RhoA GTPase regulates M-cadherin activity and myoblast fusion

Sources for skeletal muscle repair: from satellite cells to reprogramming

Skeletal muscle regeneration is the process that ensures tissue repair after damage by injury or in degenerative diseases such as muscular dystrophy. Satellite cells, the adult skeletal muscle progenitor cells, are commonly considered to be the main cell type involved in skeletal muscle regeneration. Their mechanism of action in this process is extensively characterized. However, evidence accumulated in the last decade suggests that other cell types may participate in skeletal muscle regeneration. Although their actual contribution to muscle formation and regeneration is still not clear; if properly manipulated, these cells may become new suitable and powerful sources for cell therapy of skeletal muscle degenerative diseases. Mesoangioblasts, vessel associated stem/progenitor cells with high proliferative, migratory and myogenic potential, are very good candidates for clinical applications and are already in clinical experimentation. In addition, pluripotent stem cells are very promising sources for regeneration of most tissues, including skeletal muscle. Conditions such as muscle cachexia or aging that severely alter homeostasis may be counteracted by transplantation of donor and/or recruitment and activation of resident muscle stem/progenitor cells. Advantages and limitations of different cell therapy approaches will be discussed.

RhoA mediates defective stem cell function and heterotopic ossification in dystrophic muscle of mice

Heterotopic ossification (HO) and fatty infiltration (FI) often occur in diseased skeletal muscle and have been previously described in various animal models of Duchenne muscular dystrophy (DMD); however, the pathological mechanisms remain largely unknown. Dystrophin-deficient mdx mice and dystrophin/utrophin double-knockout (dKO) mice are mouse models of DMD; however, mdx mice display a strong muscle regeneration capacity, while dKO mice exhibit a much more severe phenotype, which is similar to patients with DMD. Our results revealed that more extensive HO, but not FI, occurred in the skeletal muscle of dKO mice versus mdx mice, and RhoA activation specifically occurred at the sites of HO. Moreover, the gene expression of RhoA, BMPs, and several inflammatory factors were significantly up-regulated in muscle stem cells isolated from dKO mice; while inactivation of RhoA in the cells with RhoA/ROCK inhibitor Y-27632 led to reduced osteogenic potential and improved myogenic potential. Finally, inactivation of RhoA signaling in the dKO mice with Y-27632 improved muscle regeneration and reduced the expression of BMPs, inflammation, HO, and intramyocellular lipid accumulation in both skeletal and cardiac muscle. Our results revealed that RhoA represents a major molecular switch in the regulation of HO and muscle regeneration in dystrophic skeletal muscle of mice.

Effect of RGD functionalization and stiffness modulation of polyelectrolyte multilayer films on muscle cell differentiation

Skeletal muscle tissue engineering holds promise for the replacement of muscle damaged by injury and for the treatment of muscle diseases. Although arginylglycylaspartic acid (RGD) substrates have been widely explored in tissue engineering, there have been no studies aimed at investigating the combined effects of RGD nanoscale presentation and matrix stiffness on myogenesis. In the present work we use polyelectrolyte multilayer films made of poly(L-lysine) (PLL) and poly(L-glutamic) acid (PGA) as substrates of tunable stiffness that can be functionalized by a RGD adhesive peptide to investigate important events in myogenesis, including adhesion, migration, proliferation and differentiation. C2C12 myoblasts were used as cellular models. RGD presentation on soft films and increasing film stiffness could both induce cell adhesion, but the integrins involved in adhesion were different in the case of soft and stiff films. Soft films with RGD peptide appeared to be the most appropriate substrate for myogenic differentiation, while the stiff PLL/PGA films induced significant cell migration and proliferation and inhibited myogenic differentiation. ROCK kinase was found to be involved in the myoblast response to the different films. Indeed, its inhibition was sufficient to rescue differentiation on stiff films, but no significant changes were observed on stiff films with the RGD peptide. These results suggest that different signaling pathways may be activated depending on the mechanical and biochemical properties of multilayer films. This study emphasizes the advantage of soft PLL/PGA films presenting the RGD peptide in terms of myogenic differentiation. This soft RGD-presenting film may be further used as a coating of various polymeric scaffolds for muscle tissue engineering.

Signaling mechanisms in mammalian myoblast fusion

Myoblast fusion is a critical process that contributes to the growth of muscle during development and to the regeneration of myofibers upon injury. Myoblasts fuse with each other as well as with multinucleated myotubes to enlarge the myofiber. Initial studies demonstrated that myoblast fusion requires extracellular calcium and changes in cell membrane topography and cytoskeletal organization. More recent studies have identified several cell-surface and intracellular proteins that mediate myoblast fusion. Furthermore, emerging evidence suggests that myoblast fusion is also regulated by the activation of specific cell-signaling pathways that lead to the expression of genes whose products are essential for the fusion process and for modulating the activity of molecules that are involved in cytoskeletal rearrangement. Here, we review the roles of the major signaling pathways in mammalian myoblast fusion.

Fam65b is a new transcriptional target of FOXO1 that regulates RhoA signaling for T lymphocyte migration

Forkhead box O (FOXO) transcription factors favor both T cell quiescence and trafficking through their control of the expression of genes involved in cell cycle progression, adhesion, and homing. In this article, we report that the product of the fam65b gene is a new transcriptional target of FOXO1 that regulates RhoA activity. We show that family with sequence similarity 65 member b (Fam65b) binds the small GTPase RhoA via a noncanonical domain and represses its activity by decreasing its GTP loading. As a consequence, Fam65b negatively regulates chemokine-induced responses, such as adhesion, morphological polarization, and migration. These results show the existence of a new functional link between FOXO1 and RhoA pathways, through which the FOXO1 target Fam65b tonically dampens chemokine-induced migration by repressing RhoA activity.

Rab35 regulates cadherin-mediated adherens junction formation and myoblast fusion

The small GTPase Rab35 is identified as a regulator of cadherin trafficking and stabilization at cell–cell contacts. Rab35 function is required for PIP5KIγ accumulation at cell contacts and PI(4,5)P2 production, and hence cadherin stabilization. Rab35 regulates myoblast fusion, a cellular process under the control of cadherin-dependent signaling.

Cadherins are homophilic cell–cell adhesion molecules implicated in many fundamental processes, such as morphogenesis, cell growth, and differentiation. They accumulate at cell–cell contact sites and assemble into large macromolecular complexes named adherens junctions (AJs). Cadherin targeting and function are regulated by various cellular processes, many players of which remain to be uncovered. Here we identify the small GTPase Rab35 as a new regulator of cadherin trafficking and stabilization at cell–cell contacts in C2C12 myoblasts and HeLa cells. We find that Rab35 accumulates at cell–cell contacts in a cadherin-dependent manner. Knockdown of Rab35 or expression of a dominant-negative form of Rab35 impaired N- and M-cadherin recruitment to cell–cell contacts, their stabilization at the plasma membrane, and association with p120 catenin and led to their accumulation in transferrin-, clathrin-, and AP-2–positive intracellular vesicles. We also find that Rab35 function is required for PIP5KIγ accumulation at cell–cell contacts and phosphatidyl inositol 4,5-bisphosphate production, which is involved in cadherin stabilization at contact sites. Finally, we show that Rab35 regulates myoblast fusion, a major cellular process under the control of cadherin-dependent signaling. Taken together, these results reveal that Rab35 regulates cadherin-dependent AJ formation and myoblast fusion.

