Actin isoform utilization during differentiation and remodeling of BC3H1 myogenic cells

The novel role of IFITM1-3 in myogenic differentiation of C2C12 cells

Interferon-induced transmembrane proteins (IFITMs 1, 2, and 3) play a critical role in preventing pathogen infection in vertebrates. They are also involved in the occurrence and prognosis of cancer. Myogenesis is a complex process regulated by several factors. This study disclosed that Ifitm1-3 were upregulated in the process of myogenic differentiation of C2C12 myoblasts on days 3, 5, and 7. This positively correlated with the expression of differentiation factors MyoD, myogenin, Mrf5, and desmin. Furthermore, knockdown of Ifitm1-3 by their individual siRNAs inhibited myogenesis of C2C12 myoblasts, with relative downregulation of MyoD, myogenin, Mrf5, and desmin. Subsequently, myotube formation and fusion percentage decreased. Co-immunoprecipitation combined with LC-MS/MS analysis uncovered the interaction proteins of IFITM1 and IFITM3 in C2C12 myoblasts. A total of 84 overlapped interaction proteins of IFITM1 and IFITM3 were identified, and one of the clusters was engaged in cytoskeletal and sarcomere proteins, including desmin, myosin, actin, vimentin, nestin, ankycorbin, and nucleolin. Hence, we hypothesize that these interacting proteins may function as scaffolds for IFITM1-3, possibly through the interaction protein desmin to initiate further interaction with other proteins to participate in myogenesis; however, the molecular mechanisms remain unclear. Our study may contribute to the development of novel therapeutics for myopathic diseases.

Impact of low-volume concurrent strength training distribution on muscular adaptation

OBJECTIVES

Military-, rescue- and law-enforcement personnel require a high physical capacity including muscular strength. The present study hypothesized that 9 weeks of volume matched concurrent short frequent training sessions increases strength more efficiently than less frequent longer training sessions.

DESIGN

A randomized training intervention study with functional and physiological tests before and after the intervention.

METHODS

Military conscripts (n=290) were assigned to micro-training (four 15-min strength and four 15-min endurance bouts weekly); classical-training (one 60-min strength and one 60-min endurance training session weekly) or a control-group (two 60-min standard military physical training sessions weekly).

RESULTS

There were no group difference between micro-training and classical-training in measures of strength. Standing long jump remained similar while shotput performance was reduced (P≤0.001) in all three groups. Pull-up performance increased (P≤0.001) in micro-training (7.4±4.6 vs. 8.5±4.0 repetitions, n=59) and classical-training (5.7±4.1 vs. 7.1±4.2 repetitions, n=50). Knee extensor MVC increased (P≤0.01) in all groups (micro-training, n=30, 11.5±8.9%; classical-training, n=24, 8.3±11.5% and control, n=19, 7.5±11.8%) while elbow flexor and hand grip MVC remained similar. Micro-training increased (P≤0.05) type IIa percentage from 32.5±11.0% to 37.6±12.3% (n=20) and control-group increased (P≤0.01) type IIax from 4.4±3.0% to 11.6±7.9% (n=8). In control-group type I, fiber size increased (P≤0.05) from 5121±959μm to 6481±2084μm (n=5). Satellite cell content remained similar in all groups.

CONCLUSIONS

Weekly distribution of low-volume concurrent training completed as either eight 15-min bouts or two 60-min sessions of which 50% was strength training did not impact strength gains in a real-world setting.

Manganese influx and expression of ZIP8 is essential in primary myoblasts and contributes to activation of SOD2

Trace elements such as copper (Cu), zinc (Zn), iron (Fe), and manganese (Mn) function as enzyme cofactors and second messengers in cell signaling. Trace elements are emerging as key regulators of differentiation and development of mammalian tissues including blood, brain, and skeletal muscle. We previously reported an influx of Cu and dynamic expression of metal transporters during differentiation of skeletal muscle cells. Here, we demonstrate that during differentiation of skeletal myoblasts an increase of Mn, Fe and Zn also occurs. Interestingly the Mn increase is concomitant with increased Mn-dependent SOD2 levels. To better understand the Mn import pathway in skeletal muscle cells, we probed the functional relevance of the closely related proteins ZIP8 and ZIP14, which are implicated in Zn, Mn, and Fe transport. Partial depletion of ZIP8 severely impaired growth of myoblasts and led to cell death under differentiation conditions, indicating that ZIP8-mediated metal transport is essential in skeletal muscle cells. Moreover, knockdown of Zip8 impaired activity of the Mn-dependent SOD2. Growth defects were partially rescued only by Mn supplementation to the medium, suggesting additional functions for ZIP8 in the skeletal muscle lineage. Restoring wild type Zip8 into the knockdown cells rescued the proliferation and differentiation phenotypes. On the other hand, knockdown of Zip14, had only a mild effect on myotube size, consistent with a role for ZIP14 in muscle hypertrophy. Simultaneous knockdown of both Zip8 and Zip14 further impaired differentiation and led cell death. This is the first report on the functional relevance of two members of the ZIP family of metal transporters in the skeletal muscle lineage, and further supports the paradigm that trace metal transporters are important modulators of mammalian tissue development.

