Mutant MyoD lacking Cdc2 phosphorylation sites delays M-phase entry

Migration and differentiation of muscle stem cells are coupled by RhoA signalling during regeneration

Skeletal muscle is highly regenerative and is mediated by a population of migratory adult muscle stem cells (muSCs). Effective muscle regeneration requires a spatio-temporally regulated response of the muSC population to generate sufficient muscle progenitor cells that then differentiate at the appropriate time. The relationship between muSC migration and cell fate is poorly understood and it is not clear how forces experienced by migrating cells affect cell behaviour. We have used zebrafish to understand the relationship between muSC cell adhesion, behaviour and fate in vivo. Imaging of pax7-expressing muSCs as they respond to focal injuries in trunk muscle reveals that they migrate by protrusive-based means. By carefully characterizing their behaviour in response to injury we find that they employ an adhesion-dependent mode of migration that is regulated by the RhoA kinase ROCK. Impaired ROCK activity results in reduced expression of cell cycle genes and increased differentiation in regenerating muscle. This correlates with changes to focal adhesion dynamics and migration, revealing that ROCK inhibition alters the interaction of muSCs to their local environment. We propose that muSC migration and differentiation are coupled processes that respond to changes in force from the environment mediated by RhoA signalling.

DNA-PKcs regulates myogenesis in an Akt-dependent manner independent of induced DNA damage

Skeletal muscle regeneration relies on muscle stem (satellite) cells. We previously demonstrated that satellite cells efficiently and accurately repair radiation-induced DNA double-strand breaks (DSBs) via the DNA-dependent kinase DNA-PKcs. We show here that DNA-PKcs affects myogenesis independently of its role in DSB repair. Consequently, this process does not require the accumulation of DSBs and it is also independent of caspase-induced DNA damage. We report that in myogenic cells DNA-PKcs is essential for the expression of the differentiation factor Myogenin in an Akt2-dependent manner. DNA-PKcs interacts with the p300-containing complex that activates Myogenin transcription. We show also that SCID mice that are deficient in DNA-PKcs, and are used for transplantation and muscle regeneration studies, display altered myofiber composition and delayed myogenesis upon injury. These defects are exacerbated after repeated injury/regeneration events resulting in reduced muscle size. We thus identify a novel, caspase-independent, regulation of myogenic differentiation, and define a differentiation phase that does not involve the DNA damage/repair process.

Palmitic Acid Inhibits Myogenic Activity and Expression of Myosin Heavy Chain MHC IIb in Muscle Cells through Phosphorylation-Dependent MyoD Inactivation

Sarcopenia associated with aging and obesity is characterized by the atrophy of fast-twitch muscle fibers and an increase in intramuscular fat deposits. However, the mechanism of fast-twitch fiber-specific atrophy remains unclear. In this study, we aimed to assess the effect of palmitic acid (PA), the most common fatty acid component of human fat, on muscle fiber type, focusing on the expression of fiber-type-specific myosin heavy chain (MHC). Myotubes differentiated from C2C12 myoblasts were treated with PA. The PA treatment inhibited myotube formation and hypertrophy while reducing the gene expression of MHC IIb and IIx, specific isoforms of fast-twitch fibers. Consistent with this, a significant suppression of MHC IIb protein expression in PA-treated cells was observed. A reporter assay using plasmids containing the MHC IIb gene promoter revealed that the PA-induced reduction in MHC IIb gene expression was caused by the suppression of MyoD transcriptional activity through its phosphorylation. Treatment with a specific protein kinase C (PKC) inhibitor recovered the reduction in MHC IIb gene expression levels in PA-treated cells, suggesting the involvement of the PA-induced activation of PKC. Thus, PA selectively suppresses the mRNA and protein expression of fast-twitch MHC by modulating MyoD activity. This finding provides a potential pathogenic mechanism for age-related sarcopenia.

From cyclins to CDKIs: Cell cycle regulation of skeletal muscle stem cell quiescence and activation

After extensive proliferation during development, the adult skeletal muscle cells remain outside the cell cycle, either as post-mitotic myofibers or as quiescent muscle stem cells (MuSCs). Despite its terminally differentiated state, adult skeletal muscle has a remarkable regeneration potential, driven by MuSCs. Upon injury, MuSC quiescence is reversed to support tissue growth and repair and it is re-established after the completion of muscle regeneration. The distinct cell cycle states and transitions observed in the different myogenic populations are orchestrated by elements of the cell cycle machinery. This consists of i) complexes of cyclins and Cyclin-Dependent Kinases (CDKs) that ensure cell cycle progression and ii) their negative regulators, the Cyclin-Dependent Kinase Inhibitors (CDKIs). In this review we discuss the roles of these factors in developmental and adult myogenesis, with a focus on CDKIs that have emerging roles in stem cell functions.

