, which overgrow during cell expansion. To overcome these limitations, this study was carried out to establish a 3D culture environment using nanofiber scaffolds to enrich the myoblast population during construct formation. Poly(methyl methacrylate) (PMMA) nanofiber (PM) scaffolds were fabricated using electrospinning techniques and coated with extracellular matrix (ECM) proteins, such as collagen or laminin, in the presence or absence of genipin. A mixed population of myoblasts and fibroblasts was isolated from human skeletal muscle tissues and cultured on plain surfaces, as well as coated and non-coated PM scaffolds. PMMA can produce smooth fibers with an average diameter of 360 ± 50 nm. Adsorption of collagen and laminin on PM scaffolds is significantly enhanced in the presence of genipin, which introduces roughness to the nanofiber surface without affecting fiber diameter and mechanical properties. It was also demonstrated that laminin-coated PM scaffolds significantly enhance myoblast proliferation (0.0081 ± 0.0007 h-1) and migration (0.26 ± 0.04 μm/min), while collagen-coated PM scaffolds favors fibroblasts proliferation (0.0097 ± 0.0009 h-1) and migration (0.23 ± 0.03 μm/min). Consequently, the myoblast population was enriched on laminin-coated PM scaffolds throughout the culture process. Therefore, laminin coating of nanofiber scaffolds could be a potential scaffold for the development of a tissue-engineered muscle substitute.

Distinctive Effects of Cytochalasin B in Chick Primary Myoblasts and Fibroblasts

Actin-based structures play fundamental roles in cellular functions. However it remains controversial how cells cope with the absence of F-actin structures. This report focuses on short- and long-term effects of cytochalasin B (CB) on actin-complexes in fibroblasts and myoblasts. Thirty min of CB treatment dispersed subplasma actin cortices, lamellipodia, ruffled membranes, stress fibers and adhesion plaques into actin patches in fibroblasts and muscle cells. In contrast, 72 hrs CB treatment showed distinct morphological effects. Fibroblasts became giant multinucleated-finger shaped with 5 to 10 protrusions, 3-8 μm in width, and >200 μm in length. They lacked cortical actin, stress fibers, adhesion plaques and ruffled membranes but contained immense lamelliopodia with abnormal adhesion plaque protein complexes. Muscle cells transformed into multinucleated globular-shaped but contained normal I-Z-I and A-bands, indicating that CB did not interfere with the assembly of myofibrils. Within 30 min after CB removal, finger-shaped fibroblasts returned to their original shape and actin-containing structures rapidly reappeared, whereas muscle cells respond slowly to form elongated myotubes following CB washout. The capacity to grow, complete several nuclear cycles, assemble intermediate filaments and microtubules without a morphologically recognizable actin cytoskeleton raises interesting issues related to the role of the actin compartments in eukaryotic cells.

Creating Hierarchical Topographies on Fibrous Platforms Using Femtosecond Laser Ablation for Directing Myoblasts Behavior

Developing an artificial extracellular matrix that closely mimics the native tissue microenvironment is important for use as both a cell culture platform for controlling cell fate and an in vitro model system for investigating the role of the cellular microenvironment. Electrospinning, one of the methods for fabricating structures that mimic the native ECM, is a promising technique for creating fibrous platforms. It is well-known that align or randomly distributed electrospun fibers provide cellular contact guidance in a single pattern. However, native tissues have hierarchical structures, i.e., topographies on the micro- and nanoscales, rather than a single structure. Thus, we fabricated randomly distributed nanofibrous (720 ± 80 nm in diameter) platforms via a conventional electrospinning process, and then we generated microscale grooves using a femtosecond laser ablation process to develop engineered fibrous platforms with patterned hierarchical topographies. The engineered fibrous platforms can regulate cellular adhesive morphology, proliferation, and distinct distribution of focal adhesion proteins. Furthermore, confluent myoblasts cultured on the engineered fibrous platforms revealed that the direction of myotube assembly can be controlled. These results indicate that our engineered fibrous platforms may be useful tools in investigating the roles of nano- and microscale topographies in the communication between cells and ECM.

Down-regulation of the mitochondrial matrix peptidase ClpP in muscle cells causes mitochondrial dysfunction and decreases cell proliferation

The caseinolytic peptidase P (ClpP) is the endopeptidase component of the mitochondrial matrix ATP-dependent ClpXP protease. ClpP degrades unfolded proteins to maintain mitochondrial protein homeostasis and is involved in the initiation of the mitochondrial unfolded protein response (UPR(mt)). Outside of an integral role in the UPR(mt), the cellular function of ClpP is not well characterized in mammalian cells. To investigate the role of ClpP in mitochondrial function, we generated C2C12 muscle cells that are deficient in ClpP using siRNA or stable knockdown using lentiviral transduction. Reduction of ClpP levels by ~70% in C2C12 muscle cells resulted in a number of mitochondrial alterations including reduced mitochondrial respiration and reduced oxygen consumption rate in response to electron transport chain (ETC) complex I and II substrates. The reduction in ClpP altered mitochondrial morphology, changed the expression level of mitochondrial fission protein Drp1 and blunted UPR(mt) induction. In addition, ClpP deficient cells showed increased generation of reactive oxygen species (ROS) and decreased membrane potential. At the cellular level, reduction of ClpP impaired myoblast differentiation, cell proliferation and elevated phosphorylation of eukaryotic initiation factor 2 alpha (eIF2α) suggesting an inhibition of translation. Our study is the first to define the effects of ClpP deficiency on mitochondrial function in muscle cells in vitro. In addition, we have uncovered novel effects of ClpP on mitochondrial morphology, cell proliferation and protein translation pathways in muscle cells.

Live imaging provides new insights on dynamic F-actin filopodia and differential endocytosis during myoblast fusion in Drosophila

The process of myogenesis includes the recognition, adhesion, and fusion of committed myoblasts into multinucleate syncytia. In the larval body wall muscles of Drosophila, this elaborate process is initiated by Founder Cells and Fusion-Competent Myoblasts (FCMs), and cell adhesion molecules Kin-of-IrreC (Kirre) and Sticks-and-stones (Sns) on their respective surfaces. The FCMs appear to provide the driving force for fusion, via the assembly of protrusions associated with branched F-actin and the WASp, SCAR and Arp2/3 pathways. In the present study, we utilize the dorsal pharyngeal musculature that forms in the Drosophila embryo as a model to explore myoblast fusion and visualize the fusion process in live embryos. These muscles rely on the same cell types and genes as the body wall muscles, but are amenable to live imaging since they do not undergo extensive morphogenetic movement during formation. Time-lapse imaging with F-actin and membrane markers revealed dynamic FCM-associated actin-enriched protrusions that rapidly extend and retract into the myotube from different sites within the actin focus. Ultrastructural analysis of this actin-enriched area showed that they have two morphologically distinct structures: wider invasions and/or narrow filopodia that contain long linear filaments. Consistent with this, formin Diaphanous (Dia) and branched actin nucleator, Arp3, are found decorating the filopodia or enriched at the actin focus, respectively, indicating that linear actin is present along with branched actin at sites of fusion in the FCM. Gain-of-function Dia and loss-of-function Arp3 both lead to fusion defects, a decrease of F-actin foci and prominent filopodia from the FCMs. We also observed differential endocytosis of cell surface components at sites of fusion, with actin reorganizing factors, WASp and SCAR, and Kirre remaining on the myotube surface and Sns preferentially taken up with other membrane proteins into early endosomes and lysosomes in the myotube.

CRF type 2 receptors mediate the metabolic effects of ghrelin in C2C12 cells

OBJECTIVE

Ghrelin is known to regulate appetite control and cellular metabolism. The corticotropin-releasing factor (CRF) family is also known to regulate energy balance. In this study, the links between ghrelin and the CRF family in C2C12 cells, a mouse myoblast cell line was investigated.

DESIGN AND METHODS

C2C12 cells were treated with ghrelin in the presence or absence of CRF receptor antagonists and then subjected to different metabolic analyses.