Mitochondrial pro-apoptotic indices do not precede the transient caspase activation associated with myogenesis

Skeletal muscle differentiation requires activity of the apoptotic protease caspase-3. We attempted to identify the source of caspase activation in differentiating C2C12 skeletal myoblasts. In addition to caspase-3, caspase-2 was transiently activated during differentiation; however, no changes were observed in caspase-8 or -9 activity. Although mitochondrial Bax increased, this was matched by Bcl-2, resulting in no change to the mitochondrial Bax:Bcl-2 ratio early during differentiation. Interestingly, mitochondrial membrane potential increased on a timeline similar to caspase activation and was accompanied by an immediate, temporary reduction in cytosolic Smac and cytochrome c. Since XIAP protein expression dramatically declined during myogenesis, we investigated whether this contributes to caspase-3 activation. Despite reducing caspase-3 activity by up to 57%, differentiation was unaffected in cells overexpressing normal or E3-mutant XIAP. Furthermore, a XIAP mutant which can inhibit caspase-9 but not caspase-3 did not reduce caspase-3 activity or affect differentiation. Administering a chemical caspase-3 inhibitor demonstrated that complete enzyme inhibition was required to impair myogenesis. These results suggest that neither mitochondrial apoptotic signaling nor XIAP degradation is responsible for transient caspase-3 activation during C2C12 differentiation.

Expression of caspase family and muscle- and apoptosis-specific genes during skeletal myogenesis in mouse embryo

The caspases (Casps) are a family of cysteine proteases that are known to regulate apoptotic signaling. Apoptosis by activation of Casp is strongly associated with embryonal development and regeneration in many organs, therefore indicating that disorders caused by homozygous mutation in Casp genes can result in embryonic lethality. In the present study, the authors investigated the causative relationship between skeletal myogenesis and the activation of Casps by analyzing their dynamics during mouse embryogenesis. Individual myogenetic tissues were obtained from C57BL/6 mouse embryos aged 12.5-17.5 days post-conception (dpc), and the expression of Casps was analyzed by histochemical and molecular biological methods. Immunoreactions for Casp-3, -9 and -12 were detected first in myoblasts, increasing according to embryonal development, as a result of which myoblasts differentiated into myotube cells. On the other hand, the immunoreaction for ssDNA, which is well-known as an apoptosis marker, was little detected during the skeletal myogenesis. Quantification analysis for Casp mRNA expression by RT-PCR as well as by in situ hybridization showed a peak at 15.5 dpc but a decrease at 17.5 dpc. Similar dynamics were detected for Myod1 mRNA, one of the muscle regulatory factors, but not for Fasl, Bax and Rock1, apoptosis-associated factors during skeletal myogenesis. These results suggest that the activation of Casps in skeletal myogenesis is deeply associated with myoblast differentiation, but not directly related to apoptosis.

Barx2 controls myoblast fusion and promotes MyoD-mediated activation of the smooth muscle alpha-actin gene

Remodeling of the actin cytoskeleton is a critical early step in skeletal muscle differentiation. Smooth muscle alpha-actin (SMA) is one of the earliest markers of myoblast differentiation and is important for the migration and cell shape changes that precede fusion. We have found that satellite cell-derived primary myoblasts from mice lacking the Barx2 homeobox gene show altered patterns of actin remodeling, reduced cell migration, and delayed differentiation. Consistent with the role of SMA in these processes, Barx2(-)(/)(-) myoblasts also show reduced expression of SMA mRNA and protein. The proximal SMA promoter contains binding sites for muscle regulatory factors and serum response factor as well as a conserved homeodomain binding site (HBS). We found that Barx2 binds to the HBS element and potentiates up-regulation of SMA promoter activity by MyoD. We also show that Barx2, MyoD, and serum response factor simultaneously occupy the SMA promoter in cells and that Barx2 interacts with MyoD. Overall these data indicate that Barx2 cooperates with other muscle-expressed transcription factors to regulate the early cytoskeletal remodeling events that underlie efficient myoblast differentiation.

