The role of microRNA-1 and microRNA-133 in skeletal muscle proliferation and differentiation

Understanding the molecular mechanisms that regulate cellular proliferation and differentiation is a central theme of developmental biology. MicroRNAs (miRNAs) are a class of regulatory RNAs of approximately 22 nucleotides that post-transcriptionally regulate gene expression. Increasing evidence points to the potential role of miRNAs in various biological processes. Here we show that miRNA-1 (miR-1) and miRNA-133 (miR-133), which are clustered on the same chromosomal loci, are transcribed together in a tissue-specific manner during development. miR-1 and miR-133 have distinct roles in modulating skeletal muscle proliferation and differentiation in cultured myoblasts in vitro and in Xenopus laevis embryos in vivo. miR-1 promotes myogenesis by targeting histone deacetylase 4 (HDAC4), a transcriptional repressor of muscle gene expression. By contrast, miR-133 enhances myoblast proliferation by repressing serum response factor (SRF). Our results show that two mature miRNAs, derived from the same miRNA polycistron and transcribed together, can carry out distinct biological functions. Together, our studies suggest a molecular mechanism in which miRNAs participate in transcriptional circuits that control skeletal muscle gene expression and embryonic development.

The myofibroblast: one function, multiple origins

The crucial role played by the myofibroblast in wound healing and pathological organ remodeling is well established; the general mechanisms of extracellular matrix synthesis and of tension production by this cell have been amply clarified. This review discusses the pattern of myofibroblast accumulation and fibrosis evolution during lung and liver fibrosis as well as during atheromatous plaque formation. Special attention is paid to the specific features characterizing each of these processes, including the spectrum of different myofibroblast precursors and the distinct pathways involved in the formation of differentiated myofibroblasts in each lesion. Thus, whereas in lung fibrosis it seems that most myofibroblasts derive from resident fibroblasts, hepatic stellate cells are the main contributor for liver fibrosis and media smooth muscle cells are the main contributor for the atheromatous plaque. A better knowledge of the molecular mechanisms conducive to the appearance of differentiated myofibroblasts in each pathological situation will be useful for the understanding of fibrosis development in different organs and for the planning of strategies aiming at their prevention and therapy.

Hypoxia requires notch signaling to maintain the undifferentiated cell state

In addition to controlling a switch to glycolytic metabolism and induction of erythropoiesis and angiogenesis, hypoxia promotes the undifferentiated cell state in various stem and precursor cell populations. Here, we show that the latter process requires Notch signaling. Hypoxia blocks neuronal and myogenic differentiation in a Notch-dependent manner. Hypoxia activates Notch-responsive promoters and increases expression of Notch direct downstream genes. The Notch intracellular domain interacts with HIF-1alpha, a global regulator of oxygen homeostasis, and HIF-1alpha is recruited to Notch-responsive promoters upon Notch activation under hypoxic conditions. Taken together, these data provide molecular insights into how reduced oxygen levels control the cellular differentiation status and demonstrate a role for Notch in this process.

T-cadherin is a receptor for hexameric and high-molecular-weight forms of Acrp30/adiponectin

Acrp30/adiponectin is reduced in the serum of obese and diabetic individuals, and the genetic locus of adiponectin is linked to the metabolic syndrome. Recombinant adiponectin, administered to diet-induced obese mice, induced weight loss and improved insulin sensitivity. In muscle and liver, adiponectin stimulates AMP-activated protein kinase activation and fatty acid oxidation. To expression-clone molecules capable of binding adiponectin, we transduced a C2C12 myoblast cDNA retroviral expression library into Ba/F3 cells and panned infected cells on recombinant adiponectin linked to magnetic beads. We identified T-cadherin as a receptor for the hexameric and high-molecular-weight species of adiponectin but not for the trimeric or globular species. Only eukaryotically expressed adiponectin bound to T-cadherin, implying that posttranslational modifications of adiponectin are critical for binding. An adiponectin mutant lacking a conserved N-terminal cysteine residue required for formation of hexamer and high-molecular-weight species did not bind T-cadherin in coimmunoprecipitation studies. Although lacking known cellular functions, T-cadherin is expressed in endothelial and smooth muscle cells, where it is positioned to interact with adiponectin. Because T-cadherin is a glycosylphosphatidylinositol-anchored extracellular protein, it may act as a coreceptor for an as-yet-unidentified signaling receptor through which adiponectin transmits metabolic signals.