M-cadherin-inhibited phosphorylation of ß-catenin augments differentiation of mouse myoblasts

β-Catenin is essential for muscle development because it regulates both cadherin-mediated cell-cell adhesion and canonical Wingless and Int1 (Wnt) signaling. The phosphorylation of β-catenin by glycogen synthase kinase-3β (GSK-3β) at serine31/37/threonine41 regulates its stability and its role in canonical Wnt signaling. In this study, we have investigated whether the N-terminal phosphorylation of β-catenin is regulated by M-cadherin, and whether this regulation mediates the role of M-cadherin in myogenic differentiation. Our data show that the knockdown of M-cadherin expression by RNA interference (RNAi) in C2C12 myoblasts significantly increases the phosphorylation of β-catenin at Ser33/37/Thr41 and decreases the protein abundance of ser37/thr41-unphosphorylated active β-catenin. Furthermore, M-cadherin RNAi promotes TCF/LEF transcription activity but also blunts the initiation of the myogenic progress by Wnt pathway activator lithium chloride or Wnt-3a treatment. Knockdown of β-catenin expression by RNAi decreases myogenic induction in myoblasts. Forced expression of a phosphorylation-resistant β-catenin plasmid (S33Y-β-catenin) fails to enhance myogenic differentiation, but it partially rescues C2C12 cells from M-cadherin RNAi-induced apoptosis. These data show, for the first time, that M-cadherin-mediated signaling attenuates β-catenin phosphorylation at Ser31/37/Thr41 by GSK-3β, and that this regulation has a positive effect on myogenic differentiation induced by canonical Wnt signaling.

The alignment and fusion assembly of adipose-derived stem cells on mechanically patterned matrices

Cell patterning is typically accomplished by selectively depositing proteins for cell adhesion only on patterned regions; however in tissues, cells are also influenced by mechanical stimuli, which can also result in patterned arrangements of cells. We developed a mechanically-patterned hydrogel to observe and compare it to extracellular matrix (ECM) ligand patterns to determine how to best regulate and improve cell type-specific behaviors. Ligand-based patterning on hydrogels was not robust over prolonged culture, but cells on mechanically-patterned hydrogels differentially sorted based on stiffness preference: myocytes and adipose-derived stem cells (ASCs) underwent stiffness-mediated migration, i.e. durotaxis, and remained on myogenic hydrogel regions. Myocytes developed aligned striations and fused on myogenic stripes of the mechanically-patterned hydrogel. ASCs aligned and underwent myogenesis, but their fusion rate increased, as did the number of cells fusing into a myotube as a result of their alignment. Conversely, neuronal cells did not exhibit durotaxis and could be seen on soft regions of the hydrogel for prolonged culture time. These results suggest that mechanically-patterned hydrogels could provide a platform to create tissue engineered, innervated micro-muscles of neural and muscle phenotypes juxtaposed next to each other in order better recreate a muscle niche.

MicroRNA-148a promotes myogenic differentiation by targeting the ROCK1 gene

MicroRNAs are evolutionarily conserved small RNAs that post-transcriptionally regulate gene expression and have emerged as critical regulators of skeletal muscle development. Here, we identified miR-148a as a novel myogenic microRNA that mediated myogenic differentiation. The expression levels of miR-148a increased during C2C12 myoblast differentiation. Overexpression of miR-148a significantly promoted myogenic differentiation of both C2C12 myoblast and primary muscle cells. Blocking the function of miR-148a with a 2'-O-methylated antisense oligonucleotide inhibitor repressed C2C12 myoblast differentiation. Using a bioinformatics approach, we identified Rho-associated coiled-coil containing protein kinase 1 (ROCK1), a known inhibitor of myogenesis, as a target of miR-148a. A dual-luciferase reporter assay was used to demonstrate that miR-148a directly targeted the 3'-UTR of ROCK1. In addition, the overexpression of miR-148a decreased the protein expression of ROCK1 in C2C12 myoblast and primary muscle cells. Furthermore, ROCK1 inhibition with specific siRNA leaded to accelerated myogenic differentiation progression, underscoring a negative regulatory function of ROCK1 in myogenesis. Therefore, our results revealed a novel mechanism in which miR-148a positively regulates myogenic differentiation via ROCK1 down-regulation.

Insulin receptor substrate protein 53kDa (IRSp53) is a negative regulator of myogenic differentiation

Fusion of mononucleated myoblasts to generate multinucleated myotubes is a critical step in skeletal muscle development. Filopodia, the actin cytoskeleton based membrane protrusions, have been observed early during myoblast fusion, indicating that they could play a direct role in myogenic differentiation. The control of filopodia formation in myoblasts remains poorly understood. Here we show that the expression of IRSp53 (Insulin Receptor Substrate protein 53kDa), a known regulator of filopodia formation, is down-regulated during differentiation of both mouse primary myoblasts and a mouse myoblast cell line C2C12. Over-expression of IRSp53 in C2C12 cells led to induction of filopodia and decrease in cell adhesion, concomitantly with inhibition of myogenic differentiation. In contrast, knocking down the IRSp53 expression in C2C12 cells led to a small but significant increase in myotube development. The decreased cell adhesion of C2C12 cells over-expressing IRSp53 is correlated with a reduction in the number of vinculin patches in these cells. Mutations in the conserved IMD domain (IRSp53 and MIM (missing in metastasis) homology domain) or SH3 domain of IRSp53 abolished the ability of this protein to inhibit myogenic differentiation and reduce cell adhesion. Over-expression of the IMD domain alone was sufficient to decrease the cell-extracellular matrix adhesion and to inhibit myogenesis in a manner dependent on its function in membrane shaping. Based on our data, we propose that IRSp53 is a negative regulator of myogenic differentiation which correlates with the observed down regulation of IRSp53 expression during myoblast differentiation to myotubes.

Myoblast fusion: lessons from flies and mice

The fusion of myoblasts into multinucleate syncytia plays a fundamental role in muscle function, as it supports the formation of extended sarcomeric arrays, or myofibrils, within a large volume of cytoplasm. Principles learned from the study of myoblast fusion not only enhance our understanding of myogenesis, but also contribute to our perspectives on membrane fusion and cell-cell fusion in a wide array of model organisms and experimental systems. Recent studies have advanced our views of the cell biological processes and crucial proteins that drive myoblast fusion. Here, we provide an overview of myoblast fusion in three model systems that have contributed much to our understanding of these events: the Drosophila embryo; developing and regenerating mouse muscle; and cultured rodent muscle cells.

Proteomics-based investigation in C2C12 myoblast differentiation

Skeletal muscle cell differentiation is a multistage process extensively studied over the years. Even if great improvements have been achieved in defining biological process underlying myogenesis, many molecular mechanisms need still to be clarified.

To further highlight this process, we studied cells at undifferentiated, intermediate and highly differentiated stages, and we analyzed, for each condition, morphological and proteomic changes. We also identified the proteins that showed statistical significant changes by a ESI-Q-TOF mass spectrometer. This work provides further evidence of the involvement of particular proteins in skeletal muscle development. Furthermore, the high level of expression of many heat shock proteins, suggests a relationship between differentiation and cellular stress. Intriguingly, the discovery of myogenesis-correlated proteins, known to play a role in apoptosis, suggests a link between differentiation and this type of cell death.