RESULTS

Ghrelin enhanced glucose uptake by C2C12 cells, induced GLUT4 translocation to the cell surface and decreased RBP4 expression. A CRF-R2 selective antagonist, anti-sauvagine-30, blocked ghrelin-induced glucose uptake, Ghrelin upregulated CRF-R2 but not CRF-R1 levels. Moreover, ghrelin-treated C2C12 cells displayed a cAMP and pERK activation in response to Ucn3, a CRF-R2 specific ligand, but not in response to CRF or stressin, CRF-R1 specific ligands. Ghrelin also induced UCP2 and UCP3 expression, which were blocked by anti- sauvagine-30. Ghrelin did not induce fatty acids uptake by C2C12 cells or ACC expression. Even though C2C12 cells clearly exhibited responses to ghrelin, the known ghrelin receptor, GHSR1a, was not detectable in C2C12 cells.

CONCLUSION

The results suggest that, ghrelin plays a role in regulating muscle glucose and, raise the possibility that suppression of the CRF-R2 pathway might provide benefits in high ghrelin states.

Boron nitride nanotube-mediated stimulation of cell co-culture on micro-engineered hydrogels

In this paper, we describe the effects of the combination of topographical, mechanical, chemical and intracellular electrical stimuli on a co-culture of fibroblasts and skeletal muscle cells. The co-culture was anisotropically grown onto an engineered micro-grooved (10 µm-wide grooves) polyacrylamide substrate, showing a precisely tuned Young’s modulus (∼ 14 kPa) and a small thickness (∼ 12 µm). We enhanced the co-culture properties through intracellular stimulation produced by piezoelectric nanostructures (i.e., boron nitride nanotubes) activated by ultrasounds, thus exploiting the ability of boron nitride nanotubes to convert outer mechanical waves (such as ultrasounds) in intracellular electrical stimuli, by exploiting the direct piezoelectric effect. We demonstrated that nanotubes were internalized by muscle cells and localized in both early and late endosomes, while they were not internalized by the underneath fibroblast layer. Muscle cell differentiation benefited from the synergic combination of topographical, mechanical, chemical and nanoparticle-based stimuli, showing good myotube development and alignment towards a preferential direction, as well as high expression of genes encoding key proteins for muscle contraction (i.e., actin and myosin). We also clarified the possible role of fibroblasts in this process, highlighting their response to the above mentioned physical stimuli in terms of gene expression and cytokine production. Finally, calcium imaging-based experiments demonstrated a higher functionality of the stimulated co-cultures.

A role for β-dystroglycan in the organization and structure of the nucleus in myoblasts

We recently characterized a nuclear import pathway for β-dystroglycan; however, its nuclear role remains unknown. In this study, we demonstrate for the first time, the interaction of β-dystroglycan with distinct proteins from different nuclear compartments, including the nuclear envelope (NE) (emerin and lamins A/C and B1), splicing speckles (SC35), Cajal bodies (p80-coilin), and nucleoli (Nopp140). Electron microscopy analysis revealed that β-dystroglycan localized in the inner nuclear membrane, nucleoplasm, and nucleoli. Interestingly, downregulation of β-dystroglycan resulted in both mislocalization and decreased expression of emerin and lamin B1, but not lamin A/C, as well in disorganization of nucleoli, Cajal bodies, and splicing speckles with the concomitant decrease in the levels of Nopp140, and p80-coilin, but not SC35. Quantitative reverse transcription PCR and cycloheximide-mediated protein arrest assays revealed that β-dystroglycan deficiency did not change mRNA expression of NE proteins emerin and lamin B1 bud did alter their stability, accelerating protein turnover. Furthermore, knockdown of β-dystroglycan disrupted NE-mediated processes including nuclear morphology and centrosome-nucleus linkage, which provides evidence that β-dystroglycan association with NE proteins is biologically relevant. Unexpectedly, β-dystroglycan-depleted cells exhibited multiple centrosomes, a characteristic of cancerous cells. Overall, these findings imply that β-dystroglycan is a nuclear scaffolding protein involved in nuclear organization and NE structure and function, and that might be a contributor to the biogenesis of nuclear envelopathies.

Human myoblasts from skeletal muscle biopsies: in vitro culture preparations for morphological and cytochemical analyses at light and electron microscopy

We describe protocols for the isolation of satellite cells from human muscle biopsies, for the in vitro culture of proliferating and differentiating myoblasts, and for the preparation of cell samples suitable for morphological and cytochemical analyses at light and electron microscopy. The procedures described are especially appropriate for processing small muscle biopsies, and allow obtaining myoblast/myotube monolayers on glass coverslips, thus preserving good cell morphology and immunoreactivity for protein markers of myoblast proliferation, differentiation, and senescence.These cell preparations are suitable for cytochemical, immunocytochemical, and FISH procedures at light microscopy, and can be observed not only in bright field, phase contrast, and differential interference contrast but also in fluorescence (which can hardly be used for cells grown on conventional plastic surfaces, which generally exhibit intense autofluorescence). In their ultrastructural cytochemical application, the protocols are intended for post-embedding techniques, by which ultrathin sections from a single sample may be used for detecting a wide variety of molecular markers.

Ultraviolet B (UVB) irradiation-induced apoptosis in various cell lineages in vitro

Ultraviolet B (UVB) radiation acts as a strong apoptotic trigger in many cell types, in tumor and normal cells. Several studies have demonstrated that UVB-induced cell death occurs through the generation of reactive oxygen species. The consequent oxidative stress includes the impairment of cellular antioxidants, the induction of DNA damage and the occurrence of apoptosis. In this review, we investigated UVB apoptotic action in various cell models by using ultrastructural, molecular and cytofluorimetric techniques. Myeloid leukemia HL-60, T-lymphoblastoid Molt-4 and myelomonocytic U937 human cells, generally affected by apoptotic stimuli, were studied. Human chondrocytes and C2C12 skeletal muscle cells, known to be more resistant to damage, were also considered. All of them, when exposed to UVB radiation, revealed a number of characteristic apoptotic markers. Membrane blebbing, cytoplasm shrinkage and chromatin condensation were detected by means of electron microscopy. DNA cleavage, investigated by using agarose gel electrophoresis and TUNEL reaction, was observed in suspended cells. Differently, in chondrocytes and in skeletal muscle cells, oligonucleosomic DNA fragmentation did not appear, even if a certain TUNEL positivity was detected. These findings demonstrate that UVB radiation appears to be an ideal tool to study the apoptotic behavior.

Fine-tuning of substrate architecture and surface chemistry promotes muscle tissue development

Tissue engineering has been increasingly brought to the scientific spotlight in response to the tremendous demand for regeneration, restoration or substitution of skeletal or cardiac muscle after traumatic injury, tumour ablation or myocardial infarction. In vitro generation of a highly organized and contractile muscle tissue, however, crucially depends on an appropriate design of the cell culture substrate. The present work evaluated the impact of substrate properties, in particular morphology, chemical surface composition and mechanical properties, on muscle cell fate. To this end, aligned and randomly oriented micron (3.3±0.8 μm) or nano (237±98 nm) scaled fibrous poly(ε-caprolactone) non-wovens were processed by electrospinning. A nanometer-thick oxygen functional hydrocarbon coating was deposited by a radio frequency plasma process. C2C12 muscle cells were grown on pure and as-functionalized substrates and analysed for viability, proliferation, spatial orientation, differentiation and contractility. Cell orientation has been shown to depend strongly on substrate architecture, being most pronounced on micron-scaled parallel-oriented fibres. Oxygen functional hydrocarbons, representing stable, non-immunogenic surface groups, were identified as strong triggers for myotube differentiation. Accordingly, the highest myotube density (28±15% of total substrate area), sarcomeric striation and contractility were found on plasma-coated substrates. The current study highlights the manifold material characteristics to be addressed during the substrate design process and provides insight into processes to improve bio-interfaces.

LKB1 destabilizes microtubules in myoblasts and contributes to myoblast differentiation

BACKGROUND

Skeletal muscle myoblast differentiation and fusion into multinucleate myotubes is associated with dramatic cytoskeletal changes. We find that microtubules in differentiated myotubes are highly stabilized, but premature microtubule stabilization blocks differentiation. Factors responsible for microtubule destabilization in myoblasts have not been identified.

FINDINGS

We find that a transient decrease in microtubule stabilization early during myoblast differentiation precedes the ultimate microtubule stabilization seen in differentiated myotubes. We report a role for the serine-threonine kinase LKB1 in both microtubule destabilization and myoblast differentiation. LKB1 overexpression reduced microtubule elongation in a Nocodazole washout assay, and LKB1 RNAi increased it, showing LKB1 destabilizes microtubule assembly in myoblasts. LKB1 levels and activity increased during myoblast differentiation, along with activation of the known LKB1 substrates AMP-activated protein kinase (AMPK) and microtubule affinity regulating kinases (MARKs). LKB1 overexpression accelerated differentiation, whereas RNAi impaired it.