LBR and lamin A/C sequentially tether peripheral heterochromatin and inversely regulate differentiation

Eukaryotic cells have a layer of heterochromatin at the nuclear periphery. To investigate mechanisms regulating chromatin distribution, we analyzed heterochromatin organization in different tissues and species, including mice with mutations in the lamin B receptor (Lbr) and lamin A (Lmna) genes that encode nuclear envelope (NE) proteins. We identified LBR- and lamin-A/C-dependent mechanisms tethering heterochromatin to the NE. The two tethers are sequentially used during cellular differentiation and development: first the LBR- and then the lamin-A/C-dependent tether. The absence of both LBR and lamin A/C leads to loss of peripheral heterochromatin and an inverted architecture with heterochromatin localizing to the nuclear interior. Myoblast transcriptome analyses indicated that selective disruption of the LBR- or lamin-A-dependent heterochromatin tethers have opposite effects on muscle gene expression, either increasing or decreasing, respectively. These results show how changes in NE composition contribute to regulating heterochromatin positioning, gene expression, and cellular differentiation during development.

Initiation of myoblast to brown fat switch by a PRDM16-C/EBP-beta transcriptional complex

Brown adipose cells are specialized to dissipate chemical energy in the form of heat, as a physiological defence against cold and obesity. PRDM16 (PR domain containing 16) is a 140 kDa zinc finger protein that robustly induces brown fat determination and differentiation. Recent data suggests that brown fat cells arise in vivo from a Myf5-positive, myoblastic lineage by the action of PRDM16 (ref. 3); however, the molecular mechanisms responsible for this developmental switch is unclear. Here we show that PRDM16 forms a transcriptional complex with the active form of C/EBP-beta (also known as LAP), acting as a critical molecular unit that controls the cell fate switch from myoblastic precursors to brown fat cells. Forced expression of PRDM16 and C/EBP-beta is sufficient to induce a fully functional brown fat program in naive fibroblastic cells, including skin fibroblasts from mouse and man. Transplantation of fibroblasts expressing these two factors into mice gives rise to an ectopic fat pad with the morphological and biochemical characteristics of brown fat. Like endogenous brown fat, this synthetic brown fat tissue acts as a sink for glucose uptake, as determined by positron emission tomography with fluorodeoxyglucose. These data indicate that the PRDM16-C/EBP-beta complex initiates brown fat formation from myoblastic precursors, and may provide opportunities for the development of new therapeutics for obesity and type-2 diabetes.

Myostatin negatively regulates satellite cell activation and self-renewal

Satellite cells are quiescent muscle stem cells that promote postnatal muscle growth and repair. Here we show that myostatin, a TGF-beta member, signals satellite cell quiescence and also negatively regulates satellite cell self-renewal. BrdU labeling in vivo revealed that, among the Myostatin-deficient satellite cells, higher numbers of satellite cells are activated as compared with wild type. In contrast, addition of Myostatin to myofiber explant cultures inhibits satellite cell activation. Cell cycle analysis confirms that Myostatin up-regulated p21, a Cdk inhibitor, and decreased the levels and activity of Cdk2 protein in satellite cells. Hence, Myostatin negatively regulates the G1 to S progression and thus maintains the quiescent status of satellite cells. Immunohistochemical analysis with CD34 antibodies indicates that there is an increased number of satellite cells per unit length of freshly isolated Mstn-/- muscle fibers. Determination of proliferation rate suggests that this elevation in satellite cell number could be due to increased self-renewal and delayed expression of the differentiation gene (myogenin) in Mstn-/- adult myoblasts. Taken together, these results suggest that Myostatin is a potent negative regulator of satellite cell activation and thus signals the quiescence of satellite cells.