Is caspase-dependent apoptosis only cell differentiation taken to the extreme?

The benefits of apoptosis for a multicellular organism are obvious and fit the current dogma that the maintenance and viability of such organisms are dependent on the selective elimination of unneeded or deleterious cell types. However, self destruction at the level of the individual cell defies the most basic precepts of biology (sustaining life). If apoptosis is viewed through this construct then one question becomes paramount, i.e., why would an individual cell and its progeny develop, retain, or evolve a mechanism the sole purpose of which is to eliminate itself? In consideration of such a paradox, it is reasonable to postulate that prospective apoptotic pathways coevolved with and or were co-opted from another basic cell function(s) that did not involve the death of the cell per se. In the following article, we present the hypothesis that the conserved biochemical pathways of apoptosis are integral components of terminal cell differentiation and it is the time of engagement and activity level of these pathways that ultimately determines the choice between cell death or cell maturation.

Sphingosine 1-phosphate induces myoblast differentiation through Cx43 protein expression: a role for a gap junction-dependent and -independent function

Although sphingosine 1-phosphate (S1P) has been considered a potent regulator of skeletal muscle biology, acting as a physiological anti-mitogenic and prodifferentiating agent, its downstream effectors are poorly known. In the present study, we provide experimental evidence for a novel mechanism by which S1P regulates skeletal muscle differentiation through the regulation of gap junctional protein connexin (Cx) 43. Indeed, the treatment with S1P greatly enhanced Cx43 expression and gap junctional intercellular communication during the early phases of myoblast differentiation, whereas the down-regulation of Cx43 by transfection with short interfering RNA blocked myogenesis elicited by S1P. Moreover, calcium and p38 MAPK-dependent pathways were required for S1P-induced increase in Cx43 expression. Interestingly, enforced expression of mutated Cx43(Delta130-136) reduced gap junction communication and totally inhibited S1P-induced expression of the myogenic markers, myogenin, myosin heavy chain, caveolin-3, and myotube formation. Notably, in S1P-stimulated myoblasts, endogenous or wild-type Cx43 protein, but not the mutated form, coimmunoprecipitated and colocalized with F-actin and cortactin in a p38 MAPK-dependent manner. These data, together with the known role of actin remodeling in cell differentiation, strongly support the important contribution of gap junctional communication, Cx43 expression and Cx43/cytoskeleton interaction in skeletal myogenesis elicited by S1P.

FOXO1a acts as a selective tumor suppressor in alveolar rhabdomyosarcoma

Rhabdomyosarcoma (RMS), the most common pediatric soft-tissue sarcoma, has two major histological subtypes: embryonal RMS (ERMS), which has a favorable prognosis, and alveolar RMS (ARMS), which has a poor outcome. Although both forms of RMS express muscle cell–specific markers, only ARMS cells express PAX3-FOXO1a or PAX7-FOXO1a chimeric proteins. In mice, Pax3 and Pax7 play key roles in muscle cell development and differentiation, and FoxO1a regulates myoblast differentiation and fusion; thus, the aberrant regulation of these proteins may contribute to the development of ARMS. In this paper, we report that FOXO1a is not expressed in primary ARMS tumors or ARMS-derived tumor cell lines and that restoration of FOXO1a expression in ARMS cells is sufficient to induce cell cycle arrest and apoptosis. Strikingly, the effects of FOXO1a are selective, as enforced expression of FOXO1a in ERMS-derived tumor cell lines had no effect. Furthermore, FOXO1a induced apoptosis in ARMS by directly activating the transcription of caspase-3. We conclude that FOXO1a is a potent and specific tumor suppressor in ARMS, suggesting that agents that restore or augment FOXO1a activity may be effective as ARMS therapeutics.

FoxO1a-cyclic GMP-dependent kinase I interactions orchestrate myoblast fusion

The regulatory circuits that orchestrate mammalian myoblast cell fusion during myogenesis are poorly understood. The transcriptional activity of FoxO1a directly regulates this process, yet the molecular mechanisms governing FoxO1a activity during muscle cell differentiation remain unknown. Here we show an autoregulatory loop in which FoxO1a directly activates transcription of the cyclic GMP-dependent protein kinase I (cGKI) gene and where the ensuing cGKI activity phosphorylates FoxO1a and abolishes its DNA binding activity. These findings establish the FoxO1a-to-cGKI pathway as a novel feedback loop that allows the precise tuning of myoblast fusion. Interestingly, this pathway appears to operate independently of muscle cell differentiation programs directed by myogenic transcription factors.