Intravital imaging reveals cell cycle-dependent myogenic cell migration during muscle regeneration

During muscle regeneration, extracellular signal-regulated kinase (ERK) promotes both proliferation and migration. However, the relationship between proliferation and migration is poorly understood in this context. To elucidate this complex relationship on a physiological level, we established an intravital imaging system for measuring ERK activity, migration speed, and cell-cycle phases in mouse muscle satellite cell-derived myogenic cells. We found that in vivo, ERK is maximally activated in myogenic cells two days after injury, and this is then followed by increases in cell number and motility. With limited effects of ERK activity on migration on an acute timescale, we hypothesized that ERK increases migration speed in the later phase by promoting cell-cycle progression. Our cell-cycle analysis further revealed that in myogenic cells, ERK activity is critical for G1/S transition, and cells migrate more rapidly in S/G2 phase 3 days after injury. Finally, migration speed of myogenic cells was suppressed after CDK1/2-but not CDK1-inhibitor treatment, demonstrating a critical role of CDK2 in myogenic cell migration. Overall, our study demonstrates that in myogenic cells, the ERK-CDK2 axis promotes not only G1/S transition but also migration, thus providing a novel mechanism for efficient muscle regeneration.

CK2-Dependent Phosphorylation of the Brg1 Chromatin Remodeling Enzyme Occurs during Mitosis

Brg1 (Brahma-related gene 1) is one of two mutually exclusive ATPases that can act as the catalytic subunit of mammalian SWI/SNF (mSWI/SfigureNF) chromatin remodeling enzymes that facilitate utilization of the DNA in eukaryotic cells. Brg1 is a phospho-protein, and its activity is regulated by specific kinases and phosphatases. Previously, we showed that Brg1 interacts with and is phosphorylated by casein kinase 2 (CK2) in a manner that regulates myoblast proliferation. Here, we use biochemical and cell and molecular biology approaches to demonstrate that the Brg1-CK2 interaction occurred during mitosis in embryonic mouse somites and in primary myoblasts derived from satellite cells isolated from mouse skeletal muscle tissue. The interaction of CK2 with Brg1 and the incorporation of a number of other subunits into the mSWI/SNF enzyme complex were independent of CK2 enzymatic activity. CK2-mediated hyperphosphorylation of Brg1 was observed in mitotic cells derived from multiple cell types and organisms, suggesting functional conservation across tissues and species. The mitotically hyperphosphorylated form of Brg1 was localized with soluble chromatin, demonstrating that CK2-mediated phosphorylation of Brg1 is associated with specific partitioning of Brg1 within subcellular compartments. Thus, CK2 acts as a mitotic kinase that regulates Brg1 phosphorylation and subcellular localization.

iPTMnet: an integrated resource for protein post-translational modification network discovery

Protein post-translational modifications (PTMs) play a pivotal role in numerous biological processes by modulating regulation of protein function. We have developed iPTMnet (http://proteininformationresource.org/iPTMnet) for PTM knowledge discovery, employing an integrative bioinformatics approach—combining text mining, data mining, and ontological representation to capture rich PTM information, including PTM enzyme-substrate-site relationships, PTM-specific protein-protein interactions (PPIs) and PTM conservation across species. iPTMnet encompasses data from (i) our PTM-focused text mining tools, RLIMS-P and eFIP, which extract phosphorylation information from full-scale mining of PubMed abstracts and full-length articles; (ii) a set of curated databases with experimentally observed PTMs; and iii) Protein Ontology that organizes proteins and PTM proteoforms, enabling their representation, annotation and comparison within and across species. Presently covering eight major PTM types (phosphorylation, ubiquitination, acetylation, methylation, glycosylation, S-nitrosylation, sumoylation and myristoylation), iPTMnet knowledgebase contains more than 654 500 unique PTM sites in over 62 100 proteins, along with more than 1200 PTM enzymes and over 24 300 PTM enzyme-substrate-site relations. The website supports online search, browsing, retrieval and visual analysis for scientific queries. Several examples, including functional interpretation of phosphoproteomic data, demonstrate iPTMnet as a gateway for visual exploration and systematic analysis of PTM networks and conservation, thereby enabling PTM discovery and hypothesis generation.

Cell cycle-dependent phosphorylation and regulation of cellular differentiation

Embryogenesis requires an exquisite regulation of cell proliferation, cell cycle withdrawal and differentiation into a massively diverse range of cells at the correct time and place. Stem cells also remain to varying extents in different adult tissues, acting in tissue homeostasis and repair. Therefore, regulated proliferation and subsequent differentiation of stem and progenitor cells remains pivotal throughout life. Recent advances have characterised the cell cycle dynamics, epigenetics, transcriptome and proteome accompanying the transition from proliferation to differentiation, revealing multiple bidirectional interactions between the cell cycle machinery and factors driving differentiation. Here, we focus on a direct mechanistic link involving phosphorylation of differentiation-associated transcription factors by cell cycle-associated Cyclin-dependent kinases. We discuss examples from the three embryonic germ layers to illustrate this regulatory mechanism that co-ordinates the balance between cell proliferation and differentiation.

DCM associated LMNA mutations cause distortions in lamina structure and assembly

BACKGROUND

A and B-type lamins are integral scaffolding components of the nuclear lamina which impart rigidity and shape to all metazoan nuclei. Over 450 mutations in A-type lamins are associated with 16 human diseases including dilated cardiomyopathy (DCM). Here, we show that DCM mutants perturb the self-association of lamin A (LA) and it's binding with lamin B1 (LB1).

METHODS

We used confocal and superresolution microscopy (NSIM) to study the effect of LA mutants on the nuclear lamina. We further used circular dichroism, fluorescence spectroscopy and isothermal titration calorimetry (ITC) to probe the structural modulations, self-association and heteropolymeric association of mutant LA.