N-cadherin and M-cadherin are sequentially expressed in myoblast populations contributing to the first and second waves of myogenesis in the trout (Oncorhynchus mykiss)

The objective of this study was to investigate the expression of two promyogenic cell surface adhesion receptors, N- and M-cadherin, in developing trout (Oncorhynchus mykiss) somite, taking account of the recent identification of a dermomyotome-like epithelium in teleosts. In situ hybridization showed that N-cadherin was expressed throughout the paraxial mesoderm and nascent somite. As the somite matured, N-cadherin expression disappeared ventrally from the sclerotome, and then mediolaterally from the differentiating slow and fast muscle cells of the embryonic myotome, to become finally restricted to the undifferentiated myogenic precursors forming the dermomyotome-like epithelium that surrounds the embryonic myotome. By contrast, M-cadherin, which was transcribed in the differentiating embryonic myotome, was never expressed in the dermomyotome-like epithelium. In late-stage trout embryos, M-cadherin transcript was only detected at the periphery of the expanding myotome, where muscle cells stemming from the N-cadherin positive dermomyotome-like epithelium differentiate. Collectively, our results support the view that, in trout embryo, N-cadherin is associated with muscle cell immaturity while M-cadherin is associated with muscle cell maturation and differentiation and this during the two successive phases of myogenesis.

Rac1 and RhoA GTPases have antagonistic functions during N-cadherin-dependent cell-cell contact formation in C2C12 myoblasts

BACKGROUND INFORMATION

N-cadherin, a member of the Ca(2+)-dependent cell-cell adhesion molecule family, plays an essential role in the induction of the skeletal muscle differentiation programme. However, the molecular mechanisms which govern the formation of N-cadherin-dependent cell-cell contacts in myoblasts remain unexplored.

RESULTS

In the present study, we show that N-cadherin-dependent cell contact formation in myoblasts is defined by two stages. In the first phase, N-cadherin is highly mobile in the lamellipodia extensions between the contacting cells. The second stage corresponds to the formation of mature N-cadherin-dependent cell contacts, characterized by the immobilization of a pool of N-cadherin which appears to be clustered in the interdigitated membrane structures that are also membrane attachment sites for F-actin filaments. We also demonstrated that the formation of N-cadherin-dependent cell-cell contacts requires a co-ordinated and sequential activity of Rac1 and RhoA. Rac1 is involved in the first stage and facilitates N-cadherin-dependent cell-cell contact formation, but it is not absolutely required. Conversely, RhoA is necessary for N-cadherin-dependent cell contact formation, since, via ROCK (Rho-associated kinase) signalling and myosin 2 activation, it allows the stabilization of N-cadherin at the cell-cell contact sites.

CONCLUSIONS

We have shown that Rac1 and RhoA have opposite effects on N-cadherin-dependent cell-cell contact formation in C2C12 myoblasts and act sequentially to allow its formation.

Suppression of GSK-3β activation by M-cadherin protects myoblasts against mitochondria-associated apoptosis during myogenic differentiation

Apoptosis occurs concurrently with differentiation of muscle progenitor cells (MPCs) before they fuse to form myotubes. Dysregulated apoptosis in MPCs contributes to the low regeneration capability in aged muscle and decreases the survival rate of donor cells in stem cell-based therapies for muscular dystrophies. This study investigated the role of the M-cadherin/PI3K/Akt/GSK-3β signaling pathway in regulating apoptosis during differentiation of MPCs. Disruption of M-cadherin-dependent cell-cell adhesion by M-cadherin RNA interference in confluent C2C12 myoblasts sensitized the cells to mitochondria-associated intrinsic apoptosis induced by cell confluence or serum starvation. Further investigation of this pathway revealed that M-cadherin-mediated signaling suppressed GSK-3β activation by enhancing the PI3K/AKT-dependent inhibitory phosphorylation of Ser9 in GSK-3β. Overexpression of wild-type GSK-3β in confluent C2C12 myoblasts exacerbated the apoptosis, whereas chemical inhibition of GSK-3β using TDZD-8, or forced expression of constitutively active Akt (myrAkt), or a kinase-deficient GSK-3β mutant [GSK-3β(K85R)], attenuated apoptosis and rescued the impaired myogenic differentiation that is caused by M-cadherin RNA interference. These data suggest that M-cadherin-mediated signaling prevents acceleration of mitochondria-associated intrinsic apoptosis in MPCs by suppressing GSK-3β activation during myogenic differentiation.

Molecular and cellular mechanisms of mammalian cell fusion

The fusion of one cell with another occurs in development, injury and disease. Despite the diversity of fusion events, five steps in sequence appear common. These steps include programming fusion-competent status, chemotaxis, membrane adhesion, membrane fusion, and post-fusion resetting. Recent advances in the field start to reveal the molecules involved in each step. This review focuses on some key molecules and cellular events of cell fusion in mammals. Increasing evidence demonstrates that membrane lipid rafts, adhesion proteins and actin rearrangement are critical in the final step of membrane fusion. Here we propose a new model for the formation and expansion of membrane fusion pores based on recent observations on myotube formation. In this model, membrane lipid rafts first recruit adhesion molecules and align with opposing membranes, with the help of a cortical actin "wall" as a rigid supportive platform. Second, the membrane adhesion proteins interact with each other and trigger actin rearrangement, which leads to rapid dispersion of lipid rafts and flow of a highly fluidic phospholipid bilayer into the site. Finally, the opposing phospholipid bilayers are then pushed into direct contact leading to the formation of fusion pores by the force generated through actin polymerization. The actin polymerization generated force also drives the expansion of the fusion pores. However, several key questions about the process of cell fusion still remain to be explored. The understanding of the mechanisms of cell fusion may provide new opportunities in correcting development disorders or regenerating damaged tissues by inhibiting or promoting molecular events associated with fusion.

Skeletal muscle differentiation and fusion are regulated by the BAR-containing Rho-GTPase-activating protein (Rho-GAP), GRAF1

Although RhoA activity is necessary for promoting myogenic mesenchymal stem cell fates, recent studies in cultured cells suggest that down-regulation of RhoA activity in specified myoblasts is required for subsequent differentiation and myotube formation. However, whether this phenomenon occurs in vivo and which Rho modifiers control these later events remain unclear. We found that expression of the Rho-GTPase-activating protein, GRAF1, was transiently up-regulated during myogenesis, and studies in C2C12 cells revealed that GRAF1 is necessary and sufficient for mediating RhoA down-regulation and inducing muscle differentiation. Moreover, forced expression of GRAF1 in pre-differentiated myoblasts drives robust muscle fusion by a process that requires GTPase-activating protein-dependent actin remodeling and BAR-dependent membrane binding or sculpting. Moreover, morpholino-based knockdown studies in Xenopus laevis determined that GRAF1 expression is critical for muscle development. GRAF1-depleted embryos exhibited elevated RhoA activity and defective myofibrillogenesis that resulted in progressive muscle degeneration, defective motility, and embryonic lethality. Our results are the first to identify a GTPase-activating protein that regulates muscle maturation and to highlight the functional importance of BAR domains in myotube formation.

Toxoplasma gondii down modulates cadherin expression in skeletal muscle cells inhibiting myogenesis

Background

Toxoplasma gondii belongs to a large and diverse group of obligate intracellular parasitic protozoa. Primary culture of mice skeletal muscle cells (SkMC) was employed as a model for experimental toxoplasmosis studies. The myogenesis of SkMC was reproduced in vitro and the ability of T. gondii tachyzoite forms to infect myoblasts and myotubes and its influence on SkMC myogenesis were analyzed.

Results

In this study we show that, after 24 h of interaction, myoblasts (61%) were more infected with T. gondii than myotubes (38%) and inhibition of myogenesis was about 75%. The role of adhesion molecules such as cadherin in this event was investigated. First, we demonstrate that cadherin localization was restricted to the contact areas between myocytes/myocytes and myocytes/myotubes during the myogenesis process. Immunofluorescence and immunoblotting analysis of parasite-host cell interaction showed a 54% reduction in cadherin expression at 24 h of infection. Concomitantly, a reduction in M-cadherin mRNA levels was observed after 3 and 24 h of T. gondii-host cell interaction.

Conclusions

These data suggest that T. gondii is able to down regulate M-cadherin expression, leading to molecular modifications in the host cell surface that interfere with membrane fusion and consequently affect the myogenesis process.