Caspase 3 activity is required for skeletal muscle differentiation

The cellular alterations associated with skeletal muscle differentiation share a high degree of similarity with key phenotypic changes usually ascribed to apoptosis. For example, actin fiber disassembly/reorganization is a conserved feature of both apoptosis and differentiating myoblasts and the conserved muscle contractile protein, myosin light chain kinase, is required for the apoptotic feature of membrane blebbing. As such, these observations suggest that the induction of differentiation and apoptosis in the myogenic lineage may use overlapping cellular mechanisms. Here, we report that skeletal muscle differentiation depends on the activity of the key apoptotic protease, caspase 3. Peptide inhibition of caspase 3 activity or homologous deletion of caspase 3 leads to dramatic reduction in both myotube/myofiber formation and expression of muscle-specific proteins. Subsequently, we have identified Mammalian Sterile Twenty-like kinase as a crucial caspase 3 effector in this cellular process. Mammalian Sterile Twenty-like kinase is cleavage-activated by caspase 3, and restoration of this truncated kinase in caspase 3 null myoblasts restores the differentiation phenotype. Taken together, these results confirm a unique and unanticipated role for a caspase 3-mediated signal cascade in the promotion of myogenesis.

Mutated-gamma-actin restores growth to a yeast amino acid transport defective mutant

A mutated yeast cell 22574d lacking all three proline transporters, PUT4, UGA4, and GAP1, and incapable of growth on proline recovers its lost ability to grow on proline as sole nitrogen source when transformed with a mutagenized mouse gamma-actin cDNA (M-gamma-A). Native mouse gamma-actin cDNA is ineffective. The 3'-region of gamma-actin cDNA was mutagenized to resemble E51 cDNA previously isolated from Ehrlich tumor cells. The E51 cDNA has an extended reading frame in the 3'-region compared to that in native gamma-actin. The extension of the open reading frame in E51 cDNA, was found to be due to an additional pair of bases (TG) at position 1104 of E51 cDNA. After site-directed mutagenesis of the 3'-region of native gamma-actin cDNA to resemble that of E51 cDNA, the construct, M-gamma-A cDNA, was expressed in the 22574d yeast. While the transformation with M-gamma-A increased the uptake of both proline and gamma-amino butyric acid, the transport of five other solutes was not changed by this transformation. Northern blotting of the nontransformed and the M-gamma-A-transformed 22574d cells with gene-specific probes for the three proline transporters showed the expression of an mRNA for UGA4 in both transformed and nontransformed cells but no evidence for the expression of GAP1 or PUT4. The mRNA for UGA4 was expressed at a lower level in strain 22574d than in the parent yeast sigma1278b. Furthermore, the message in the mutated cells is smaller in size by about 15%. These results are consistent with the synthesis of a mutated transporter which requires the coexpression of M-gamma-A, but not native gamma-actin, to restore physiological function, i.e., proline or gamma-amino acid transport.

Subcutaneous tissue fibroblasts transfected with muscle and nonmuscle actins: A good in vitro model to study fibroblastic cell plasticity

Cultured fibroblasts develop several biochemical and morphological properties of smooth muscle cells, particularly the expression of alpha-smooth muscle actin, the actin isoform typical of vascular smooth muscle cells. They resemble modified fibroblasts or myofibroblasts observed in granulation tissue during wound repair and in fibrotic situations. We have analysed by immunolabeling the fate of exogenous epitope-tagged actin isoforms by transfection of the corresponding cDNAs into fibroblasts cultured from rat subcutaneous tissue. Tagged muscle actins were efficiently integrated into stress fibers and did not produce obvious changes in cell shape of transfected cells. Transfected nonmuscle actins in contrast changed the morphology and were not or poorly incorporated into stress fibers. These cultured subcutaneous fibroblasts behave similarly to smooth muscle cells when transfected with the same actin encoding cDNAs, indicating another common characteristic of these two cell types in sorting and targeting actin isoforms. Subcutaneous fibroblasts transfected with muscle and nonmuscle actin isoforms provide a good in vitro model to analyze the intracellular sorting of isoactins and to improve our knowledge of myofibroblast characterization and differentiation during tissue repair as well as to understand the relationships between modifications of actin cytoskeleton, adhesion and extracellular matrix proteins.