Biological progression from adult bone marrow to mononucleate muscle stem cell to multinucleate muscle fiber in response to injury

Adult bone marrow-derived cells (BMDC) are shown to contribute to muscle tissue in a step-wise biological progression. Following irradiation-induced damage, transplanted GFP-labeled BMDC become satellite cells: membrane-ensheathed mononucleate muscle stem cells. Following a subsequent exercise-induced damage, GFP-labeled multinucleate myofibers are detected. Isolated GFP-labeled satellite cells are heritably myogenic. They express three characteristic muscle markers, are karyotypically diploid, and form clones that can fuse into multinucleate cells in culture or into myofibers after injection into mouse muscles. These results suggest that two temporally distinct injury-related signals first induce BMDC to occupy the muscle stem cell niche and then to help regenerate mature muscle fibers. The stress-induced progression of BMDC to muscle satellite cell to muscle fiber results in a contribution to as many as 3.5% of muscle fibers and is due to developmental plasticity in response to environmental cues.

APPL1 binds to adiponectin receptors and mediates adiponectin signalling and function

Adiponectin, also known as Acrp30, is an adipose tissue-derived hormone with anti-atherogenic, anti-diabetic and insulin sensitizing properties. Two seven-transmembrane domain-containing proteins, AdipoR1 and AdipoR2, have recently been identified as adiponectin receptors, yet signalling events downstream of these receptors remain poorly defined. By using the cytoplasmic domain of AdipoR1 as bait, we screened a yeast two-hybrid cDNA library derived from human fetal brain. This screening led to the identification of a phosphotyrosine binding domain and a pleckstrin homology domain-containing adaptor protein, APPL1 (adaptor protein containing pleckstrin homology domain, phosphotyrosine binding (PTB) domain and leucine zipper motif). APPL1 interacts with adiponectin receptors in mammalian cells and the interaction is stimulated by adiponectin. Overexpression of APPL1 increases, and suppression of APPL1 level reduces, adiponectin signalling and adiponectin-mediated downstream events (such as lipid oxidation, glucose uptake and the membrane translocation of glucose transport 4 (GLUT4)). Adiponectin stimulates the interaction between APPL1 and Rab5 (a small GTPase) interaction, leading to increased GLUT4 membrane translocation. APPL1 also acts as a critical regulator of the crosstalk between adiponectin signalling and insulin signalling pathways. These results demonstrate a key function for APPL1 in adiponectin signalling and provide a molecular mechanism for the insulin sensitizing function of adiponectin.

Electrospinning polyaniline-contained gelatin nanofibers for tissue engineering applications

Polyaniline (PANi), a conductive polymer, was blended with a natural protein, gelatin, and co-electrospun into nanofibers to investigate the potential application of such a blend as conductive scaffold for tissue engineering purposes. Electrospun PANi-contained gelatin fibers were characterized using scanning electron microscopy (SEM), electrical conductivity measurement, mechanical tensile testing, and differential scanning calorimetry (DSC). SEM analysis of the blend fibers containing less than 3% PANi in total weight, revealed uniform fibers with no evidence for phase segregation, as also confirmed by DSC. Our data indicate that with increasing the amount of PANi (from 0 to approximately 5%w/w), the average fiber size was reduced from 803+/-121 nm to 61+/-13 nm (p<0.01) and the tensile modulus increased from 499+/-207 MPa to 1384+/-105 MPa (p<0.05). The results of the DSC study further strengthen our notion that the doping of gelatin with a few % PANi leads to an alteration of the physicochemical properties of gelatin. To test the usefulness of PANi-gelatin blends as a fibrous matrix for supporting cell growth, H9c2 rat cardiac myoblast cells were cultured on fiber-coated glass cover slips. Cell cultures were evaluated in terms of cell proliferation and morphology. Our results indicate that all PANi-gelatin blend fibers supported H9c2 cell attachment and proliferation to a similar degree as the control tissue culture-treated plastic (TCP) and smooth glass substrates. Depending on the concentrations of PANi, the cells initially displayed different morphologies on the fibrous substrates, but after 1 week all cultures reached confluence of similar densities and morphology. Taken together these results suggest that PANi-gelatin blend nanofibers might provide a novel conductive material well suited as biocompatible scaffolds for tissue engineering.