Comparison of satellite cell-derived myoblasts and C2C12 differentiation in two- and three-dimensional cultures: changes in adhesion protein expression

Changes in the expression of adhesion proteins involved in myoblast differentiation were investigated in monolayer (two-dimensional) and 3D (three-dimensional) cell cultures. The expression of integrin alpha3 subunit, integrin beta1 subunit, ADAM12 (a disintegrin and metalloproteinase 12), tetraspanins CD9 and CD81 and M-cadherin were examined in the murine myoblast cell line C2C12 and in a primary culture of rat satellite cells. Myoblasts in monolayer and 3D cultures showed significant differences in their morphology and cytoskeletal organization. All of the studied proteins participated in myoblast fusion in each culture examined, but differences in their levels of expression were observed. Satellite cell-derived myoblasts exhibited higher expression of adhesion protein mRNAs than C2C12 cells. Also, C2C12 cells from a 3D culture showed slightly higher expression of adhesion protein transcripts than the same cells cultured as a monolayer. Significantly, the levels of adhesion protein mRNAs were found to change in parallel in all cell culture types. Despite this finding, it is important that differences between satellite cell-derived myoblasts and cell line C2C12 grown in monolayer and 3D cultures are taken into account when studying processes of myoblast differentiation in vitro.

Molecular mechanisms of myoblast fusion across species

Skeletal muscle development, growth and regeneration depend on the ability of progenitor myoblasts to fuse to one another in a series of ordered steps. Whereas the cellular steps leading to the formation of a multinucleated myofiber are conserved in several model organisms, the molecular regulatory factors may vary. Understanding the common and divergent mechanisms regulating myoblast fusion in Drosophila melanogaster (fruit fly), Danio rerio (zebrafish) and Mus musculus (mouse) provides a better insight into the process of myoblast fusion than any of these models could provide alone. Deciphering the mechanisms of myoblast fusion from simpler to more complex organisms is of fundamental interest to skeletal muscle biology and may provide therapeutic avenues for various diseases that affect muscle.

Xenopus Kazrin interacts with ARVCF-catenin, spectrin and p190B RhoGAP, and modulates RhoA activity and epithelial integrity

In common with other p120-catenin subfamily members, Xenopus ARVCF (xARVCF) binds cadherin cytoplasmic domains to enhance cadherin metabolic stability or, when dissociated, modulates Rho-family GTPases. We report here that xARVCF binds and is stabilized by Xenopus KazrinA (xKazrinA), a widely expressed conserved protein that bears little homology to established protein families, and which is known to influence keratinocyte proliferation and differentiation and cytoskeletal activity. Although we found that xKazrinA binds directly to xARVCF, we did not resolve xKazrinA within a larger ternary complex with cadherin, nor did it co-precipitate with core desmosomal components. Instead, screening revealed that xKazrinA binds spectrin, suggesting a potential means by which xKazrinA localizes to cell-cell borders. This was supported by the resolution of a ternary biochemical complex of xARVCF-xKazrinA-xβ2-spectrin and, in vivo, by the finding that ectodermal shedding followed depletion of xKazrin in Xenopus embryos, a phenotype partially rescued with exogenous xARVCF. Cell shedding appeared to be the consequence of RhoA activation, and thereby altered actin organization and cadherin function. Indeed, we also revealed that xKazrinA binds p190B RhoGAP, which was likewise capable of rescuing Kazrin depletion. Finally, xKazrinA was found to associate with δ-catenins and p0071-catenins but not with p120-catenin, suggesting that Kazrin interacts selectively with additional members of the p120-catenin subfamily. Taken together, our study supports the essential role of Kazrin in development, and reveals the biochemical and functional association of KazrinA with ARVCF-catenin, spectrin and p190B RhoGAP.

The effects of 1,25-dihydroxyvitamin D3 on colon cancer cells depend on RhoA-ROCK-p38MAPK-MSK signaling

Many studies support a protective action of vitamin D against colon cancer. 1alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) exerts wide gene regulatory effects in human colon cancer cells. We previously reported that 1,25(OH)2D3 increases cytosolic Ca2+ concentration and transiently activates RhoA and its effector the Rho-associated coiled-kinase (ROCK), and later p38MAPK-MSK. We found that the inhibition of ROCK signaling by Y27632 or that of MSK by Ro318220 prevent the formation of epithelioid islands of SW480-ADH cells by 1,25(OH)2D3 and disrupts the adhesive phenotype of HT29 cells. ROCK and MSK inhibition also abrogates the induction of 1,25(OH)2D3 24-hydroxylase (CYP24), E-cadherin, and vinculin and the repression of cyclin D1 by 1,25(OH)2D3. Moreover, 1,25(OH)2D3 does not promote the localization of the tight junction protein occludin at the plasma membrane in cells expressing a dominant negative RhoA (N19-RhoA). In addition, 1,25(OH)2D3 specifically increases the level of the cysteine protease-inhibitor cystatin D, whereas that of cystatin SN is unaffected. The increase of cystatin D protein caused by 1,25(OH)2D3 is abrogated in N19-RhoA cells. Thus, activation of the RhoA-ROCK-p38MAPK-MSK signaling pathway is essential for the regulation of the phenotype and of the CST5/cystatin D candidate tumor suppressor and other target genes by 1,25(OH)2D3 in colon cancer cells.

Stem cell shape regulates a chondrogenic versus myogenic fate through Rac1 and N-cadherin

Human mesenchymal stem cells (hMSCs) are multipotent cells that can differentiate into many cell types. Chondrogenesis is induced in hMSCs cultured as a micromass pellet to mimic cellular condensation during cartilage development, and exposed to transforming growth factor beta (TGFbeta). Interestingly, TGFbeta can also induce hMSC differentiation to smooth-muscle-like cell types, but it remains unclear what directs commitment between these two lineages. Our previous work revealed that cell shape regulates hMSC commitment between osteoblasts and adipocytes through RhoA signaling. Here we show that cell shape also confers a switch between chondrogenic and smooth muscle cell (SMC) fates. Adherent and well-spread hMSCs stimulated with TGF beta 3 upregulated SMC genes, whereas cells allowed to attach onto micropatterned substrates, but prevented from spreading and flattening, upregulated chondrogenic genes. Interestingly, cells undergoing SMC differentiation exhibited little change in RhoA, but significantly higher Rac1 activity than chondrogenic cells. Rac1 activation inhibited chondrogenesis and was necessary and sufficient for inducing SMC differentiation. Furthermore, TGF beta 3 and Rac1 signaling upregulated N-cadherin, which was required for SMC differentiation. These results demonstrate a chondrogenic-SMC fate decision mediated by cell shape, Rac1, and N-cadherin, and highlight the tight coupling between lineage commitment and the many changes in cell shape, cell-matrix adhesion, and cell-cell adhesion that occur during morphogenesis.

Spatial and functional restriction of regulatory molecules during mammalian myoblast fusion

Myoblast fusion is a highly regulated process that is key for forming skeletal muscle during development and regeneration in mammals. Much remains to be understood about the molecular regulation of myoblast fusion. Some molecules that influence mammalian muscle fusion display specific cellular localization during myogenesis. Such molecules can be localized to the contact region between two fusing cells either in both cells or only in one of the cells. How distinct localization of molecules contributes to fusion is not clear. Further complexity exists as other molecules are functionally restricted to myoblasts at later stages of myogenesis to regulate their fusion with multinucleated myotubes. This review examines these three categories of molecules and discusses how spatial and functional restriction may contribute to the formation of a multinucleated cell. Understanding how and why molecules become restricted in location or function is likely to provide further insights into the mechanisms regulating mammalian muscle fusion.