CONCLUSIONS

Reduced microtubule stability precedes myoblast differentiation and the associated ultimate microtubule stabilization seen in myotubes. LKB1 plays a positive role in microtubule destabilization in myoblasts and in myoblast differentiation. This work suggests a model by which LKB1-induced microtubule destabilization facilitates the cytoskeletal changes required for differentiation. Transient destabilization of microtubules might be a useful strategy for enhancing and/or synchronizing myoblast differentiation.

An invasive podosome-like structure promotes fusion pore formation during myoblast fusion

Recent studies in Drosophila have implicated actin cytoskeletal remodeling in myoblast fusion, but the cellular mechanisms underlying this process remain poorly understood. Here we show that actin polymerization occurs in an asymmetric and cell type-specific manner between a muscle founder cell and a fusion-competent myoblast (FCM). In the FCM, a dense F-actin-enriched focus forms at the site of fusion, whereas a thin sheath of F-actin is induced along the apposing founder cell membrane. The FCM-specific actin focus invades the apposing founder cell with multiple finger-like protrusions, leading to the formation of a single-channel macro fusion pore between the two muscle cells. Two actin nucleation-promoting factors of the Arp2/3 complex, WASP and Scar, are required for the formation of the F-actin foci, whereas WASP but not Scar promotes efficient foci invasion. Our studies uncover a novel invasive podosome-like structure (PLS) in a developing tissue and reveal a previously unrecognized function of PLSs in facilitating cell membrane juxtaposition and fusion.

Heavy ion irradiation induces autophagy in irradiated C2C12 myoblasts and their bystander cells

Autophagy is one of the major processes involved in the degradation of intracellular materials. Here, we examined the potential impact of heavy ion irradiation on the induction of autophagy in irradiated C2C12 mouse myoblasts and their non-targeted bystander cells. In irradiated cells, ultrastructural analysis revealed the accumulation of autophagic structures at various stages of autophagy (i.e. phagophores, autophagosomes and autolysosomes) within 20 min after irradiation. Multivesicular bodies (MVBs) and autolysosomes containing MVBs (amphisomes) were also observed. Heavy ion irradiation increased the staining of microtubule-associated protein 1 light chain 3 and LysoTracker Red (LTR). Such enhanced staining was suppressed by an autophagy inhibitor 3-methyladenine. In addition to irradiated cells, bystander cells were also positive with LTR staining. Altogether, these results suggest that heavy ion irradiation induces autophagy not only in irradiated myoblasts but also in their bystander cells.

Foxj3 transcriptionally activates Mef2c and regulates adult skeletal muscle fiber type identity

The mechanisms that regulate skeletal muscle differentiation, fiber type diversity and muscle regeneration are incompletely defined. Forkhead transcription factors are critical regulators of cellular fate determination, proliferation, and differentiation. We identified a forkhead/winged helix transcription factor, Foxj3, which was expressed in embryonic and adult skeletal muscle. To define the functional role of Foxj3, we examined Foxj3 mutant mice. Foxj3 mutant mice are viable but have significantly fewer Type I slow-twitch myofibers and have impaired skeletal muscle contractile function compared to their wild type controls. In response to a severe injury, Foxj3 mutant mice have impaired muscle regeneration. Foxj3 mutant myogenic progenitor cells have perturbed cell cycle kinetics and decreased expression of Mef2c. Examination of the skeletal muscle 5' upstream enhancer of the Mef2c gene revealed an evolutionary conserved forkhead binding site (FBS). Transcriptional assays in C2C12 myoblasts revealed that Foxj3 transcriptionally activates the Mef2c gene in a dose dependent fashion and binds to the conserved FBS. Together, these studies support the hypothesis that Foxj3 is an important regulator of myofiber identity and muscle regeneration through the transcriptional activation of the Mef2c gene.

Valosin containing protein associated inclusion body myopathy: abnormal vacuolization, autophagy and cell fusion in myoblasts

Inclusion body myopathy associated with Paget's disease and frontotemporal dementia (IBMPFD) is caused by mutations in the valosin containing protein (VCP) gene. The disease is associated with progressive proximal muscle weakness, inclusions and vacuoles in muscle fibers, malfunction in the bone remodeling process resulting in Paget's disease, and premature frontotemporal dementia. VCP is involved in several cellular processes related to the endoplasmic reticulum associated degradation of proteins. To understand the pathological mechanisms underlying the myopathy in IBMPFD, we have studied the cellular consequences of VCP mutations in human primary myoblasts. Our results revealed that patients' myoblasts accumulate large vacuoles. Lysosomal membrane proteins Lamp1 and Lamp2 show increased molecular weights in patients' myoblasts due to differential N-glycosylation. Additionally, mutant myoblasts show increased autophagy when cultured in the absence of nutrients, as well as defective cell fusion and increased apoptosis. Our results elucidate that VCP mutations result in disturbances in several cellular processes, which will help us in the understanding of the pathological mechanisms resulting in muscle weakness and other features of VCP associated disease.

Grp94 is Tyr-phosphorylated by Fyn in the lumen of the endoplasmic reticulum and translocates to Golgi in differentiating myoblasts

The endoplasmic-reticulum chaperone Grp94 is required for the cell surface export of molecules involved in the native immune response, in mesoderm induction and muscle development, but the signals responsible for Grp94 recruitment are still obscure. Here we show for the first time that Grp94 undergoes Tyr-phosphorylation in differentiating myogenic C2C12 cells. By means of phospho-proteomic and immunoprecipitation analyses, and the use of Src-specific inhibitors we demonstrate that the Src-tyrosine-kinase Fyn becomes active early after induction of C2C12 cell differentiation, in parallel with the recruitment and the Tyr-phosphorylation of Grp94, which peaks at 6-hour differentiation. Grp94 is Tyr-phosphorylated inside the endoplasmic reticulum by a lumenal Fyn, as indicated by fluorescence and electronmicroscopy immunolocalization, co-immunoprecipitation after chemical cross-linking and by treatment of intact endoplasmic-reticulum vesicles with proteinase K. Furthermore, fractionation of cellular membrane compartments and double-immunofluorescence studies showed that Tyr-phosphorylation of Grp94 is necessary for the protein translocation from the endoplasmic reticulum to the Golgi apparatus. These results indicate that Fyn-catalyzed Tyr-phosphorylation of Grp94 is an event required to promote the chaperone export from the endoplasmic reticulum occurring in the early phase of myoblast differentiation.

Ultrastructural analysis of myoblast fusion in Drosophila

Myoblast fusion in Drosophila has become a powerful genetic system with which to unravel the mechanisms underlying cell fusion. The identification of important components of myoblast fusion by genetic analysis has led to a molecular pathway toward our understanding of this cellular process. In addition to the application of immunohistochemistry and live imaging techniques to visualize myoblast fusion at the light microscopic level, ultrastructural analysis using electron microscopy remains an indispensable tool to reveal fusion intermediates and specific membrane events at sites of fusion. In this chapter, we describe conventional chemical fixation and high-pressure freezing/freeze substitution methods for visualizing fusion intermediates during Drosophila myoblast fusion. Furthermore, we describe an immunoelectron microscopic method for localizing specific proteins relative to the fusion apparatus.

Myoblast fusion in Drosophila

Myogenic differentiation in Drosophila melanogaster, as in many other organisms, involves the generation of multinucleate muscle fibers through the fusion of myoblasts. Prior to fusion, the myoblasts become specified as one of two distinct cell types. They then become competent to fuse and express genes associated with cell recognition and adhesion. Initially, cell-type- specific adhesion molecules mediate recognition and fusion between these two distinct populations of myoblasts. Intracellular proteins that are essential for the fusion process are then recruited to points of cell-cell contact at the membrane, where the cell surface molecules have become localized. Many of these cytosolic proteins contribute to reorganization of the cytoskeleton through activation of small guanosine triphosphatases and recruitment of actin nucleating proteins. Following the initial fusion event, the ultimate size of the syncytia is achieved through multiple rounds of fusion between the developing syncytia and mononucleate myoblasts. Ultrastructural changes associated with cell fusion include recruitment of electron-dense vesicles to points of cell-cell contact, resolution of these vesicles into fusion plaques, fusion pore formation, and membrane vesiculation. This chapter reviews our current understanding of the genes, pathways, and ultrastructural events associated with fusion in the Drosophila embryo, giving rise to multinucleate syncytia that will be used throughout larval life.