Controlling alginate gel degradation utilizing partial oxidation and bimodal molecular weight distribution

Degradability is often a critical property of materials utilized in tissue engineering. Although alginate, a naturally derived polysaccharide, is an attractive material due to its biocompatibility and ability to form hydrogels, its slow and uncontrollable degradation can be an undesirable feature. In this study, we characterized gels formed using a combination of partial oxidation of polymer chains and a bimodal molecular weight distribution of polymer. Specifically, alginates were partially oxidized to a theoretical extent of 1% with sodium periodate, which created acetal groups susceptible to hydrolysis. The ratio of low MW to high MW alginates used to form gels was also varied, while maintaining the gel forming ability of the polymer. The rate of degradation was found to be controlled by both the oxidation and the ratio of high to low MW alginates, as monitored by the reduction of mechanical properties and corresponding number of crosslinks, dry weight loss, and molecular weight decrease. It was subsequently examined whether these modifications would lead to reduced biocompatibility by culturing C2C12 myoblast on these gels. Myoblasts adhered, proliferated, and differentiated on the modified gels at a comparable rate as those cultured on the unmodified gels. Altogether, this data indicates these hydrogels exhibit tunable degradation rates and provide a powerful material system for tissue engineering.

The microRNA miR-181 targets the homeobox protein Hox-A11 during mammalian myoblast differentiation

Deciphering the mechanisms underlying skeletal muscle-cell differentiation in mammals is an important challenge. Cell differentiation involves complex pathways regulated at both transcriptional and post-transcriptional levels. Recent observations have revealed the importance of small (20-25 base pair) non-coding RNAs (microRNAs or miRNAs) that are expressed in both lower organisms and in mammals. miRNAs modulate gene expression by affecting mRNA translation or stability. In lower organisms, miRNAs are essential for cell differentiation during development; some miRNAs are involved in maintenance of the differentiated state. Here, we show that miR-181, a microRNA that is strongly upregulated during differentiation, participates in establishing the muscle phenotype. Moreover, our results suggest that miR-181 downregulates the homeobox protein Hox-A11 (a repressor of the differentiation process), thus establishing a functional link between miR-181 and the complex process of mammalian skeletal-muscle differentiation. Therefore, miRNAs can be involved in the establishment of a differentiated phenotype - even when they are not expressed in the corresponding fully differentiated tissue.

Assembly of centrosomal proteins and microtubule organization depends on PCM-1

The protein PCM-1 localizes to cytoplasmic granules known as "centriolar satellites" that are partly enriched around the centrosome. We inhibited PCM-1 function using a variety of approaches: microinjection of antibodies into cultured cells, overexpression of a PCM-1 deletion mutant, and specific depletion of PCM-1 by siRNA. All approaches led to reduced targeting of centrin, pericentrin, and ninein to the centrosome. Similar effects were seen upon inhibition of dynactin by dynamitin, and after prolonged treatment of cells with the microtubule inhibitor nocodazole. Inhibition or depletion of PCM-1 function further disrupted the radial organization of microtubules without affecting microtubule nucleation. Loss of microtubule organization was also observed after centrin or ninein depletion. Our data suggest that PCM-1-containing centriolar satellites are involved in the microtubule- and dynactin-dependent recruitment of proteins to the centrosome, of which centrin and ninein are required for interphase microtubule organization.

Novel insights into the regulation of skeletal muscle protein synthesis as revealed by a new nonradioactive in vivo technique

In this study, the principles of surface sensing of translation (SUnSET) were used to develop a nonradioactive method for ex vivo and in vivo measurements of protein synthesis (PS). Compared with controls, we first demonstrate excellent agreement between SUnSET and a [(3)H]phenylalanine method when detecting synergist ablation-induced increases in skeletal muscle PS ex vivo. We then show that SUnSET can detect the same synergist ablation-induced increase in PS when used in vivo (IV-SUnSET). In addition, IV-SUnSET detected food deprivation-induced decreases in PS in the heart, kidney, and skeletal muscles, with similar changes being visualized with an immunohistochemical version of IV-SUnSET (IV-IHC-SUnSET). By combining IV-IHC-SUnSET with in vivo transfection, we demonstrate that constitutively active PKB induces a robust increase in skeletal muscle PS. Furthermore, transfection with Ras homolog enriched in brain (Rheb) revealed that a PKB-independent activation of mammalian target of rapamycin is also sufficient to induce an increase in skeletal muscle PS. Finally, IV-IHC-SUnSET exposed the existence of fiber type-dependent differences in skeletal muscle PS, with PS in type 2B and 2X fibers being significantly lower than that in type 2A fibers within the same muscle. Thus, our nonradioactive method allowed us to accurately visualize and quantify PS under various ex vivo and in vivo conditions and revealed novel insights into the regulation of PS in skeletal muscle.