ADP-ribosylation factor 6 regulates mammalian myoblast fusion through phospholipase D1 and phosphatidylinositol 4,5-bisphosphate signaling pathways

Here we show that ARF6 is associated with the multiproteic complex that contains M-cadherin, Trio, and Rac1 and accumulates at sites of myoblast fusion. ARF6 silencing inhibits the association of Trio and Rac1 with M-cadherin. Moreover, we demonstrate that ARF6 regulates myoblast fusion through Phospholipase D activation and PI(4,5)P2 production.

Myoblast fusion is an essential step during myoblast differentiation that remains poorly understood. M-cadherin–dependent pathways that signal through Rac1 GTPase activation via the Rho-guanine nucleotide exchange factor (GEF) Trio are important for myoblast fusion. The ADP-ribosylation factor (ARF)6 GTPase has been shown to bind to Trio and to regulate Rac1 activity. Moreover, Loner/GEP₁₀₀/BRAG2, a GEF of ARF6, has been involved in mammalian and Drosophila myoblast fusion, but the specific role of ARF6 has been not fully analyzed. Here, we show that ARF6 activity is increased at the time of myoblast fusion and is required for its implementation in mouse C2C12 myoblasts. Specifically, at the onset of myoblast fusion, ARF6 is associated with the multiproteic complex that contains M-cadherin, Trio, and Rac1 and accumulates at sites of myoblast fusion. ARF6 silencing inhibits the association of Trio and Rac1 with M-cadherin. Moreover, we demonstrate that ARF6 regulates myoblast fusion through phospholipase D (PLD) activation and phosphatidylinositol 4,5-bis-phosphate production. Together, these data indicate that ARF6 is a critical regulator of C2C12 myoblast fusion and participates in the regulation of PLD activities that trigger both phospholipids production and actin cytoskeleton reorganization at fusion sites.

Regulation of promyogenic signal transduction by cell-cell contact and adhesion

Skeletal myoblast differentiation involves acquisition of the muscle-specific transcriptional program and morphological changes, including fusion into multinucleated myofibers. Differentiation is regulated by extracellular signaling cues, including cell-cell contact and adhesion. Cadherin and Ig adhesion receptors have been implicated in distinct but overlapping stages of myogenesis. N-cadherin signals through the Ig receptor Cdo to activate p38 MAP kinase, while the Ig receptor neogenin signals to activate FAK; both processes promote muscle-specific gene expression and myoblast fusion. M-cadherin activates Rac1 to enhance fusion. Specific Ig receptors (Kirre and Sns) are essential for myoblast fusion in Drosophila, also signaling through Rac, and vertebrate orthologs of Kirre and Sns have partially conserved function. Mice lacking specific cytoplasmic signaling factors activated by multiple receptors (e.g., Rac1) have strong muscle phenotypes in vivo. In contrast, mice lacking individual adhesion receptors that lie upstream of these factors have modest phenotypes. Redundancy among receptors may account for this. Many of the mammalian Ig receptors and cadherins associate with each other, and multivalent interactions within these complexes may require removal of multiple components to reveal dramatic defects in vivo. Nevertheless, it is possible that the murine adhesion receptors rate-limiting in vivo have not yet been identified or fully assessed.

Eccentric exercise activates novel transcriptional regulation of hypertrophic signaling pathways not affected by hormone changes

Unaccustomed eccentric exercise damages skeletal muscle tissue, activating mechanisms of recovery and remodeling that may be influenced by the female sex hormone 17β-estradiol (E2). Using high density oligonucleotide based microarrays, we screened for differences in mRNA expression caused by E2 and eccentric exercise. After random assignment to 8 days of either placebo (CON) or E2 (EXP), eighteen men performed 150 single-leg eccentric contractions. Muscle biopsies were collected at baseline (BL), following supplementation (PS), +3 hours (3H) and +48 hours (48H) after exercise. Serum E2 concentrations increased significantly with supplementation (P<0.001) but did not affect microarray results. Exercise led to early transcriptional changes in striated muscle activator of Rho signaling (STARS), Rho family GTPase 3 (RND3), mitogen activated protein kinase (MAPK) regulation and the downstream transcription factor FOS. Targeted RT-PCR analysis identified concurrent induction of negative regulators of calcineurin signaling RCAN (P<0.001) and HMOX1 (P = 0.009). Protein contents were elevated for RND3 at 3H (P = 0.02) and FOS at 48H (P<0.05). These findings indicate that early RhoA and NFAT signaling and regulation are altered following exercise for muscle remodeling and repair, but are not affected by E2.

Identification of novel interacting protein partners of SMN using tandem affinity purification

Mutations in the survival motor neuron (SMN) gene cause spinal muscular atrophy (SMA), a neuromuscular disease associated with muscle weakness that progresses to paralysis, respiratory distress, and ultimately death. Both neurons and muscle are severely affected in this disease. Tandem affinity purification (TAP) has emerged as a useful tool for studying protein complexes in vitro. We have used this purification system to discover novel SMN interacting partners in C2C12 muscle and PC12 neuronal cells. To do so, we fused a TAP-tag, consisting of a HIS hexamer and FLAG epitope separated by the tobacco etch virus (TEV) protease cleavage site, to either the N- or C-terminal region of SMN. Interestingly, the profile of SMN interacting proteins varies depending on the cell type and stage. We have identified a number of novel SMN interacting proteins in both C2C12 and PC12 cells, and from among these we have validated Annexin II and myosin regulatory light chain (MRLC). The discovery of these proteins will lead to a better understanding of the mechanisms underlying the pathophysiology of SMA.

Time-lapse imaging of in vitro myogenesis using atomic force microscopy

Myoblast therapy relies on the integration of skeletal muscle stem cells into distinct muscular compartments for the prevention of clinical conditions such as heart failure, or bladder dysfunction. Understanding the fundamentals of myogenesis is hence crucial for the success of these potential medical therapies. In this report, we followed the rearrangement of the surface membrane structure and the actin cytoskeletal organization in C2C12 myoblasts at different stages of myogenesis using atomic force microscopy (AFM) and confocal laser scanning microscopy (CLSM). AFM imaging of living myoblasts undergoing fusion unveiled that within minutes of making cell-cell contact, membrane tubules appear that unite the myoblasts and increase in girth as fusion proceeds. CLSM identified these membrane tubules as built on scaffolds of actin filaments that nucleate at points of contact between fusing myoblasts. In contrast, similarly behaving membrane tubules are absent during cytokinesis. The results from our study in combination with recent findings in literature further expand the understanding of the biochemical and membrane structural rearrangements involved in the two fundamental cellular processes of division and fusion.

S100B protein in myoblasts modulates myogenic differentiation via NF-kappaB-dependent inhibition of MyoD expression

S100B, a Ca(2+)-binding protein of the EF-hand type, is expressed in myoblasts, the precursors of skeletal myofibers, and muscle satellite cells (this work). S100B has been shown to participate in the regulation of several intracellular processes including cell cycle progression and differentiation. We investigated regulatory activities of S100B within myoblasts by stable overexpression of S100B and by inhibition of S100B expression. Overexpression of S100B in myoblast cell lines and primary myoblasts resulted in inhibition of myogenic differentiation, evidenced by lack of expression of myogenin and myosin heavy chain (MyHC) and absence of myotube formation. S100B-overexpressing myoblasts showed reduced MyoD expression levels and unchanged Myf5 expression levels, compared with control myoblasts, and transient transfection of S100B-overexpressing myoblasts with MyoD, but not Myf5, restored differentiation and fusion in part. The transcriptional activity of NF-kappaB, a negative regulator of MyoD expression, was enhanced in S100B-overexpressing myoblasts, and blocking NF-kappaB activity resulted in reversal of S100B's inhibitory effects. Yin Yang1, a transcriptional repressor that is induced by NF-kappaB (p65) and mediates NF-kappaB inhibitory effects on several myofibrillary genes, also was upregulated in S100B-overexpressing myoblasts. Conversely, silencing S100B expression in myoblast cell lines by RNA interference resulted in reduced NF-kappaB activity and enhanced MyoD, myogenin and MyHC expression and myotube formation. Thus, intracellular S100B might modulate myoblast differentiation by interfering with MyoD expression in an NF-kappaB-dependent manner.