Cellular diversity of human placental stem villi: an ultrastructural and immunohistochemical study

The aim of the study was to investigate the distribution and differentiation of cell types in the stroma of human placental stem villi (SV). A total of 14 human term placental tissues were studied. Double immunolabeling was performed for desmin-vimentin, desmin-alpha-smooth actin and vimentin-alpha-smooth actin. Cytokeratin 7, proliferating cell nuclear antigen immunolabeling was also performed. Parallel tissue samples were examined by transmission electron microscopy. HSCORE was performed for the semi-quantitative analysis of distribution of cells in the stroma of SV. Vimentin-labeled cells were mostly distributed in the subtrophoblastic area. Desmin-vimentin double immunolabeling was mainly localized in the triangular area and to a lesser degree in the perivascular area and vessel walls (p=or<0.001). However, desmin-alpha smooth actin labeling was observed predominantly in the vessel wall and perivascular area. Vimentin-alpha smooth actin immunoreactivity was significantly stronger in the triangular and perivascular areas compared to the vessel walls (p=0.003). Ultrastructurally, cells in the stroma of SV were mesenchyme cells, reticulum cells, fibroblasts, myofibroblasts, smooth muscle cells, and Hofbauer cells, filamented and vacuolated cells. The differentiation of myofibroblasts in the triangular and perivascular areas may play a role in maturation of SV and villous contractility, modulation of the intervillous space and this may have effects on maternofetal placental circulation.

Activation of p38/MEF2C pathway by all-trans retinoic acid in cardiac myoblasts

Myocyte enhancer factor 2C (MEF2C) is a transcription factor particularly expressed in cardiac muscle. While the effects of all-trans retinoic acid (atRA) on embryonic heart are well described, the mechanism of atRA action on MEF2C activity in cardiomyocytes is less known. The aim of the present study was to investigate whether and how atRA regulates MEF2C activity in H9c2 rat ventricular cells. Here, our results, obtained from Western blot and protein kinase assays, showed that the phosphorylation of p38 mitogen-activated protein kinase (MAPK) and MEF2C was induced by atRA in H9c2 myocardial cells. And the result from luciferase assays showed that the transactivation activity of MEF2C was upregulated by p38. Furthermore, using confocal microscopy and immunoprecipitation, we found that atRA hastened p38 translocation into nuclei to interact with MEF2C, and SB202190 inhibited nuclear translocation of p38. These results suggest that atRA may mediate p38/MEF2C signaling pathway during heart development.

Muscle intermediate filaments and their links to membranes and membranous organelles

Intermediate filaments (IFs) play a key role in the integration of structure and function of striated muscle, primarily by mediating mechanochemical links between the contractile apparatus and mitochondria, myonuclei, the sarcolemma and potentially the vesicle trafficking apparatus. Linkage of all these membranous structures to the contractile apparatus, mainly through the Z-disks, supports the integration and coordination of growth and energy demands of the working myocyte, not only with force transmission, but also with de novo gene expression, energy production and efficient protein and lipid trafficking and targeting. Desmin, the most abundant and intensively studied muscle intermediate filament protein, is linked to proper costamere organization, myoblast and stem cell fusion and differentiation, nuclear shape and positioning, as well as mitochondrial shape, structure, positioning and function. Similar links have been established for lysosomes and lysosome-related organelles, consistent with the presence of widespread links between IFs and membranous structures and the regulation of their fusion, morphology and stabilization necessary for cell survival.

Vital imaging of H9c2 myoblasts exposed to tert-butylhydroperoxide--characterization of morphological features of cell death

Background

When exposed to oxidative conditions, cells suffer not only biochemical alterations, but also morphologic changes. Oxidative stress is a condition induced by some pro-oxidant compounds, such as by tert-butylhydroperoxide (tBHP) and can also be induced in vivo by ischemia/reperfusion conditions, which is very common in cardiac tissue. The cell line H9c2 has been used as an in vitro cellular model for both skeletal and cardiac muscle. Understanding how these cells respond to oxidative agents may furnish novel insights into how cardiac and skeletal tissues respond to oxidative stress conditions. The objective of this work was to characterize, through vital imaging, morphological alterations and the appearance of apoptotic hallmarks, with a special focus on mitochondrial changes, upon exposure of H9c2 cells to tBHP.

Results

When exposed to tBHP, an increase in intracellular oxidative stress was detected in H9c2 cells by epifluorescence microscopy, which was accompanied by an increase in cell death that was prevented by the antioxidants Trolox and N-acetylcysteine. Several morphological alterations characteristic of apoptosis were noted, including changes in nuclear morphology, translocation of phosphatidylserine to the outer leaflet of the cell membrane, and cell blebbing. An increase in the exposure period or in tBHP concentration resulted in a clear loss of membrane integrity, which is characteristic of necrosis. Changes in mitochondrial morphology, consisting of a transition from long filaments to small and round fragments, were also detected in H9c2 cells after treatment with tBHP. Bax aggregates near mitochondrial networks were formed after short periods of incubation.

Conclusion

Vital imaging of alterations in cell morphology is a useful method to characterize cellular responses to oxidative stress. In the present work, we report two distinct patterns of morphological alterations in H9c2 cells exposed to tBHP, a pro-oxidant agent frequently used as model to induce oxidative stress. In particular, dynamic changes in mitochondrial networks could be visualized, which appear to be centrally involved in how these cells respond to oxidative stress. The data also indicate that the cause of H9c2 cell death following tBHP exposure is increased intracellular oxidative stress.

Blown fuse regulates stretching and outgrowth but not myoblast fusion of the circular visceral muscles in Drosophila

Circular visceral muscles of Drosophila are binuclear syncytia arising from fusion of two different kinds of myoblasts: a circular visceral founder cell and one visceral fusion-competent myoblast. In contrast to fusion leading to the somatic body-wall musculature, myoblast fusion for the circular visceral muscles does not result in massive syncytia but instead in syncytia interconnected with multiple cytoplasmic bridges, which differentiate into large web-shaped muscles. Here, we show that these syncytial circular visceral muscles build a gut-enclosing network with the interwoven longitudinal visceral muscles. At the ultrastructural level, during circular visceral myoblast fusion and the first step of somatic myoblast fusion prefusion complexes and electron-dense plaques were not detectable which was surprising as these structures are characteristic for the second step of somatic myoblast fusion. Moreover, we demonstrate that Blown fuse (Blow), a cytoplasmic protein essential for the second step of somatic myoblast fusion, plays a different role in circular visceral myogenesis. Blow is known to be essential for progression beyond the prefusion complex in the somatic mesoderm; however, analysis of blow mutants established that it has a restricted role in stretching and outgrowth of the syncytia in the circular visceral muscles. Furthermore, we also found that in the visceral mesoderm, Blow is expressed in both the fusion-competent myoblasts and circular visceral founders, while expression in the somatic mesoderm is initially restricted to fusion-competent myoblasts. We also demonstrate that different enhancer elements in the first intron of blow are responsible for this distinct expression pattern. Thus, we propose a model for Blow in which this protein is involved in at least two clearly differing processes during Drosophila muscle formation, namely somatic myoblast fusion on the one hand and stretching and outgrowth of circular visceral muscles on the other.

Cytoskeletal structure in cells harboring two mutations: R133C in NOTCH3 and 5650G>A in mitochondrial DNA

We have previously described a patient with cerebral autosomal-dominant arteriopathy with subcortical infarcts and leucoencephalopathy (CADASIL) caused by R133C mutation in NOTCH3 and with a concomitant myopathy caused by a G to A point mutation at base pair 5650 (5650G>A) in the gene encoding tRNA(Ala) in mitochondrial DNA (mtDNA). In the present study, we have examined the morphology of the cytoskeletal components in fibroblasts and myoblasts of this patient. Immunolabeling revealed that tubulin network was sparse and formed asters in these cells, whereas no changes were found in actin and vimentin networks in comparison to the control cell lines. Furthermore, mitochondria were less abundant and the branches of the mitochondrial network were reduced in number. Muscle histochemical analysis showed ragged red fibres (RRFs) and cytochrome c oxidase (COX)-negative fibres. The mean proportion of mtDNA with 5650G>A was lower in histologically normal muscle fibres than in the COX-negative fibres and in the RRFs. These findings suggest that 5650G>A is a pathogenic mtDNA mutation. However, the changes in tubulin network and mitochondrial distribution in patient fibroblasts and myoblasts cannot solely be explained by this mutation.