Microenvironmental VEGF concentration, not total dose, determines a threshold between normal and aberrant angiogenesis

Use of long-term constitutive expression of VEGF for therapeutic angiogenesis may be limited by the growth of abnormal blood vessels and hemangiomas. We investigated the relationship between VEGF dosage and the morphology and function of newly formed blood vessels by implanting retrovirally transduced myoblasts that constitutively express VEGF164 into muscles of adult mice. Reducing VEGF dosage by decreasing the total number of VEGF myoblasts implanted did not prevent vascular abnormalities. However, when clonal populations of myoblasts homogeneously expressing different levels of VEGF were implanted, a threshold between normal and aberrant angiogenesis was found. Clonal myoblasts that expressed low to medium levels of VEGF induced growth of stable, pericyte-coated capillaries of uniform size that were not leaky and became VEGF independent, as shown by treatment with the potent VEGF blocker VEGF-TrapR1R2. In contrast, clones that expressed high levels of VEGF induced hemangiomas. Remarkably, when different clonal populations were mixed, even a small proportion of cells with high production of VEGF was sufficient to cause hemangioma growth. These results show for the first time to our knowledge that the key determinant of whether VEGF-induced angiogenesis is normal or aberrant is the microenvironmental amount of growth factor secreted, rather than the overall dose. Long-term continuous delivery of VEGF, when maintained below a threshold microenvironmental level, can lead to normal angiogenesis without other exogenous growth factors.

IL-4 acts as a myoblast recruitment factor during mammalian muscle growth

Skeletal muscle formation and growth require the fusion of myoblasts to form multinucleated myofibers or myotubes, but few molecules are known to regulate myoblast fusion in mammals. The transcription factor NFATc2 controls myoblast fusion at a specific stage of myogenesis after the initial formation of a myotube and is necessary for further cell growth. By examining genes regulated by NFATc2 in muscle, this study identifies the cytokine IL-4 as a molecular signal that controls myoblast fusion with myotubes. Muscle cells lacking IL-4 or the IL-4alpha receptor subunit form normally but are reduced in size and myonuclear number. IL-4 is expressed by a subset of muscle cells in fusing muscle cultures and requires the IL-4alpha receptor subunit on myoblasts to promote fusion and growth. These data demonstrate that following myotube formation, myotubes recruit myoblast fusion by secretion of IL-4, leading to muscle growth.

Energy landscapes and functions of supramolecular systems

By means of two supramolecular systems--peptide amphiphiles engaged in hydrogen-bonded β-sheets, and chromophore amphiphiles driven to assemble by π-orbital overlaps--we show that the minima in the energy landscapes of supramolecular systems are defined by electrostatic repulsion and the ability of the dominant attractive forces to trap molecules in thermodynamically unfavourable configurations. These competing interactions can be selectively switched on and off, with the order of doing so determining the position of the final product in the energy landscape. Within the same energy landscape, the peptide-amphiphile system forms a thermodynamically favoured product characterized by long bundled fibres that promote biological cell adhesion and survival, and a metastable product characterized by short monodisperse fibres that interfere with adhesion and can lead to cell death. Our findings suggest that, in supramolecular systems, functions and energy landscapes are linked, superseding the more traditional connection between molecular design and function.

Biophysically defined and cytocompatible covalently adaptable networks as viscoelastic 3D cell culture systems

Presented here is a cytocompatible covalently adaptable hydrogel uniquely capable of mimicking the complex biophysical properties of native tissue and enabling natural cell functions without matrix degradation. Demonstrated is both the ability to control elastic modulus and stress relaxation time constants by more than an order of magnitude while predicting these values based on fundamental theoretical understanding and the simulation of muscle tissue and the encapsulation of myoblasts.