Autocrine and paracrine angiopoietin 1/Tie-2 signaling promotes muscle satellite cell self-renewal

Mechanisms governing muscle satellite cell withdrawal from cell cycle to enter into quiescence remain poorly understood. We studied the role of angiopoietin 1 (Ang1) and its receptor Tie-2 in the regulation of myogenic precursor cell (mpc) fate. In human and mouse, Tie-2 was preferentially expressed by quiescent satellite cells in vivo and reserve cells (RCs) in vitro. Ang1/Tie-2 signaling, through ERK1/2 pathway, decreased mpc proliferation and differentiation, increased the number of cells in G0, increased expression of RC-associated markers (p130, Pax7, Myf-5, M-cadherin), and downregulated expression of differentiation-associated markers. Silencing Tie-2 had opposite effects. Cells located in the satellite cell neighborhood (smooth muscle cells, fibroblasts) upregulated RC-associated markers by secreting Ang1 in vitro. In vivo, Tie-2 blockade and Ang1 overexpression increased the number of cycling and quiescent satellite cells, respectively. We propose that Ang1/Tie-2 signaling regulates mpc self-renewal by controlling the return to quiescence of a subset of satellite cells.

A cell active chemical GEF inhibitor selectively targets the Trio/RhoG/Rac1 signaling pathway

RhoGEFs (guanine nucleotide exchange factors of the Rho GTPase family) are upstream regulators of cell adhesion and migration pathways, thus representing attractive yet relatively unexplored targets for the development of anti-invasive drugs. We screened for chemical inhibitors of TrioN, the N-terminal GEF domain of the multidomain Trio protein, and identified ITX3 as a nontoxic inhibitor. In transfected mammalian cells, ITX3 blocked TrioN-mediated dorsal membrane ruffling and Rac1 activation while having no effect on GEF337-, Tiam1-, or Vav2-mediated RhoA or Rac1 activation. ITX3 specifically inhibited endogenous TrioN activity, as evidenced by its ability to inhibit neurite outgrowth in nerve growth factor (NGF)-stimulated PC12 cells or C2C12 differentiation into myotubes. This study introduces a selective cell active inhibitor of the Trio/RhoG/Rac1 pathway and validates RhoGEFs as druggable targets.

The low-affinity receptor for neurotrophins p75NTR plays a key role for satellite cell function in muscle repair acting via RhoA

Regeneration of muscle fibers, lost during pathological muscle degeneration or after injuries, is mediated by the production of new myofibres. This process, sustained by the resident stem cells of the muscle, the satellite cells, is finely regulated by local cues, in particular by cytokines and growth factors. Evidence in the literature suggests that nerve growth factor (NGF) is involved in muscle fiber regeneration; however, its role and mechanism of action were unclear. We have investigated this issue in in vivo mouse models of muscle regeneration and in primary myogenic cells. Our results demonstrate that NGF acts through its low-affinity receptor p75(NTR) in a developmentally regulated signaling pathway necessary to myogenic differentiation and muscle repair in vivo. We also demonstrate that this action of NGF is mediated by the down-regulation of RhoA-GTP signaling in myogenic cells.

The rho-guanine nucleotide exchange factor domain of obscurin activates rhoA signaling in skeletal muscle

Obscurin is a large ( approximately 800-kDa), modular protein of striated muscle that concentrates around the M-bands and Z-disks of each sarcomere, where it is well positioned to sense contractile activity. Obscurin contains several signaling domains, including a rho-guanine nucleotide exchange factor (rhoGEF) domain and tandem pleckstrin homology domain, consistent with a role in rho signaling in muscle. We investigated the ability of obscurin's rhoGEF domain to interact with and activate small GTPases. Using a combination of in vitro and in vivo approaches, we found that the rhoGEF domain of obscurin binds selectively to rhoA, and that rhoA colocalizes with obscurin at the M-band in skeletal muscle. Other small GTPases, including rac1 and cdc42, neither associate with the rhoGEF domain of obscurin nor concentrate at the level of the M-bands. Furthermore, overexpression of the rhoGEF domain of obscurin in adult skeletal muscle selectively increases rhoA expression and activity in this tissue. Overexpression of obscurin's rhoGEF domain and its effects on rhoA alter the expression of rho kinase and citron kinase, both of which can be activated by rhoA in other tissues. Injuries to rodent hindlimb muscles caused by large-strain lengthening contractions increases rhoA activity and displaces it from the M-bands to Z-disks, similar to the effects of overexpression of obscurin's rhoGEF domain. Our results suggest that obscurin's rhoGEF domain signals at least in part by inducing rhoA expression and activation, and altering the expression of downstream kinases in vitro and in vivo.

Coordinated patterns of gene expressions for adult muscle build-up in transgenic mice expressing myostatin propeptide

Background

Skeletal muscle growth and maintenance are essential for human health. One of the muscle regulatory genes, namely myostatin, a member of transforming growth factor-β, plays a dominant role in the genetic control of muscle mass. Myostatin is synthesized as a precursor protein, which generates the N-terminal propeptide and the C-terminal mature myostatin peptide by a post-translational cleavage event. Previously, transgenic over-expression of myostatin propeptide in skeletal muscle results in significant muscle growth in early stages of development. The objectives of present study were to further characterize muscle growth in later stages of life and to identify genes and their expression patterns that are responsible for adult muscle build-up by myostatin propeptide.

Results

Immunohistochemical staining with an antibody to the N-terminus indicates a high level of myostatin propeptide present in the muscles of transgenic mice while there were no apparent differences in myostatin protein distribution in the muscle fibers between the transgenic and wild-type mice. Main individual muscles increased by 76–152% in the transgenic mice over their wild-type littermate mice at 12 months of age. A large number of nuclei were localized in the central and basal lamina of the myofibers in the transgenic mice as the number of nuclei per fiber and 100 μm² area was significantly higher in transgenic mice than wild-type mice. By systemic comparisons of global mRNA expression patterns between transgenic mice and wild-type littermates using microarray and qRT-PCR techniques, we have identified distinct gene expression patterns to support adult muscle build-up by myostatin propeptide, which are comprised of enhanced expressions of myogenic regulatory factors and extracelullar matrix components, and differentially down-regulated expressions of genes related to protein degradation and mitochondrial ATP synthesis.

Conclusion

The results present a coordinated pattern of gene expressions for reduced energy utilization during muscle build-up in adult stage. Enhanced muscle buildup by myostatin propeptide is sustained by reduced ATP synthesis as a result of a decreased activity of protein degradation. Myostatin propeptide may have a therapeutic application to the treatment of clinical muscle wasting problems by depressing myostatin activity.