The pesticide methoxychlor decreases myotube formation in cell culture by slowing myoblast proliferation

We studied the effect of the estrogenic pesticide methoxychlor (MXC) on skeletal muscle development using C2C12 cell culture. Myoblast cultures were exposed to various concentrations of MXC at various times during the process of myoblast fusion into myotubes. We observed that MXC exposure decreased myotube formation. In addition, we observed myoblasts with cytoplasmic vacuoles in cultures exposed to MXC. Because cytoplasmic vacuoles can be characteristic of cell death, apoptosis assays and trypan blue exclusion assays were performed. We found no difference in the frequency of apoptosis or in the frequency of cell death for cultures exposed to MXC and untreated cultures. Collectively, these results indicate that MXC exposure decreases myotube formation without causing cell death. In contrast, when cell proliferation was assessed, untreated cultures had a myoblast proliferation rate 50% greater than cultures exposed to MXC. We conclude that MXC decreases myotube formation at least in part by slowing myoblast proliferation. Furthermore, we suggest that direct exposure to MXC could affect skeletal muscle development in animals or humans, in addition to the defects in reproductive development that have previously been reported.

In-vivo observation of cells with a combined high-resolution multiphoton–acoustic scanning microscope

We present a combined multiphoton-acoustic microscope giving collocated access to the local morphological as well as mechanical properties of living cells. Both methods relay on intrinsic contrast mechanisms and dispense with the need of staining. In the acoustic part of the microscope, a gigahertz ultrasound wave is generated by an acoustic lens and the reflected sound energy is detected by the identical lens in a confocal setup. The achieved lateral resolution is in the range of 1 mum. Contrast in the images arises mainly from the local absorption of sound in the cells related to viscose damping. Additionally, acoustic microscopy can access the sound speed as well as the acoustic impedance of the cell membrane and the cell shape, as it is an intrinsic volume scanning technique. The multiphoton image formation bases on the detection of autofluorescence due to endogenous fluorophores. The nonlinearity of two-photon absorption provides submicron lateral and axial resolution without the need of confocal optical detection. In addition, in the near-IR cell damages are drastically reduced in comparison with direct excitation in the visible or UV. The presented setup was aligned with a dedicated procedure to ensure identical image areas. Combined multiphoton/acoustic images of living myoblast cells are discussed focusing on the reliability of the method.

Small heat shock protein alphaB-crystallin binds to p53 to sequester its translocation to mitochondria during hydrogen peroxide-induced apoptosis

Apoptosis is a highly conserved procedure of cell death and occurs under various stimuli, including oxidative stress. A small heat shock protein, alphaB-crystallin, is found to process resistance to apoptosis in some cells and tissues. But the mechanisms under this protective role are not fully understood. In the present study, we reported the early protective role of alphaB-crystallin against hydrogen peroxide-induced apoptosis in mice myogenic C(2)C(12) cells. alphaB-Crystallin interacted with p53, a proapoptotic protein, during cell apoptosis and such protein interaction mainly occurred in the cytoplasm of the cells, suggesting that the interaction of alphaB-crystallin with p53 might prevent the translocation of p53 from cytoplasm to mitochondria. Hence, this study provides a hint that alphaB-crystallin affects on p53 mitochondrial translocation during oxidative stress-induced apoptosis.

Mitofusin 2 (Mfn2): a key player in insulin-dependent myogenesis in vitro

We have previously shown that mitochondrial activity increases in response to insulin in differentiating muscle cells. Moreover, the protein kinase kinase/extracellular-signal-regulated kinase (MAPKK/ERK-MEK) inhibitor PD98059 accelerates insulin-mediated myogenesis, whereas the phosphatidylinositol 3-kinase (PI3-K) inhibitor LY294002 or blockade of mitochondrial respiration abrogates insulin-mediated myogenesis. Our present study focuses on the mitochondrial transmembrane protein, hyperplasia suppressor gene/mitofusin2 (HSG/Mfn2), which regulates both mitochondrial fusion (as demonstrated by perinuclear mitochondria clustering) and insulin-dependent myogenesis in vitro. Increased mitochondrial length and interconnectivity are not observed after the inhibition of PI3-K activity with LY294002. Insulin induces Mfn2 and subunits I and IV of cytochrome-c oxidase (MTCOI and NCOIV) in L6 myoblasts. Inhibition of the MEK-dependent signalling pathway elevates the Mfn-2 protein level. The molecular mechanism of this phenomenon is unknown, although immunoprecipitation studies indicate that, during insulin-mediated myogenesis, Ras protein (an upstream activator of the MAPK/ERK1/2 cascade) interacts with HSG/Mfn2 in muscle cells. Interaction of Ras with Mfn2 continues unless insulin is present and is reduced after PD98059 co-treatment indicating that insulin-mediated myogenesis is increased by the inhibition of MEK, most probably by the lack of mitogenic signals opposing muscle differentiation. We conclude that insulin-mediated myogenesis depends on PI3-K activity, which stimulates mitochondrial activity and the extensive fusion of mitochondria. We further suggest that insulin stimulates the expression of Mfn2 protein, which in turn binds to Ras and inhibits the MEK-dependent signalling pathway. At the same time, the PI3-K-dependent signalling pathway is boosted, mitochondrial respiration increases and the rate of myogenesis is accelerated.

Mechanism of Ca2+ increase in myoblasts derived from chicken embryos

The mechanism of intracellular calcium ions (Ca(2+)) increase in chicken myoblasts was studied using histological, immunohistochemical, immunoblotting and Ca(2+) imaging techniques. Mononuclear myoblasts at embryonic day 12 (E12) contained myofibrils in the peripheral cytoplasm, and the sarcoplasmic reticulum was observed in the cytoplasm. Several Ca(2+)-related receptors, namely acetylcholine (ACh) receptors, dihydropyridine receptors (DHPRs) and ryanodine receptors (RyRs), were detected in the tissue as early as E12. Western blotting analyses detected one band corresponding to RyR subtype 3 (RyR3) at E12 and two bands corresponding to RyR1 and RyR3 after E13. Ca(2+) imaging of mononuclear myoblasts in vitro revealed an intense Ca(2+)-increase response to ACh stimulation, and this effect was abolished after EGTA addition to the culture medium. Nifedipine treatment also led to a lack of Ca(2+) increase in response to ACh stimulation, while ryanodine treatment led to a weak Ca(2+)-increase response. On the other hand, multinuclear myoblasts showed a Ca(2+)-increase response to ACh stimulation in the presence of not only EGTA but also nifedipine, although ryanodine treatment led to a lack of Ca(2+) increase. These results suggest that the mechanism of Ca(2+) increase in mononuclear myoblasts involves extracellular Ca(2+) entry through DHPRs, which is amplified by Ca(2+) release from the cytoplasmic Ca(2+) store, while multinuclear myoblasts mainly depend on Ca(2+) release from the cytoplasmic Ca(2+) store.