Myoblasts transplanted into rat infarcted myocardium are functionally isolated from their host

Survival and differentiation of myogenic cells grafted into infarcted myocardium have raised the hope that cell transplantation becomes a new therapy for cardiovascular diseases. The approach was further supported by transplantation of skeletal myoblasts, which was shown to improve cardiac performance in several animal species. Despite the success of myoblast transplantation and its recent trial in human, the mechanism responsible for the functional improvement remains unclear. Here, we used intracellular recordings coupled to video and fluorescence microscopy to establish whether myoblasts, genetically labeled with enhanced GFP and transplanted into rat infarcted myocardium, retain excitable and contractile properties, and participate actively to cardiac function. Our results indicate that grafted myoblasts differentiate into peculiar hyperexcitable myotubes with a contractile activity fully independent of neighboring cardiomyocytes. We conclude that mechanisms other than electromechanical coupling between grafted and host cells are involved in the improvement of cardiac function.

Single hematopoietic stem cells generate skeletal muscle through myeloid intermediates

Recent studies have shown that cells from the bone marrow can give rise to differentiated skeletal muscle fibers. However, the mechanisms and identities of the cell types involved have remained unknown, and the validity of the observation has been questioned. Here, we use transplantation of single CD45+ hematopoietic stem cells (HSCs) to demonstrate that the entire circulating myogenic activity in bone marrow is derived from HSCs and their hematopoietic progeny. We also show that ongoing muscle regeneration and inflammatory cell infiltration are required for HSC-derived contribution, which does not occur through a myogenic stem cell intermediate. Using a lineage tracing strategy, we show that myofibers are derived from mature myeloid cells in response to injury. Our results indicate that circulating myeloid cells, in response to inflammatory cues, migrate to regenerating skeletal muscle and stochastically incorporate into mature myofibers.

Pax7 activates myogenic genes by recruitment of a histone methyltransferase complex

Satellite cells purified from adult skeletal muscle can participate extensively in muscle regeneration and can also re-populate the satellite cell pool, suggesting that they have direct therapeutic potential for treating degenerative muscle diseases. The paired-box transcription factor Pax7 is required for satellite cells to generate committed myogenic progenitors. In this study we undertook a multi-level approach to define the role of Pax7 in satellite cell function. Using comparative microarray analysis, we identified several novel and strongly regulated targets; in particular, we identified Myf5 as a gene whose expression was regulated by Pax7. Using siRNA, fluorescence-activated cell sorting (FACS) and chromatin immunoprecipitation (ChIP) studies we confirmed that Myf5 is directly regulated by Pax7 in myoblasts derived from satellite cells. Tandem affinity purification (TAP) and mass spectrometry were used to purify Pax7 together with its co-factors. This revealed that Pax7 associates with the Wdr5-Ash2L-MLL2 histone methyltransferase (HMT) complex that directs methylation of histone H3 lysine 4 (H3K4, refs 4-10). Binding of the Pax7-HMT complex to Myf5 resulted in H3K4 tri-methylation of surrounding chromatin. Thus, Pax7 induces chromatin modifications that stimulate transcriptional activation of target genes to regulate entry into the myogenic developmental programme.

Interstitial fluid flow induces myofibroblast differentiation and collagen alignment in vitro

The differentiation of fibroblasts to contractile myofibroblasts, which is characterized by de novo expression of alpha-smooth muscle actin (alpha-SMA), is crucial for wound healing and a hallmark of tissue scarring and fibrosis. These processes often follow inflammatory events, particularly in soft tissues such as skin, lung and liver. Although inflammatory cells and damaged epithelium can release transforming growth factor beta1 (TGF-beta1), which largely mediates myofibroblast differentiation, the biophysical environment of inflammation and tissue regeneration, namely increased interstitial flow owing to vessel hyperpermeability and/or angiogenesis, may also play a role. We demonstrate that low levels of interstitial (3D) flow induce fibroblast-to-myofibroblast differentiation as well as collagen alignment and fibroblast proliferation, all in the absence of exogenous mediators. These effects were associated with TGF-beta1 induction, and could be eliminated with TGF-beta1 blocking antibodies. Furthermore, alpha1beta1 integrin was seen to play an important role in the specific response to flow, as its inhibition prevented fibroblast differentiation and subsequent collagen alignment but did not block their ability to contract the gel in a separate floating gel assay. This study suggests that the biophysical environment that often precedes fibrosis, such as swelling, increased microvascular permeability and increased lymphatic drainage--all which involve interstitial fluid flow--may itself play an important role in fibrogenesis.