Degenerative muscle fiber accelerates adipogenesis of intramuscular cells via RhoA signaling pathway

In some pathological conditions such as Duchenne muscular dystrophy, it has been known that a fatty infiltration in skeletal muscle is often observed and that is also one of primary factors to induce marked decline of muscular strength. However, the mechanism of fatty infiltration, cellular origin of accumulated adipocytes and its significance are not fully understood. The fact that persistent degenerative muscle fibers are present on dystrophic muscle leads us to hypothesize that muscle fiber condition affects fatty infiltration in skeletal muscle. We employed a single fiber culture system to determine whether fiber condition affects an appearance of adipocytes on the fibers. Artificially hyper-contracted muscle fibers (HCF), generated from isolated intact fibers (IF) of rat extensor digitrum longus muscle, were maintained as non-adherent cultures for 5-7 days. Interestingly, there appeared to be considerable numbers of mature adipocytes on HCF, whereas no adipocytes were seen on IF, indicating that cells on HCF spontaneously differentiated into mature adipocytes. Activation of RhoA signaling by the addition of thrombin decreased the number of adipocytes on HCF in a dose-dependent manner, whereas the number of MyoD-positive myoblasts increased. In contrast, Y-27632, a specific inhibitor of Rho kinases (ROCK), induced adipogenic differentiation of cells derived from IF. In addition, administration of Y-27632 into mouse regenerating muscle resulted in fat accumulation in the muscle. Taken together, the present studies clearly demonstrated that muscle fiber condition affects fat accumulation in skeletal muscle and that is possibly mediated by the RhoA signaling pathway.

Engineering a multi-nucleated myotube, the role of the actin cytoskeleton

Myoblasts in vitro form characteristic arrays of bipolar-shaped cells prior to fusion. We have shown that the actin cytoskeleton re-organizes in these fusing cells and that the interaction of non-muscle myosin 2A with actin at the plasma membrane helps to generate the bipolar shape of myoblasts, which is key for fusion. Here we discuss how fusion occurs, and in particular how the actin cytoskeleton is involved. Myoblast fusion is essential to form the multi-nucleated muscle fibres that make up the skeletal muscle. Skeletal muscle fibres contain many nuclei, roughly one nucleus to every 15 sarcomeres (35 microm) in adult muscle, although this varies with muscle type (Bruusgaard et al., 2006). Thus a muscle fibre 30 cm long contains about 8000 nuclei and is formed by the fusion of about 8000 cells during development. The formation of multi-nucleated myotubes has been intensively studied for many years using a number of different systems. Many different proteins have been identified using Drosophila as a model system (e.g. see reviews by Taylor, 2000, 2002) that have given an insight into what happens in mammals. However, the process of fusion of mammalian cells is less well understood, and this paper will cover some of the aspects of mammalian myoblast fusion, with a particular focus on the role of the actin cytoskeleton.

Rho/Rho-associated kinase signal regulates myogenic differentiation via myocardin-related transcription factor-A/Smad-dependent transcription of the Id3 gene

RhoA is known to be involved in myogenic differentiation, but whether it acts as a positive or negative regulator is controversial. To resolve this issue, we investigated the differentiation stage-specific roles of RhoA and its effector, Rho-associated kinase, using C2C12 myoblasts. We found that proliferating myoblasts show high levels of RhoA and serum-response factor activities and strong expression of the downstream target of RhoA, myocardin-related transcription factor-A (MRTF-A or MAL); these activities and expression are markedly lower in differentiating myocytes. We further demonstrated that, in proliferating myoblasts, an increase in MRTF-A, which forms a complex with Smad1/4, strikingly activates the expression level of the Id3 gene; the Id3 gene product is a potent inhibitor of myogenic differentiation. Finally, we found that during differentiation, one of the forkhead transcription factors translocates into the nucleus and suppresses Id3 expression by preventing the association of the MRTF-A-Smad complex with the Id3 promoter, which leads to the enhancement of myogenic differentiation. We conclude that RhoA/Rho-associated kinase signaling plays positive and negative roles in myogenic differentiation, mediated by MRTF-A/Smad-dependent transcription of the Id3 gene in a differentiation stage-specific manner.

Inhibition of ROCK activity allows InlF-mediated invasion and increased virulence of Listeria monocytogenes

Listeria monocytogenes is an intracellular bacterial pathogen that causes life-threatening disease. The mechanisms used by L. monocytogenes to invade non-professional phagocytic cells are not fully understood. In addition to the requirement of bacterial determinants, host cell conditions profoundly influence infection. Here, we have shown that inhibition of the RhoA/ROCK pathway by pharmacological inhibitors or RNA interference results in increased L. monocytogenes invasion of murine fibroblasts and hepatocytes. InlF, a member of the internalin multigene family with no known function, was identified as a L. monocytogenes-specific factor mediating increased host cell binding and entry. Conversely, activation of RhoA/ROCK activity resulted in decreased L. monocytogenes adhesion and invasion. Furthermore, virulence of wild-type bacteria during infection of mice was significantly increased upon inhibition of ROCK activity, whereas colonization and virulence of an inlF deletion mutant was not affected, thus supporting a role for InlF as a functional virulence determinant in vivo under specific conditions. In addition, inhibition of ROCK activity in human-derived cells enhanced either bacterial adhesion or adhesion and entry in an InlF-independent manner, further suggesting a host species or cell type-specific role for InlF and that additional bacterial determinants are involved in mediating ROCK-regulated invasion of human cells.

Functional role of sortilin in myogenesis and development of insulin-responsive glucose transport system in C2C12 myocytes

Sortilin has been implicated in the formation of insulin-responsive GLUT4 storage vesicles in adipocytes by regulating sorting events between the trans-Golgi-network and endosomes. We herein show that sortilin serves as a potent myogenic differentiation stimulator for C2C12 myocytes by cooperatively functioning with p75NTR, which subsequently further contributes to development of the insulin-responsive glucose transport system in C2C12 myotubes. Sortilin expression was up-regulated upon C2C12 differentiation, and overexpression of sortilin in C2C12 cells significantly stimulated myogenic differentiation, a response that was completely abolished by either anti-p75NTR- or anti-nerve growth factor (NGF)-neutralizing antibodies. Importantly, small interference RNA-mediated suppression of endogenous sortilin significantly inhibited C2C12 differentiation, indicating the physiological significance of sortilin expression in the process of myogenesis. Although sortilin overexpression in C2C12 myotubes improved insulin-induced 2-deoxyglucose uptake, as previously reported, this effect apparently resulted from a decrease in the cellular content of GLUT1 and an increase in GLUT4 via differentiation-dependent alterations at both the gene transcriptional and the post-translational level. In addition, cellular contents of Ubc9 and SUMO-modified proteins appeared to be increased by sortilin overexpression. Taken together, these data demonstrate that sortilin is involved not only in development of the insulin-responsive glucose transport system in myocytes, but is also directly involved in muscle differentiation via modulation of proNGF-p75NTR.

Proteome analysis of differentiating human myoblasts by dialysis-assisted two-dimensional gel electrophoresis (DAGE)

In the present study, modifications in cytosolic expressed proteins during human myoblast differentiation were studied by dialysis-assisted 2-DE (DAGE, [1]). About 1000 spots were analysed on the 5th and 13th day of differentiation with a dynamic range of protein expression exceeding 1000-fold. During myogenic differentiation, the number of nonmatching spots as well as the extent of quantitative differences between matched spots significantly increased. Over one hundred differentially expressed spots were excised and identified by MALDI-TOF MS. The differentiation-associated expression pattern of eight proteins was validated by Western blot analysis. Differential expression of several proteins was demonstrated for the first time in human myotubes. Interestingly, Ingenuity pathway analysis grouped 30 of these proteins into two overlapping networks containing as principal nodes IGF-1 and tumour necrosis factor, two proteins known to play a crucial role in cytogenesis. Our results illustrate the large rearrangement of the proteome during the differentiation of human myoblasts and provide evidence for new partners involved in this complex process.