RESULTS

Transfection of mutants in cultured cell lines result in the formation of nuclear aggregates of varied size and distribution. Endogenous LB1 is sequestered into these aggregates. This is consistent with the ten-fold increase in association constant of the mutant proteins compared to the wild type. These mutants exhibit differential heterotypic interaction with LB1, along with significant secondary and tertiary structural alterations of the interacting proteins. Thermodynamic studies demonstrate that the mutants bind to LB1 with different stoichiometry, affinity and energetics.

CONCLUSIONS

In this report we show that increased self-association propensity of mutant LA modulates the LA-LB1 interaction and precludes the formation of an otherwise uniform laminar network.

GENERAL SIGNIFICANCE

Our results might highlight the role of homotypic and heterotypic interactions of LA in the pathogenesis of DCM and hence laminopathies in the broader sense.

MyoD phosphorylation on multiple C terminal sites regulates myogenic conversion activity

MyoD is a master regulator of myogenesis with a potent ability to redirect the cell fate of even terminally differentiated cells. Hence, enhancing the activity of MyoD is an important step to maximising its potential utility for in vitro disease modelling and cell replacement therapies. We have previously shown that the reprogramming activity of several neurogenic bHLH proteins can be substantially enhanced by inhibiting their multi-site phosphorylation by proline-directed kinases. Here we have used Xenopus embryos as an in vivo developmental and reprogramming system to investigate the multi-site phospho-regulation of MyoD during muscle differentiation. We show that, in addition to modification of a previously well-characterised site, Serine 200, MyoD is phosphorylated on multiple additional serine/threonine sites during primary myogenesis. Through mutational analysis, we derive an optimally active phospho-mutant form of MyoD that has a dramatically enhanced ability to drive myogenic reprogramming in vivo. Mechanistically, this is achieved through increased protein stability and enhanced chromatin association. Therefore, multi-site phospho-regulation of class II bHLH proteins is conserved across cell lineages and germ layers, and manipulation of phosphorylation of these key regulators may have further potential for enhancing mammalian cell reprogramming.

Phosphorylation-dependent degradation of MEF2C contributes to regulate G2/M transition

The Myocyte Enhancer Factor 2C (MEF2C) transcription factor plays a critical role in skeletal muscle differentiation, promoting muscle-specific gene transcription. Here we report that in proliferating cells MEF2C is degraded in mitosis by the Anaphase Promoting Complex/Cyclosome (APC/C) and that this downregulation is necessary for an efficient progression of the cell cycle. We show that this mechanism of degradation requires the presence on MEF2C of a D-box (R-X-X-L) and 2 phospho-motifs, pSer98 and pSer110. Both the D-box and pSer110 motifs are encoded by the ubiquitous alternate α1 exon. These two domains mediate the interaction between MEF2C and CDC20, a co-activator of APC/C. We further report that in myoblasts, MEF2C regulates the expression of G2/M checkpoint genes (14–3–3γ, Gadd45b and p21) and the sub-cellular localization of CYCLIN B1. The importance of controlling MEF2C levels during the cell cycle is reinforced by the observation that modulation of its expression affects the proliferation rate of colon cancer cells. Our findings show that beside the well-established role as pro-myogenic transcription factor, MEF2C can also function as a regulator of cell proliferation.

Direct reprogramming of fibroblasts to myocytes via bacterial injection of MyoD protein

Forced exogenous gene expression has been well characterized as an effective method for directing both cellular differentiation and dedifferentiation. However, transgene expression is not amenable for therapeutic application due to potential insertional mutagenesis. Protein-based techniques provide a safe alternative, but current protein delivery methods are quite limited by labor-intensive purification processes, low protein yield, and inefficient intracellular targeting. Such limitations may be overcome by using a naturally occurring bacterial protein injection system, called the type III secretion system (T3SS), which injects bacterial proteins directly into the eukaryotic cell cytoplasm. Using a genetically attenuated strain of Pseudomonas aeruginosa, we have previously described the ability of this system to easily deliver a high quantity of protein to both differentiated and pluripotent cells. MyoD is a key muscle regulatory factor, the overexpression of which is able to induce transdifferentiation of numerous cell types into functional myocytes. Here we demonstrate transient injection of MyoD protein by P. aeruginosa to be sufficient to induce myogenic conversion of mouse embryonic fibroblasts. In addition to clear morphological changes, muscle-specific gene expression has been observed both at mRNA and protein levels. These studies serve as a foundation for the bacterial delivery of transcription factors to efficiently modulate concentration-dependent and temporal activation of gene expression that directs cell fate without jeopardizing genomic integrity.

HUWE1 ubiquitinates MyoD and targets it for proteasomal degradation

MyoD is a tissue-specific transcriptional activator that acts as a master switch for muscle development. It activates a broad array of muscle-specific genes, which leads to conversion of proliferating myoblasts into mature myotubes. The ubiquitin proteasome system (UPS) plays an important role in controlling MyoD. Both its N-terminal residue and internal lysines can be targeted by ubiquitin, and both modifications appear to direct it for proteasomal degradation. The protein is short-lived and has a half-life of ∼45min in different cells. It was reported that MyoD can be ubiquitinated by MAFbx/AT-1, but accumulating lines of experimental evidence showed that other ligase(s) may also participate in its targeting. Here we describe the involvement of HUWE1 in the ubiquitination and proteasomal degradation of MyoD. Furthermore, we show that the ligase can ubiquitinate the protein in its N-terminal residue.