Co-electrospun poly(lactide-co-glycolide), gelatin, and elastin blends for tissue engineering scaffolds

In this study, we describe composite scaffolds composed of synthetic and natural materials with physicochemical properties suitable for tissue engineering applications. Fibrous scaffolds were co-electrospun from a blend of a synthetic biodegradable polymer (poly(lactic-co-glycolic acid), PLGA, 10% solution) and two natural proteins, gelatin (denatured collagen, 8% solution) and alpha-elastin (20% solution) at ratios of 3:1:2 and 2:2:2 (v/v/v). The resulting PLGA-gelatin-elastin (PGE) fibers were homogeneous in appearance with an average diameter of 380 +/- 80 nm, which was considerably smaller than fibers made under identical conditions from the starting materials (PLGA, 780 +/- 200 nm; gelatin, 447 +/- 123 nm; elastin, 1060 +/- 170 nm). Upon hydration, PGE fibers swelled to an average fiber diameter of 963 +/- 132 nm, but did not disintegrate. Importantly, PGE scaffolds were stable in an aqueous environment without crosslinking and were more elastic than those made of pure elastin fibers. To investigate the cytocompatibility of PGE, we cultured H9c2 rat cardiac myoblasts and rat bone marrow stromal cells (BMSCs) on fibrous PGE scaffolds. We found that myoblasts grew equally as well or slightly better on the scaffolds than on tissue-culture plastic. Microscopic evaluation confirmed that myoblasts reached confluence on the scaffold surfaces while simultaneously growing into the scaffolds. Histological characterization of the PGE constructs indicated that BMSCs penetrated into the center of scaffolds and began proliferating shortly after seeding. Our results suggest that fibrous scaffolds made of PGE and similar biomimetic blends of natural and synthetic polymers may be useful for engineering soft tissues, such as heart, lung, and blood vessels.

Non-muscle myosins 2A and 2B drive changes in cell morphology that occur as myoblasts align and fuse

The interaction of non-muscle myosins 2A and 2B with actin may drive changes in cell movement, shape and adhesion. To investigate this, we used cultured myoblasts as a model system. These cells characteristically change shape from triangular to bipolar when they form groups of aligned cells. Antisense oligonucleotide knockdown of non-muscle myosin 2A, but not non-muscle myosin 2B, inhibited this shape change, interfered with cell-cell adhesion, had a minor effect on tail retraction and prevented myoblast fusion. By contrast, non-muscle myosin 2B knockdown markedly inhibited tail retraction, increasing cell length by over 200% by 72 hours compared with controls. In addition it interfered with nuclei redistribution in myotubes. Non-muscle myosin 2C is not involved as western analysis showed that it is not expressed in myoblasts, but only in myotubes. To understand why non-muscle myosins 2A and 2B have such different roles, we analysed their distributions by immuno-electron microscopy, and found that non-muscle myosin 2A was more tightly associated with the plasma membrane than non-muscle myosin 2B. This suggests that non-muscle myosin 2A is more important for bipolar shape formation and adhesion owing to its preferential interaction with membrane-associated actin, whereas the role of non-muscle myosin 2B in retraction prevents over-elongation of myoblasts.

A seeding device for tissue engineered tubular structures

One of the challenges in the tissue engineering of tubular tissues and organs is the efficient seeding of porous scaffolds with the desired cell type and density in a short period of time, without affecting cell viability. Though different seeding techniques have been investigated, a fast, reproducible, and efficient bulk seeding method with uniform cellular distribution has yet to be reported. In this paper, a novel seeding device utilizing the synergistic effects of vacuum, centrifugal force and flow has been developed and analyzed. The device allows porous tubular scaffolds to be uniformly bulk seeded as well as luminally surface-seeded with cells. Porous tubular polymer scaffolds were bulk and surface-seeded with cell suspensions, and cell viability and seeding efficiency were subsequently assessed. A rigorous quantitative analysis of shear stresses acting on the cells during the seeding process, and of cell location within the scaffolds following seeding was also performed. Our results showed that the scaffolds were uniformly seeded along the longitudinal and circumferential directions within the tube wall without affecting cell viability or exposing them to excessive shear stresses.

Dysfunction of endocytic and autophagic pathways in a lysosomal storage disease

OBJECTIVE

To understand the mechanisms of skeletal muscle destruction and resistance to enzyme replacement therapy in Pompe disease, a deficiency of lysosomal acid alpha-glucosidase (GAA), in which glycogen accumulates in lysosomes primarily in cardiac and skeletal muscles.

METHODS

We have analyzed compartments of the lysosomal degradative pathway in GAA-deficient myoblasts and single type I and type II muscle fibers isolated from wild-type, untreated, and enzyme replacement therapy-treated GAA knock-out mice.

RESULTS

Studies in myoblasts from GAA knock-out mice showed a dramatic expansion of vesicles of the endocytic/autophagic pathways, decreased vesicular movement in overcrowded cells, and an acidification defect in a subset of late endosomes/lysosomes. Analysis by confocal microscopy of isolated muscle fibers demonstrated that the consequences of the lysosomal glycogen accumulation are strikingly different in type I and II muscle fibers. Only type II fibers, which are the most resistant to therapy, contain large regions of autophagic buildup that span the entire length of the fibers.

INTERPRETATION

The vastly increased autophagic buildup may be responsible for skeletal muscle damage and prevent efficient trafficking of replacement enzyme to lysosomes.

Thrombopoietin protects against in vitro and in vivo cardiotoxicity induced by doxorubicin

BACKGROUND

Doxorubicin (DOX) is an important antineoplastic agent. However, the associated cardiotoxicity, possibly mediated by the production of reactive oxygen species, has remained a significant and dose-limiting clinical problem. Our hypothesis is that the hematopoietic/megakaryocytopoietic growth factor thrombopoietin (TPO) protects against DOX-induced cardiotoxicity and might involve antiapoptotic mechanism exerted on cardiomyocytes.

METHODS AND RESULTS

In vitro investigations on H9C2 cell line and spontaneously beating cells of primary, neonatal rat ventricle, as well as an in vivo study in a mouse model of DOX-induced acute cardiomyopathy, were performed. Our results showed that pretreatment with TPO significantly increased viability of DOX-injured H9C2 cells and beating rates of neonatal myocytes, with effects similar to those of dexrazoxane, a clinically approved cardiac protective agent. TPO ameliorated DOX-induced apoptosis of H9C2 cells as demonstrated by assays of annexin V, active caspase-3, and mitochondrial membrane potential. In the mouse model, administration of TPO (12.5 microg/kg IP for 3 alternate days) significantly reduced DOX-induced (20 mg/kg) cardiotoxicity, including low blood cell count, cardiomyocyte lesions (apoptosis, vacuolization, and myofibrillar loss), and animal mortality. Using Doppler echocardiography, we observed increased heart rate, fractional shortening, and cardiac output in animals pretreated with TPO compared with those receiving DOX alone.

CONCLUSIONS

These data have provided the first evidence that TPO is a protective agent against DOX-induced cardiac injury. We propose to further explore an integrated program, incorporating TPO with other protocols, for treatment of DOX-induced cardiotoxicity and other forms of cardiomyopathy.

Cartap-induced cytotoxicity in mouse C2C12 myoblast cell line and the roles of calcium ion and oxidative stress on the toxic effects

Our previous study has demonstrated that instead of neuromuscular blockage cartap, an organonitrogen insecticide, could cause a marked irreversible Ca2+-dependent contracture in both isolated mouse and rabbit phrenic nerve-diaphragms. We further examined the potential of direct myocytotoxicity of cartap and the possible roles of calcium ion and oxidative stress on cartap-induced muscle cell injury using the mouse myoblast cell line, C2C12. Cartap exerted a dose- and time-dependent cytotoxic effect in C2C12 cells measured by MTT colorimetric assay and trypan blue dye exclusion. The extracellular activities of both creatine kinase (CK) and lactate dehydrogenase (LDH) were elevated in the cartap-treated groups at or greater than 100 microM. The isoenzymatic profiles showed that the elevations were mainly due to CK-3, LDH-3, and LDH-4. Following the addition of 0.5-2.5mM EGTA, a Ca2+ chelator, or 30-100 microM verapamil, an L-type Ca2+ channel blocker, the cartap-induced reduction in MTT metabolic rate of C2C12 cells was significantly restored in a dose-dependent manner in both EGTA and verapamil-treated cells. Furthermore, EGTA could significantly reduce the cartap-induced elevation in the levels of total extracellular CK and LDH activities. Additionally, cartap significantly increased the level of endogenous reactive oxygen species (ROS) in C2C12 cells in a dose- and time-dependent manner. The cartap-induced ROS generation could be significantly inhibited by antioxidants, including Vitamins C and E, catalase, and superoxide dismutase, with catalase the most effective. EGTA could significantly inhibit cartap-induced ROS generation in a dose-dependent manner. The results suggested that cartap could induce ROS generation in C2C12 cells via a Ca2+-dependent mechanism resulting in subsequent cytotoxicity, at least partially, to C2C12 cells. It is speculated that both Ca2+ and Ca2+-induced ROS may also play the central role on the myogenic contracture and myofiber injury of the diaphragm leading to respiratory failure and subsequent death in rabbits exposed ocularly to cartap.