RhoE controls myoblast alignment prior fusion through RhoA and ROCK

Differentiation of skeletal myoblasts into multinucleated myotubes is a multi-step process orchestrated by several signaling pathways. The Rho small G protein family plays critical roles both during myogenesis induction and myoblast fusion. We report here that in C2C12 myoblasts, expression of RhoE, an atypical member of this family, increases until the onset of myoblast fusion before resuming its basal level once fusion has occurred. We show that RhoE accumulates in elongated, aligned myoblasts prior to fusion and that its expression is also increased during injury-induced skeletal muscle regeneration. Moreover, although RhoE is not required for myogenesis induction, it is essential for myoblast elongation and alignment before fusion and for M-cadherin expression and accumulation at the cell-cell contact sites. Myoblasts lacking RhoE present with defective p190RhoGAP activation and RhoA inhibition at the onset of myoblast fusion. RhoE interacts also with the RhoA effector Rho-associated kinase (ROCK)I whose activity must be downregulated to allow myoblast fusion. Consistently, we show that pharmacological inactivation of RhoA or ROCK restores myoblast fusion in RhoE-deficient myoblasts. RhoE physiological upregulation before myoblast fusion is responsible for the decrease in RhoA and ROCKI activities, which are required for the fusion process. Therefore, we conclude that RhoE is an essential regulator of myoblast fusion.

Stem cell trafficking in tissue development, growth, and disease

Regulated movement of stem cells is critical for organogenesis during development and for homeostasis and repair in adulthood. Here we analyze the biological significance and molecular mechanisms underlying stem cell trafficking in the generation of the germline, and the generation and regeneration of blood and muscle. Comparison across organisms and lineages reveals remarkable conservation as well as specialization in homing and migration mechanisms used by mature leukocytes, adult and fetal stem cells, and cancer stem cells. In vivo trafficking underpins the successful therapeutic application of hematopoietic stem cells for bone-marrow transplant, and further elucidation of homing and migration pathways in other systems will enable broader application of stem cells for targeted cell therapy and drug delivery.

Molecular control of mammalian myoblast fusion

The fusion of postmitotic mononucleated myoblasts to form syncytial myofibers is a critical step in the formation of skeletal muscle. Myoblast fusion occurs both during development and throughout adulthood, as skeletal muscle growth and regeneration require the accumulation of additional nuclei within myofibers. Myoblasts must undergo a complex series of molecular and morphological changes prior to fusing with one another. Although many molecules regulating myoblast fusion have been identified, the precise mechanism by which these molecules act in concert to control fusion remains to be elucidated. A comprehensive understanding of how myo-blast fusion is controlled may contribute to the treatment of various disorders associated with loss of muscle mass. In this chapter, we examine progress made toward elucidating the cellular and molecular pathways involved in mammalian myoblast fusion. Special emphasis is placed on the molecules that regulate myofiber formation without discernibly affecting biochemical differentiation.

A role for the Myoblast city homologues Dock1 and Dock5 and the adaptor proteins Crk and Crk-like in zebrafish myoblast fusion

Myoblast fusion follows a defined sequence of events that is strikingly similar in vertebrates and invertebrates. Genetic analysis in Drosophila has identified many of the molecules that mediate the different steps in the fusion process; by contrast, the molecular basis of myoblast fusion during vertebrate embryogenesis remains poorly characterised. A key component of the intracellular fusion pathway in Drosophila is the protein encoded by the myoblast city (mbc) gene, a close homologue of the vertebrate protein dedicator of cytokinesis 1 (DOCK1, formerly DOCK180). Using morpholino antisense-oligonucleotide-mediated knockdown of gene activity in the zebrafish embryo, we show that the fusion of embryonic fast-twitch myoblasts requires the activities of Dock1 and the closely related Dock5 protein. In addition, we show that the adaptor proteins Crk and Crk-like (Crkl), with which Dock proteins are known to interact physically, are also required for myoblast fusion.

XRab40 and XCullin5 form a ubiquitin ligase complex essential for the noncanonical Wnt pathway

Rab GTPases are key regulators of intracellular membrane trafficking. We sought to elucidate the roles of Rab GTPases in Xenopus gastrulation, and found that a Xenopus homolog of Rab40 (XRab40) is required for normal gastrulation. XRab40 is localized at the Golgi apparatus and interacts with ElonginB/C and Cullin5 to form a ubiquitin ligase. XRab40/XCullin5 functions cooperatively and regulates the ubiquitination and localization of Rap2 GTPase. Furthermore, XRab40/XCullin5 regulates the membrane localization of Dishevelled (Dsh), a key signaling molecule in the Wnt pathway, through Rap2 and its effector Misshapen/Nck-interacting kinase (XMINK). XMINK interacts with Dsh, and is translocated to the plasma membrane by Wnt activation. We propose a novel signaling cascade consisting of XRab40/XCullin5, Rap2 and XMINK, which plays a crucial role in the regulation of the noncanonical Wnt pathway.

ROCK2 and its alternatively spliced isoform ROCK2m positively control the maturation of the myogenic program

Signal transduction cascades involving Rho-associated kinases (ROCK), the serine/threonine kinases downstream effectors of Rho, have been implicated in the regulation of diverse cellular functions including cytoskeletal organization, cell size control, modulation of gene expression, differentiation, and transformation. Here we show that ROCK2, the predominant ROCK isoform in skeletal muscle, is progressively up-regulated during mouse myoblast differentiation and is highly expressed in the dermomyotome and muscle precursor cells of mouse embryos. We identify a novel and evolutionarily conserved ROCK2 splicing variant, ROCK2m, that is preferentially expressed in skeletal muscle and strongly up-regulated during in vivo and in vitro differentiation processes. The specific knockdown of ROCK2 or ROCK2m expression in C2C12 myogenic cells caused a significant and selective impairment of the expression of desmin and of the myogenic regulatory factors Mrf4 and MyoD. We demonstrate that in myogenic cells, ROCK2 and ROCK2m are positive regulators of the p42 and p44 mitogen-activated protein kinase-p90 ribosomal S6 kinase-eucaryotic elongation factor 2 intracellular signaling pathways and, thereby, positively regulate the hypertrophic effect elicited by insulin-like growth factor 1 and insulin, linking the multifactorial functions of ROCK to an important control of the myogenic maturation.

First step of the cell-penetrating peptide mechanism involves Rac1 GTPase-dependent actin-network remodelling

BACKGROUND INFORMATION

Application of CPPs (cell-penetrating peptides) constitutes a promising strategy for the intracellular delivery of therapeutic molecules. The non-covalent approach based on the amphipathic peptide MPG has been successfully used to improve the delivery of biologically active macromolecules, both in cellulo and in vivo, through a mechanism independent of the endosomal pathway and mediated by the membrane potential.

RESULTS

In the present study, we have investigated the first step of the cellular uptake mechanism of MPG and shown that both MPG and MPG-cargo complexes interact with the extracellular matrix through the negatively charged heparan sulfate proteoglycans. We demonstrated that initiation of cellular uptake constitutes a highly dynamic mechanism where the binding of MPG or the MPG-cargo to the extracellular matrix is rapidly followed by a remodelling of the actin network associated with the activation of the GTPase Rac1. We suggest that MPG-induced clustering of the glycosaminoglycan platform constitutes the 'onset' of the cellular uptake mechanism, thereby increasing membrane dynamics and membrane fusion processes. This process favours cell entry of MPG or MPG-DNA complexes, which is further controlled by the ability of MPG to induce a local membrane destabilization.

CONCLUSIONS

Although CPPs are taken up through different pathways and mechanisms, the initial step involves electrostatic interactions with the glycosaminoglycan platform, and the dynamics of associated membrane microdomains can be generalized to most non-viral delivery systems.