Secretome Analysis of Skeletal Myogenesis Using SILAC and Shotgun Proteomics

Myogenesis, the formation of skeletal muscle, is a multistep event that commences with myoblast proliferation, followed by cell-cycle arrest, and finally the formation of multinucleated myotubes via fusion of mononucleated myoblasts. Each step is orchestrated by well-documented intracellular factors, such as cytoplasmic signalling molecules and nuclear transcription factors. Regardless, the key step in getting a more comprehensive understanding of the regulation of myogenesis is to explore the extracellular factors that are capable of eliciting the downstream intracellular factors. This could further provide valuable insight into the acute cellular response to extrinsic cues in maintaining normal muscle development. In this paper, we survey the intracellular factors that respond to extracellular cues that are responsible for the cascades of events during myogenesis: myoblast proliferation, cell-cycle arrest of myoblasts, and differentiation of myoblasts into myotubes. This focus on extracellular perspective of muscle development illustrates our mass spectrometry-based proteomic approaches to identify differentially expressed secreted factors during skeletal myogenesis.

Conditional TGF-β1 treatment increases stem cell-like cell population in myoblasts

The limitation in successfully acquiring large populations of stem cell has impeded their application. A new method based on the dedifferentiation of adult somatic cells to generate induced multipotent stem cells would allow us to obtain a large amount of autologous stem cells for regenerative medicine. The current work was proposed to induce a sub-population of cells with characteristics of muscle stem cells from myoblasts through conditional treatment of transforming growth factor (TGF)-β(1) . Our results show that a lower concentration of TGF-β(1) is able to promote C2C12 myoblasts to express stem cell markers as well as to repress myogenic proteins, which involves a mechanism of dedifferentiation. Moreover, TGF-β(1) treatment promoted the proliferation-arrested C2C12 myoblasts to re-enter the S-phase. We also investigated the multi-differentiation potentials of the dedifferentiated cells. TGF-β(1) pre-treated C2C12 myoblasts were implanted into mice to repair dystrophic skeletal muscle or injured bone. In addition to the C2C12 myoblasts, similar effects of TGF-β(1) were also observed in the primary myoblasts of mice. Our results suggest that TGF-β(1) is effective as a molecular trigger for the dedifferentiation of skeletal muscle myoblasts and could be used to generate a large pool of progenitor cells that collectively behave as multipotent stem cell-like cells for regenerative medicine applications.

Human NDR kinases control G(1)/S cell cycle transition by directly regulating p21 stability

The G(1) phase of the cell cycle is an important integrator of internal and external cues, allowing a cell to decide whether to proliferate, differentiate, or die. Multiple protein kinases, among them the cyclin-dependent kinases (Cdks), control G(1)-phase progression and S-phase entry. With the regulation of apoptosis, centrosome duplication, and mitotic chromosome alignment downstream of the HIPPO pathway components MST1 and MST2, mammalian NDR kinases have been implicated to function in cell cycle-dependent processes. Although they are well characterized in terms of biochemical regulation and upstream signaling pathways, signaling mechanisms downstream of mammalian NDR kinases remain largely unknown. We identify here a role for human NDR in regulating the G(1)/S transition. In G(1) phase, NDR kinases are activated by a third MST kinase (MST3). Significantly, interfering with NDR and MST3 kinase expression results in G(1) arrest and subsequent proliferation defects. Furthermore, we describe the first downstream signaling mechanisms by which NDR kinases regulate cell cycle progression. Our findings suggest that NDR kinases control protein stability of the cyclin-Cdk inhibitor protein p21 by direct phosphorylation. These findings establish a novel MST3-NDR-p21 axis as an important regulator of G(1)/S progression of mammalian cells.

Mammalian target of rapamycin regulates miRNA-1 and follistatin in skeletal myogenesis

Mammalian target of rapamycin (mTOR) has emerged as a key regulator of skeletal muscle development by governing distinct stages of myogenesis, but the molecular pathways downstream of mTOR are not fully understood. In this study, we report that expression of the muscle-specific micro-RNA (miRNA) miR-1 is regulated by mTOR both in differentiating myoblasts and in mouse regenerating skeletal muscle. We have found that mTOR controls MyoD-dependent transcription of miR-1 through its upstream enhancer, most likely by regulating MyoD protein stability. Moreover, a functional pathway downstream of mTOR and miR-1 is delineated, in which miR-1 suppression of histone deacetylase 4 (HDAC4) results in production of follistatin and subsequent myocyte fusion. Collective evidence strongly suggests that follistatin is the long-sought mTOR-regulated fusion factor. In summary, our findings unravel for the first time a link between mTOR and miRNA biogenesis and identify an mTOR-miR-1-HDAC4-follistatin pathway that regulates myocyte fusion during myoblast differentiation in vitro and skeletal muscle regeneration in vivo.