Biological organization of the extraocular muscles

Extraocular muscle is fundamentally distinct from other skeletal muscles. Here, we review the biological organization of the extraocular muscles with the intent of understanding this novel muscle group in the context of oculomotor system function. The specific objectives of this review are threefold. The first objective is to understand the anatomic arrangement of the extraocular muscles and their compartmental or layered organization in the context of a new concept of orbital mechanics, the active pulley hypothesis. The second objective is to present an integrated view of the morphologic, cellular, and molecular differences between extraocular and the more traditional skeletal muscles. The third objective is to relate recent data from functional and molecular biology studies to the established extraocular muscle fiber types. Developmental mechanisms that may be responsible for the divergence of the eye muscles from a skeletal muscle prototype also are considered. Taken together, a multidisciplinary understanding of extraocular muscle biology in health and disease provides insights into oculomotor system function and malfunction. Moreover, because the eye muscles are selectively involved or spared in a variety of neuromuscular diseases, knowledge of their biology may improve current pathogenic models of and treatments for devastating systemic diseases.

Common themes and cell type specific variations of higher order chromatin arrangements in the mouse

Background

Similarities as well as differences in higher order chromatin arrangements of human cell types were previously reported. For an evolutionary comparison, we now studied the arrangements of chromosome territories and centromere regions in six mouse cell types (lymphocytes, embryonic stem cells, macrophages, fibroblasts, myoblasts and myotubes) with fluorescence in situ hybridization and confocal laser scanning microscopy. Both species evolved pronounced differences in karyotypes after their last common ancestors lived about 87 million years ago and thus seem particularly suited to elucidate common and cell type specific themes of higher order chromatin arrangements in mammals.

Results

All mouse cell types showed non-random correlations of radial chromosome territory positions with gene density as well as with chromosome size. The distribution of chromosome territories and pericentromeric heterochromatin changed during differentiation, leading to distinct cell type specific distribution patterns. We exclude a strict dependence of these differences on nuclear shape. Positional differences in mouse cell nuclei were less pronounced compared to human cell nuclei in agreement with smaller differences in chromosome size and gene density. Notably, the position of chromosome territories relative to each other was very variable.

Conclusion

Chromosome territory arrangements according to chromosome size and gene density provide common, evolutionary conserved themes in both, human and mouse cell types. Our findings are incompatible with a previously reported model of parental genome separation.

Desmin filaments are stably associated with the outer nuclear surface in chick myoblasts

Eukaryotic cells have highly organized, interconnected intracellular compartments. The nuclear surface and cytoplasmic cytoskeletal filaments represent compartments involved in such an association. Intermediate filaments are the major cytoskeletal elements in this association. Desmin is a muscle-specific structural protein and one of the earliest known muscle-specific genes to be expressed during cardiac and skeletal muscle development. Desmin filaments have been shown to be associated with the nuclear surface in the myogenic cell line C2C12. Previous studies have revealed that mice lacking desmin develop imperfect muscle, exhibiting the loss of nuclear shape and positioning. In the present work, we have analyzed the association between desmin filaments and the outer nuclear surface in nuclei isolated from pectoral skeletal muscle of chick embryos and in primary chick myogenic cell cultures by using immunofluorescence microscopy, negative staining, immunogold, and transmission electron microscopy. We show that desmin filaments remain firmly attached to the outer nuclear surface after the isolation of nuclei. Furthermore, positive localization of desmin persists after gentle washing of the nuclei with high ionic strength solutions. These data suggest that desmin intermediate filaments are stably and firmly connected to the outer nuclear surface in skeletal muscles cells in vivo and in vitro.

Subcellular recruitment by TSG118 and TSPYL implicates a role for zinc finger protein 106 in a novel developmental pathway

To gain insight into the function of zinc finger protein 106 (ZFP106), we analyzed its subcellular targeting and identified its interacting proteins. Although ZFP106 was detected predominantly in the fibrillar component of the nucleolus and co-localized with the nucleolar transcriptional machinery, its overexpression did not affect transcription of pre-ribosomal RNA genes. The nucleolar association of ZFP106 did neither require ongoing ribosomal RNA synthesis nor nucleolar chromatin indicating that a protein-protein interaction confines ZFP106 to the nucleolus. Deletion analysis revealed that the C-terminal WD40 repeat region functions in nucleolar targeting. This domain interacts with the product of testis-specific gene 118 (TSG118), which also co-localizes with ZFP106 in the nucleolus. Rapid downregulation of TSG118 expression during in vitro terminal differentiation coincides with a loss of nucleolar ZFP106. By its structural features and expression, TSG118 mimics nucleostemin, a nucleolar protein linked to the proliferation potential of stem cells. A two-hybrid screen with the N-terminal region of ZFP106 as bait led to the isolation of testis-specific Y-encoded-like protein (TSPYL), a member of the nucleosome assembly protein family. A frame-shift mutation in TSPYL has recently been found to cause a sudden infant death syndrome with testis dysgenesis. Specific recruitment of ZFP106 via amino acids 412-781 into TSPYL-positive nucleoplasmic bodies requires a TSPYL domain absent in the mutant protein of patients with testis dysgenesis. These results identify ZFP106 as a potential player in a novel pathway involved in testis development.

Regulation of skeletal muscle fiber type and slow myosin heavy chain 2 gene expression by inositol trisphosphate receptor 1

Innervation-dependent signaling cascades that control activation of downstream transcription factors regulate expression of skeletal muscle fiber type-specific genes. Many of the innervation-regulated signaling cascades in skeletal muscle are dependent on intracellular calcium and the mechanisms by which calcium is released from the sarcoplasmic reticulum (SR). We report that the inositol trisphosphate receptor 1 (IP3R1), responsible for calcium release from the SR as a slow wave, was more abundant in fast contracting compared to slow contracting avian muscle fibers. Furthermore, inhibition of IP3R1 activity by 2-aminoethoxydiphenylborate (2-APB) and xestospongin D induced a fiber type transition and expression of the slow myosin heavy chain 2 (slow MyHC2) gene in innervated fast muscle fibers. Activation of the slow MyHC2 promoter by IP3R1 inhibition was accompanied by a reduction in protein kinase C activity. In addition, inhibition of IP3R1 activity resulted in a reduction of nuclear factor of activated T cells (NFAT)-dependent transcription and nuclear localization, indicating that IP3R1 activity regulated NFAT transcription factor activity in skeletal muscle fibers. Myocyte enhancer factor 2 (MEF2)-dependent transcriptional activity was increased by innervation, but unaffected by IP3R1 activity. The results indicate that IP3R1 activity regulates muscle fiber type-specific gene expression in innervated muscle fibers.

Divergent mitochondrial and endoplasmic reticulum association of DMPK splice isoforms depends on unique sequence arrangements in tail anchors

Myotonic dystrophy protein kinase (DMPK) is a Ser/Thr-type protein kinase with unknown function, originally identified as the product of the gene that is mutated by triplet repeat expansion in patients with myotonic dystrophy type 1 (DM1). Alternative splicing of DMPK transcripts results in multiple protein isoforms carrying distinct C termini. Here, we demonstrate by expressing individual DMPKs in various cell types, including C(2)C(12) and DMPK(-/-) myoblast cells, that unique sequence arrangements in these tails control the specificity of anchoring into intracellular membranes. Mouse DMPK A and C were found to associate specifically with either the endoplasmic reticulum (ER) or the mitochondrial outer membrane, whereas the corresponding human DMPK A and C proteins both localized to mitochondria. Expression of mouse and human DMPK A-but not C-isoforms in mammalian cells caused clustering of ER or mitochondria. Membrane association of DMPK isoforms was resistant to alkaline conditions, and mutagenesis analysis showed that proper anchoring was differentially dependent on basic residues flanking putative transmembrane domains, demonstrating that DMPK tails form unique tail anchors. This work identifies DMPK as the first kinase in the class of tail-anchored proteins, with a possible role in organelle distribution and dynamics.