MicroRNAs regulate the expression of the alternative splicing factor nPTB during muscle development

Alternative pre-mRNA splicing determines many changes in gene expression during development. Two regulators known to control splicing patterns during neuron and muscle differentiation are the polypyrimidine tract-binding protein (PTB) and its neuronal homolog nPTB. These proteins repress certain exons in early myoblasts, but upon differentiation of mature myotubes PTB/nPTB expression is reduced, leading to increased inclusion of their target exons. We show here that the repression of nPTB expression during myoblast differentiation results from its targeting by the muscle-restricted microRNA miR-133. During differentiation of C2C12 myoblasts, nPTB protein but not mRNA expression is strongly reduced, concurrent with the up-regulation of miR-133 and the induction of splicing for several PTB-repressed exons. Introduction of synthetic miR-133 into undifferentiated C2C12 cells leads to a decrease in endogenous nPTB expression. Both the miR-133 and the coexpressed miR-1/206 microRNAs are extremely conserved across animal species, and PTB proteins are predicted targets for these miRNAs in Drosophila, mice, and humans. There are two potential miR-133-responsive elements (MRE) within the nPTB 3' untranslated region (UTR), and a luciferase reporter carrying this 3' UTR is repressed by miR-133 in an MRE-dependent manner. Transfection of locked nucleic acid (LNA) oligonucleotides designed to block the function of miR-133 and miR-1/206 increases expression of nPTB and decreases the inclusion of PTB dependent exons. These results indicate that miR-133 directly down-regulates a key splicing factor during muscle development and establishes a role for microRNAs in the control of a developmentally dynamic splicing program.

Beta1 integrins regulate myoblast fusion and sarcomere assembly

The mechanisms that regulate the formation of multinucleated muscle fibers from mononucleated myoblasts are not well understood. We show here that extracellular matrix (ECM) receptors of the beta1 integrin family regulate myoblast fusion. beta1-deficient myoblasts adhere to each other, but plasma membrane breakdown is defective. The integrin-associated tetraspanin CD9 that regulates cell fusion is no longer expressed at the cell surface of beta1-deficient myoblasts, suggesting that beta1 integrins regulate the formation of a protein complex important for fusion. Subsequent to fusion, beta1 integrins are required for the assembly of sarcomeres. Other ECM receptors such as the dystrophin glycoprotein complex are still expressed but cannot compensate for the loss of beta1 integrins, providing evidence that different ECM receptors have nonredundant functions in skeletal muscle fibers.

Mir193b-365 is essential for brown fat differentiation

Mammals have two principal types of fat. White adipose tissue primarily serves to store extra energy as triglycerides, whereas brown adipose tissue is specialized to burn lipids for heat generation and energy expenditure as a defence against cold and obesity. Recent studies have demonstrated that brown adipocytes arise in vivo from a Myf5-positive, myoblastic progenitor by the action of Prdm16 (PR domain containing 16). Here, we identified a brown-fat-enriched miRNA cluster, MiR-193b-365, as a key regulator of brown fat development. Blocking miR-193b and/or miR-365 in primary brown preadipocytes markedly impaired brown adipocyte adipogenesis by enhancing Runx1t1 (runt-related transcription factor 1; translocated to, 1) expression, whereas myogenic markers were significantly induced. Forced expression of Mir193b and/or Mir365 in C2C12 myoblasts blocked the entire programme of myogenesis, and, in adipogenic conditions, miR-193b induced myoblasts to differentiate into brown adipocytes. Mir193b-365 was upregulated by Prdm16 partially through Pparα. Our results demonstrate that Mir193b-365 serves as an essential regulator for brown fat differentiation, in part by repressing myogenesis.