Down-regulation of MyoD by calpain 3 promotes generation of reserve cells in C2C12 myoblasts

Calpain 3 is a calcium-dependent cysteine protease that is primarily expressed in skeletal muscle and is implicated in limb girdle muscular dystrophy type 2A. To date, its best characterized function is located within the sarcomere, but this protease is found in other cellular compartments, which suggests that it exerts multiple roles. Here, we present evidence that calpain 3 is involved in the myogenic differentiation process. In the course of in vitro culture of myoblasts to fully differentiated myotubes, a population of quiescent undifferentiated "reserve cells" are maintained. These reserve cells are closely related to satellite cells responsible for adult muscle regeneration. In the present work, we observe that reserve cells express higher levels of endogenous Capn3 mRNA than proliferating myoblasts. We show that calpain 3 participates in the establishment of the pool of reserve cells by decreasing the transcriptional activity of the key myogenic regulator MyoD via proteolysis independently of the ubiquitin-proteasome degradation pathway. Our results identify calpain 3 as a potential new player in the muscular regeneration process by promoting renewal of the satellite cell compartment.

Phosphorylation-dependent regulation of SATB1, the higher-order chromatin organizer and global gene regulator

The chromatin organizer SATB1 regulates distant genes by selectively tethering matrix attachment regions (MARs) to the nuclear matrix. Post-translational modifications (PTMs) are important regulators of functional activities of proteins. Recently, a phosphorylation-dependent molecular switch that provided insights into the molecular mechanism of transcriptional repression and activation by SATB1 was discovered. SATB1 is specifically phosphorylated by protein kinase C (PKC) at serine 185 in vivo, and this modification leads to repression of transcription by SATB1 via increased association with the histone deacetylase 1 (HDAC1) corepressor. In this chapter, we describe methods for overexpression and purification of full length SATB1 protein and for its in vitro phosphorylation. We also describe method for in vivo phosphorylation of SATB1 upon immunoprecipitation using anti-SATB1. Finally, we describe a functional assay to monitor the effect of phosphorylation on transcription activity of SATB1 in vivo using MAR-linked reporter assay, in the presence and absence of PKC inhibitors.

p38-{gamma}-dependent gene silencing restricts entry into the myogenic differentiation program

The regenerative capacity of muscle is regulated by p38-γ, which phosphorylates MyoD and leads to formation of a complex that represses myogenin transcription.

The mitogen-activated protein kinase p38-γ is highly expressed in skeletal muscle and is associated with the dystrophin glycoprotein complex; however, its function remains unclear. After induced damage, muscle in mice lacking p38-γ generated significantly fewer myofibers than wild-type muscle. Notably, p38-γ-deficient muscle contained 50% fewer satellite cells that exhibited premature Myogenin expression and markedly reduced proliferation. We determined that p38-γ directly phosphorylated MyoD on Ser199 and Ser200, which results in enhanced occupancy of MyoD on the promoter of myogenin together with markedly decreased transcriptional activity. This repression is associated with extensive methylation of histone H3K9 together with recruitment of the KMT1A methyltransferase to the myogenin promoter. Notably, a MyoD S199A/S200A mutant exhibits markedly reduced binding to KMT1A. Therefore, p38-γ signaling directly induces the assembly of a repressive MyoD transcriptional complex. Together, these results establish a hitherto unappreciated and essential role for p38-γ signaling in positively regulating the expansion of transient amplifying myogenic precursor cells during muscle growth and regeneration.

Cytologic, histologic, and immunohistochemical features of maxillofacial alveolar rhabdomyosarcoma in a juvenile dog

A 15-month-old castrated male dog with a history of intermittent epistaxis and sneezing was admitted for the examination of a maxillofacial mass. An impression smear of a biopsy sample from the cauliflower-shaped gingival mass contained numerous round cells, 5-25 microm in diameter, which contained a moderate amount of clear to pale blue cytoplasm and resembled lymphoid cells. Mitotic figures were frequently observed. The mass was diagnosed as malignant round cell neoplasia. On histologic examination the tumor was composed of diffusely arranged, small, atypical round cells with a small amount of fibrovascular stroma. Immunohistochemically, the cells were negative for CD3, CD18, CD20, CD79alpha, cytokeratin, melan-A, chromogranin A, alpha-smooth muscle actin, and myoglobin but positive for vimentin and desmin. The cells also had strong positive nuclear staining for myogenin and MyoD1. A diagnosis of solid-pattern alveolar rhabdomyosarcoma was made on the basis of morphologic and immunohistochemical results. Alveolar rhabdomyosarcoma should be considered in the differential diagnosis of tumors in juvenile dogs, especially when cytologic findings reveal round, undifferentiated cells.

The Carboxy-Terminal Modulator Protein (CTMP) regulates mitochondrial dynamics

Background

Mitochondria are central to the metabolism of cells and participate in many regulatory and signaling events. They are looked upon as dynamic tubular networks. We showed recently that the Carboxy-Terminal Modulator Protein (CTMP) is a mitochondrial protein that may be released into the cytosol under apoptotic conditions.

Methodology/Principal Findings

Here we report an unexpected function of CTMP in mitochondrial homeostasis. In this study, both full length CTMP, and a CTMP mutant refractory to N-terminal cleavage and leading to an immature protein promote clustering of spherical mitochondria, suggesting a role for CTMP in the fission process. Indeed, cellular depletion of CTMP led to accumulation of swollen and interconnected mitochondria, without affecting the mitochondrial fusion process. Importantly, in vivo results support the relevance of these findings, as mitochondria from livers of adult CTMP knockout mice had a similar phenotype to cells depleted of CTMP.