Muscle-specific interaction of caveolin isoforms: differential complex formation between caveolins in fibroblastic vs. muscle cells

It is generally well accepted that caveolin-3 expression is muscle specific, whereas caveolin-1 and -2 are coexpressed in a variety of cell types, including adipocytes, endothelial cells, epithelial cells, and fibroblasts. Caveolin-1 and -2 are known to form functional hetero-oligomeric complexes in cells where they are coexpressed, whereas caveolin-3 forms homo-oligomeric high molecular mass complexes. Although caveolin-2 might be expected to interact in a similar manner with caveolin-3, most studies indicate that this is not the case. However, this view has recently been challenged as it has been demonstrated that caveolin-2 and -3 are coexpressed in primary cultures of cardiac myocytes, where these two proteins can be coimmunoprecipitated. Thus it remains controversial whether caveolin-2 interacts with caveolin-3. Here, we directly address the issue of caveolin isoform protein-protein interactions by means of three distinct molecular genetic approaches. First, using caveolin-1-deficient mouse embryonic fibroblasts, in which we have stably expressed caveolin-1, -2, or -3, we find that caveolin-1 interacts with caveolin-2 in this setting, whereas caveolin-3 does not, in agreement with most published observations. Next, we used a transfected L6 myoblast cell system expressing all three caveolin proteins. Surprisingly, we found that caveolin-1, -2, and -3 all coimmunoprecipitate in this cell type, suggesting that this interaction is muscle cell specific. Similar results were obtained when the skeletal muscle of caveolin-1 transgenic animals was analyzed for caveolin-1 and caveolin-3 coimmunoprecipitation. Thus we conclude that all three caveolins can interact to form a discrete hetero-oligomeric complex, but that such complex formation is clearly muscle specific.

Actin filament organization in aligned prefusion myoblasts

The organization of the actin cytoskeleton in prefusion aligning myoblasts is likely to be important for their shape and interaction. We investigated actin filament organization and polarity by transmission electron microscopy (TEM) in these cells. About 84% of the filaments counted were either found in a subplasmalemma sheet up to 0.5 microm thick that was aligned with the long axis of the cell, or in protrusions. The remaining filaments were found in the cytoplasm, where they were randomly orientated and not organized into bundles. The polarity of the subplasmalemma filaments changed progressively from one end of the cell to the other. At the ends of the cells and in protrusions, the majority of filaments were organized such that their barbed ends faced the tip of the protrusion. We did not find any actin filament bundles or stress fibres in these cells. Time-lapse phase microscopy demonstrated that aligned cells were still actively migrating at the time of our TEM observations, and their direction of movement was restricted to the long axis of the cell group. The ability of these cells to locomote actively in the absence of actin filament bundles suggests that in these cells the subplasmalemma actin sheet contributes not only to cell shape but also to cell locomotion.

Differential effect of experimental hypertension and hypercholesterolemia on adventitial remodeling

OBJECTIVE

Intima-media remodeling, as frequently assessed by changes in the external elastic lamina-to-lumen area (EELLA), is well-described in coronary artery disease in contrast to adventitial remodeling, especially in the early disease stage.

METHOD AND RESULTS

Female domestic pigs were randomized to one of the following 12-week treatment groups: normal diet (N; n=6), high-cholesterol diet (HC; n=6), or renovascular hypertension (HT; n=4). Low-density lipoprotein (LDL) cholesterol serum concentration was higher in HC than in N and HT (395.5+/-106 versus 38.6+/-14 and 37.2+/-6.8 mg/dL; P<0.05 for both). Mean arterial pressure was higher in HT than in N and HC (141.3+/-21 versus 107.4+/-8.9 and 109.4+/-7.8 mm Hg; P<0.05 for both). EELLA ratio, as assessed by morphometry, was similar in N, HC, and HTN (1.03+/-0.32 versus 0.95+/-0.29 and 1.01+/-0.09; P<0.05 for both). Coronary vasa vasorum density, as assessed by 3-dimensional micro-computed tomography, was higher in HC than in N and HT (3.4+/-1.0 versus 1.9+/-0.3 and 2.0+/-1.2; P<0.05 for both). In contrast, immunostaining showed a higher collagen III content and the presence of adventitial myofibroblasts in HT compared with N and HC.

CONCLUSIONS

The current study suggests that adventitial remodeling precedes intima and media remodeling of coronary arteries early after exposure to hypercholesterolemia and hypertension, with distinct qualitative differences between them. Intima-media remodeling is well-described in coronary artery disease in contrast to adventitial remodeling. Results of the current study on coronary arteries of pigs, randomized to 12 weeks of normal diet (N), hypercholesterolemic diet (HC), or renovascular hypertensive (HT), indicate that adventitial remodeling precedes intima-media remodeling early after risk factor exposure with distinct qualitative differences.

kette and blown fuse interact genetically during the second fusion step of myogenesis in Drosophila

Drosophila myoblast fusion proceeds in two steps. The first one gives rise to small syncytia, the muscle precursor cells, which then recruit further fusion competent myoblasts to reach the final muscle size. We have identified Kette as an essential component for myoblast fusion. In kette mutants, founder cells and fusion-competent myoblasts are determined correctly and overcome the very first fusion. But then, at the precursor cell stage, fusion is interrupted. At the ultrastructural level, fusion is characterised by cell-cell recognition, alignment, formation of prefusion complexes, electron dense plaques and membrane breakdown. In kette mutants, electron dense plaques of aberrant length accumulate and fusion is interrupted owing to a complete failure of membrane breakdown. Furthermore, we show that kette interacts genetically with blown fuse (blow) which is known to be required to proceed from prefusion complexes to the formation of the electron dense plaques. Interestingly, a surplus of Kette can replace Blow function during myogenesis. We propose a model in which Dumbfounded/Sticks and stones-dependent cell adhesion is mediated over Rolling Pebbles, Myoblast city, Crk, Blown fuse and Kette, and thus induces membrane fusion.

Mitochondrial-dependent regulation of myoblast proliferation

The aim of the present study was to determine whether mitochondrial activity could regulate myoblast proliferation. We demonstrate that an increase in mitochondrial activity of L6E9 myoblasts can be easily obtained by simply raising extracellular pyruvate concentration in the culture dish. Under this condition, L6E9 myoblasts underwent a rapid growth arrest in G1 + S phases concomitant to a marked cellular hypertrophy. No sign of myoblast fusion was evident. This was accompanied by the down-regulation of proliferating cell nuclear antigen expression and an increase in p21 expression. Mitochondrial biogenesis was also stimulated, as indicated by a twofold increase in mitochondrial content. These cells exhibited a large increase in the production of reactive oxygen species that could contribute to the observed phenotypic alterations. However, exposure of pyruvate-treated cells to antioxidants did not reverse growth arrest. Similarly, exposure of control cells to oxidants did not induce growth arrest. Our observations suggest that mitochondrial activity appears to play a central role in regulating myoblast proliferation. They also argue strongly in favor of a retrograde communication establishing a mitochondrial control of nuclear gene expression that could be modulated by mitochondrial activity.

Interaction of the disintegrin and cysteine-rich domains of ADAM12 with integrin alpha7beta1

We describe a novel interaction between the disintegrin and cysteine-rich (DC) domains of ADAM12 and the integrin alpha7beta1. Integrin alpha7beta1 extracted from human embryonic kidney 293 cells transfected with alpha7 cDNA was retained on an affinity column containing immobilized DC domain of ADAM12. 293 cells stably transfected with alpha7 cDNA adhered to DC-coated wells, and this adhesion was partially inhibited by 6A11 integrin alpha7 function-blocking antibody. The X1 and the X2 extracellular splice variants of integrin alpha7 supported equally well adhesion to the DC protein. Integrin alpha7beta1-mediated cell adhesion to DC had different requirements for Mn2+ than adhesion to laminin. Furthermore, integrin alpha7beta1-mediated cell adhesion to laminin, but not to DC, resulted in efficient cell spreading and phosphorylation of focal adhesion kinase (FAK) at Tyr397. We also show that adhesion of L6 myoblasts to DC is mediated in part by the endogenous integrin alpha7beta1 expressed in these cells. Since integrin alpha7 plays an important role in muscle cell growth, stability, and survival, and since ADAM12 has been implicated in muscle development and regeneration, we postulate that the interaction between ADAM12 and integrin alpha7beta1 may be relevant to muscle development, function, and disease. We also conclude that laminin and the DC domain of ADAM12 represent two functional ligands for integrin alpha7beta1, and adhesion to each of these two ligands via integrin alpha7beta1 triggers different cellular responses.