Conclusions/Significance

Together, these results lead us to propose that CTMP has a major function in mitochondrial dynamics and could be involved in the regulation of mitochondrial functions.

Inhibition of atrogin-1/MAFbx mediated MyoD proteolysis prevents skeletal muscle atrophy in vivo

Ubiquitin ligase Atrogin1/Muscle Atrophy F-box (MAFbx) up-regulation is required for skeletal muscle atrophy but substrates and function during the atrophic process are poorly known. The transcription factor MyoD controls myogenic stem cell function and differentiation, and seems necessary to maintain the differentiated phenotype of adult fast skeletal muscle fibres. We previously showed that MAFbx mediates MyoD proteolysis in vitro. Here we present evidence that MAFbx targets MyoD for degradation in several models of skeletal muscle atrophy. In cultured myotubes undergoing atrophy, MAFbx expression increases, leading to a cytoplasmic-nuclear shuttling of MAFbx and a selective suppression of MyoD. Conversely, transfection of myotubes with sh-RNA-mediated MAFbx gene silencing (shRNAi) inhibited MyoD proteolysis linked to atrophy. Furthermore, overexpression of a mutant MyoDK133R lacking MAFbx-mediated ubiquitination prevents atrophy of mouse primary myotubes and skeletal muscle fibres in vivo. Regarding the complex role of MyoD in adult skeletal muscle plasticity and homeostasis, its rapid suppression by MAFbx seems to be a major event leading to skeletal muscle wasting. Our results point out MyoD as the second MAFbx skeletal muscle target by which powerful therapies could be developed.

Carboxy-Terminal Modulator Protein (CTMP) is a mitochondrial protein that sensitizes cells to apoptosis

The Carboxy-Terminal Modulator Protein (CTMP) protein was identified as a PKB inhibitor that binds to its hydrophobic motif. Here, we report mitochondrial localization of endogenous and exogenous CTMP. CTMP exhibits a dual sub-mitochondrial localization as a membrane-bound pool and a free pool of mature CTMP in the inter-membrane space. CTMP is released from the mitochondria into the cytosol early upon apoptosis. CTMP overexpression is associated with an increase in mitochondrial membrane depolarization and caspase-3 and polyADP-ribose polymerase (PARP) cleavage. In contrast, CTMP knock-down results in a marked reduction in the loss of mitochondrial membrane potential as well as a decrease in caspase-3 and PARP activation. Mutant CTMP retained in the mitochondria loses its capacity to sensitize cells to apoptosis. Thus, proper maturation of CTMP is essential for its pro-apoptotic function. Finally, we demonstrate that CTMP delays PKB phosphorylation following cell death induction, suggesting that CTMP regulates apoptosis via inhibition of PKB.

Differential regulation of multiple alternatively spliced transcripts of MyoD

Splice variants of the basic helix-loop-helix myoblast determination factor (myoD) have not been previously found in vertebrates. Here we report the identification and characterization of three alternative transcripts of a myoD paralogue from the tiger pufferfish (Takifugu rubripes). The T. rubripes myoD1 gene (TmyoD1) has 3 exons and 2 introns and it is present on scaffold 104, in a region of conserved synteny with zebrafish. The isoform TMyoD1-alpha is a putative protein of 281 residues that contains the basic, helix-loop-helix and helix III domains and shares 61%, 56%, 51%, 49% and 56% overall identity with zebrafish, Xenopus, mouse, human and chicken MyoD1, respectively. TMyoD1-beta arises from an alternative 3' splice site and differs from TMyoD1-alpha by a 26-residue insertion adjacent to helix III, which is one of the functional domains required for chromatin remodelling. The third alternative transcript, TmyoD1-gamma, retains intron I and has two premature termination codons far from the 3'-most exon-exon junction. TmyoD1-gamma is therefore likely to be degraded by nonsense-mediated decay, an important widespread post-transcriptional mechanism that regulates transcript levels. Analysis of gene expression by qPCR revealed that TmyoD1-alpha was the most abundant transcript in fast and slow myotomal muscle. TmyoD1-alpha expression was 2-fold higher in fast muscle of juvenile fish that were actively producing new myotubes compared to adult stages that had stopped recruiting fast muscle fibres. A similar expression pattern was observed for TmyoD1-alpha in slow muscle but the differences were not significant. Transcript levels of TmyoD1-gamma only varied significantly in fast muscle and were 5-fold higher in adult compared to juvenile stages. Significant differences in expression of TmyoD1 splice variants were also observed during embryonic development. The differential expression of three alternative transcripts of myoD1 in developing and adult myotomal muscle of T. rubripes supports the hypothesis that diversity generated by alternative splicing may be of functional significance in muscle development in this species.

Chromatin modification and muscle differentiation

Skeletal muscle differentiation is a multistep process, which begins with the commitment of multi-potent mesodermal precursor cells to the muscle fate. These committed cells, the myoblasts, then differentiate and fuse into multinucleated myotubes. The final step of muscle differentiation is the maturation of differentiated myotubes into myofibres. Skeletal muscle development requires the coordinated expression of various transcription factors like the members of the myocyte enhancer binding-factor 2 family and the muscle regulatory factors. These transcription factors, in collaboration with chromatin-remodelling complexes, act in specific combinations and within complex transcriptional regulatory networks to achieve skeletal myogenesis. Additional factors involved in the epigenetic regulation of this process continue to be discovered. In this review, the authors discuss the recent discoveries in the epigenetic regulation of myogenesis. They also summarise the role of chromatin-modifying enzymes regulating muscle gene expression. These different factors are often involved in multiple steps of muscle differentiation and have redundant activities. Altogether, the recent findings have allowed a better understanding of myogenesis and have raised new hopes for the pharmacological development of new therapies aimed at muscle degeneration diseases, such as myotonic dystrophy or Duchenne muscular dystrophy.

Molecular cloning and expression of grass carp MyoD in yeast Pichia pastoris

MyoD, expressed in skeletal muscle lineages of vertebrate embryo, is one of muscle-specific basic helix-loop-helix (bHLH) transcription factors, which plays a key role in the determination and differentiation of all skeletal muscle lineages. In this study, a cDNA of grass carp MyoD was cloned and characterized from total RNA of grass carp embryos by RT-PCR. The full-length cDNA of grass carp MyoD is 1597 bp. The cDNA sequence analysis reveals an open reading frame of 825 bp coding for a protein of 275 amino acids, which includes a bHLH domain composed of basic domain (1-84(th) amino acids) and HLH domain (98-142(th) amino acids), without signal peptide. Then the MyoD cDNA of grass carp was cloned to yeast expression vector pPICZalphaA and transformed into P. pastoris GS115 strain, the recombinant MyoD protein with a molecular weight of about 31KD was obtained after inducing for 2d with 0.5% methanol in pH 8.0 BMGY medium, and the maximum yield was about 250 mg/L in shaking-flask fermentation. The results were expected to benefit for further studies on the crystal structure and physiological function of fish MyoD.

MyoD undergoes a distinct G2/M-specific regulation in muscle cells

The transcription factors MyoD and Myf5 present distinct patterns of expression during cell cycle progression and development. In contrast to the mitosis-specific disappearance of Myf5, which requires a D-box-like motif overlapping the basic domain, here we describe a stable and inactive mitotic form of MyoD phosphorylated on its serine 5 and serine 200 residues by cyclin B-cdc2. In mitosis, these modifications are required for releasing MyoD from condensed chromosomes and inhibiting its DNA-binding and transcriptional activation ability. Then, nuclear MyoD regains instability in the beginning of G1 phase due to rapid dephosphorylation events. Moreover, a non-phosphorylable MyoD S5A/S200A is not excluded from condensed chromatin and alters mitotic progression with apparent abnormalities. Thus, the drop of MyoD below a threshold level and its displacement from the mitotic chromatin could present another window in the cell cycle for resetting the myogenic transcriptional program and to maintain the myogenic determination of the proliferating cells.

Multiple phosphorylation events control mitotic degradation of the muscle transcription factor Myf5

Background

The two myogenic regulatory factors Myf5 and MyoD are basic helix-loop-helix muscle transcription factors undergoing differential cell cycle dependent proteolysis in proliferating myoblasts. This regulated degradation results in the striking expression of these two factors at distinct phases of the cell cycle, and suggests that their precise and alternated disappearance is an important feature of myoblasts, maybe connected to the maintenance of the proliferative status and/or commitment to the myogenic lineage of these cells. One way to understand the biological function(s) of the cyclic expression of these proteins is to specifically alter their degradation, and to analyze the effects of their stabilization on cells. To this aim, we undertook the biochemical analysis of the mechanisms governing Myf5 mitotic degradation, using heterologous systems.

Results

We show here that mitotic degradation of Myf5 is conserved in non-myogenic cells, and is thus strictly under the control of the cell cycle apparatus. Using Xenopus egg extracts as an in vitro system to dissect the main steps of Myf5 mitotic proteolysis, we show that (1) Myf5 stability is regulated by a complex interplay of phosphorylation/dephosphorylation, probably involving various kinases and phosphatases, (2) Myf5 is ubiquitylated in mitotic extracts, and this is a prerequisite to its degradation by the proteasome and (3) at least in the Xenopus system, the E3 responsible for its mitotic degradation is not the APC/C (the major E3 during mitosis).

Conclusion

Altogether, our data strongly suggest that the mitotic degradation of Myf5 by the ubiquitin-proteasome system is precisely controlled by multiple phosphorylation of the protein, and that the APC/C is not involved in this process.

Signaling to the chromatin during skeletal myogenesis: novel targets for pharmacological modulation of gene expression

Cellular differentiation entails an extensive reprogramming of the genome toward the expression of discrete subsets of genes, which establish the tissue-specific phenotype. This program is achieved by epigenetic marks of the chromatin at particular loci, and is regulated by environmental cues, such as soluble factors and cell-to-cell interactions. How the intracellular cascades convert the myriad of external stimuli into the nuclear information necessary to reprogram the genome toward specific responses is a question of biological and medical interest. The elucidation of the signaling converting cues from outside the cells into chromatin modifications at individual promoters holds the promise to unveil the targets for selective pharmacological interventions to modulate gene expression for therapeutic purposes. Enhancing muscle regeneration and preventing muscle breakdown are important goals in the therapy of muscular diseases, cancer-associated cachexia and aging-associated sarcopenia. We will summarize the recent progress of our knowledge of the regulation of gene expression by intracellular cascades elicited by external cues during skeletal myogenesis. And will illustrate the potential importance of targeting the chromatin signaling in regenerative medicine--e.g. to boost muscle regeneration.

Conditional transgenic system for mouse aurora a kinase: degradation by the ubiquitin proteasome pathway controls the level of the transgenic protein

Aurora A is a mitotic kinase that localizes to centrosomes. Expression of this protein is normally limited to the mitotic stage (G(2)-M) of the cell cycle, whereas human cancer cells frequently exhibit overexpression of Aurora A protein regardless of the cell cycle stage. In the present study, Aurora A transgenic mouse lines were generated with a new conditional expression system (cytomegalovirus immediate early enhancer-chicken beta-actin hybrid promoter-Z-enhanced green fluorescent protein) in order to analyze the function of this protein. Although transcripts for Aurora A were elevated in multiple organs of the transgenic mice, the corresponding protein was not detected in extracts analyzed by immunoblotting. The treatment of transgenic-derived embryonic fibroblasts (MEF) with proteasome inhibitors markedly increased the protein level of transgenic Aurora A, indicating that the transgenic Aurora A protein is readily degraded in normal mouse tissues. Under the exponential growth conditions of MEF cells, transgenic Aurora A was detected within the mitotic stage of the cell cycle and localized to centrosomes. In contrast, the marker of the transgenic promoter (enhanced green fluorescent protein) was continuously expressed throughout the cell cycle, indicating the constitutive transcription of transgenic mRNA. These results indicate that transgenic Aurora A is protected from degradation within G(2)-M but is immediately degraded after translation in the G(1)-S stage of the cell cycle. The findings obtained with this transgenic model and derived cells support that the transition from protection to degradation by the ubiquitin proteasome system at the end of mitosis is an important step in controlling the level of Aurora A protein during the cell cycle.

Differential effects of Hsp90 inhibition on protein kinases regulating signal transduction pathways required for myoblast differentiation

As derivatives of the Hsp90-inhibitor and tumoricidal agent geldanamycin move into phase II clinical trials, its potential for triggering adverse effects in non-tumor cell populations requires closer examination. In this report, the effect of geldanamycin on the differentiation and survival of C2C12 myoblasts was investigated. Treatment of differentiating C2C12 myoblasts with geldanamycin blocked myogenin expression, inhibited myotubule formation, and led to the depletion of three Hsp90-dependent protein kinases, ErbB2, Fyn, and Akt, and induction of apoptosis. ErbB2 levels declined rapidly, while Fyn and Akt levels decreased at a slower rate. Geldanamycin blocked the interaction of Hsp90 and its "kinase-specific" co-chaperone Cdc37 with Fyn, indicating that Fyn is an Hsp90-dependent kinase. Pulse-chase experiments indicated that geldanamycin caused newly synthesized Akt and Fyn to be degraded rapidly, but geldanamycin had little effect on the turnover rate of mature Fyn and Akt. Curiously, total cellular Src (c-Src) protein levels and the turnover rate of newly synthesized c-Src were unaffected by geldanamycin. While, geldanamycin had no effect on the levels of the putative Hsp90 client protein MyoD expressed in C2C12 cells, geldanamycin disrupted the interaction of Cdc37 with MyoD. Thus, inhibition of Hsp90 caused C2C12 cells to become depleted of multiple signal transduction proteins whose functions are essential for myoblast differentiation, and muscle cell survival, suggesting that geldanamycin derivatives may have the prospective of adversely affecting the physiology of certain sensitive muscle cell populations in vivo.

Degradation of MyoD Mediated by the SCF (MAFbx) Ubiquitin Ligase

MyoD controls myoblast identity and differentiation and is required for myogenic stem cell function in adult skeletal muscle. MyoD is degraded by the ubiquitin-proteasome pathway mediated by different E3 ubiquitin ligases not identified as yet. Here we report that MyoD interacts with Atrogin-1/MAFbx (MAFbx), a striated muscle-specific E3 ubiquitin ligase dramatically up-regulated in atrophying muscle. A core LXXLL motif sequence in MyoD is necessary for binding to MAFbx. MAFbx associates with MyoD through an inverted LXXLL motif located in a series of helical leucine-charged residue-rich domains. Mutation in the LXXLL core motif represses ubiquitination and degradation of MyoD induced by MAFbx. Overexpression of MAFbx suppresses MyoD-induced differentiation and inhibits myotube formation. Finally the purified recombinant SCF(MAFbx) complex (SCF, Skp1, Cdc53/Cullin 1, F-box protein) mediated MyoD ubiquitination in vitro in a lysine-dependent pathway. Mutation of the lysine 133 in MyoD prevented its ubiquitination by the recombinant SCF(MAFbx) complex. These observations thus demonstrated that MAFbx functions in ubiquitinating MyoD via a sequence found in transcriptional coactivators. These transcriptional coactivators mediate the binding to liganded nuclear receptors. We also identified a novel protein-protein interaction module not yet identified in F-box proteins. MAFbx may play an important role in the course of muscle differentiation by determining the abundance of MyoD.