The inhibition of differentiation caused by TGFbeta in fetal myoblasts is dependent upon selective expression of PKCtheta: a possible molecular basis for myoblast diversification during limb histogenesis

AEB-071 Ameliorates Muscle Weakness by Altering Helper T Lymphocytes in an Experimental Autoimmune Myasthenia Gravis Rat Model

Background

Myasthenia gravis (MG) is an autoimmune neurological disorder of neuromuscular junctions. In this study we established experimental autoimmune myasthenia gravis (EAMG) rat models to investigate the effects of AEB-071 (AEB), which is a specific inhibitor of protein kinase C that prevents T lymphocyte activation.

Material/Methods

We utilized animals divided into 4 groups: (1) control rats, (2) EAMG, (3) AEB-071+EAMG, and (4) AZP+EAMG. Drug treatment was continued for 10 days. Ten weeks after immunization we measured body weights, assessed mortality rates, and used Lennon scores to evaluate EAMG grades. We also assessed the proportions of T_reg, T_h1, T_h2, T_h17, and lymphocytes using flow cytometry.

Results

In the absence of drug treatment, we found a significant decline in body weights in the EAMG group in comparison to control rats, and EAMG group rats also had higher Lennon scores (P<0.05). Interestingly, we found that AEB-071 restored the body weight of EAMG rats and the decreased mortality rate compared to AZP treatment. Although a decrease in the number of T_reg cells was observed, the proportion of T_h lymphocytes was significantly increased in the EAMG group, and AEB-071 treatment decreased the proportion of T_h lymphocytes.

Conclusions

We concluded that AEB-071 treatment imparts beneficial effects in EAMG rat models by reducing mortality rate and restoring T_h lymphocyte balance, and thus may be an attractive candidate for use in MG treatment.

Lack of PKCθ Promotes Regenerative Ability of Muscle Stem Cells in Chronic Muscle Injury

Duchenne muscular dystrophy (DMD) is a genetic disease characterized by muscle wasting and chronic inflammation, leading to impaired satellite cells (SCs) function and exhaustion of their regenerative capacity. We previously showed that lack of PKCθ in mdx mice, a mouse model of DMD, reduces muscle wasting and inflammation, and improves muscle regeneration and performance at early stages of the disease. In this study, we show that muscle regeneration is boosted, and fibrosis reduced in mdxθ^−/− mice, even at advanced stages of the disease. This phenotype was associated with a higher number of Pax7 positive cells in mdxθ^−/− muscle compared with mdx muscle, during the progression of the disease. Moreover, the expression level of Pax7 and Notch1, the pivotal regulators of SCs self-renewal, were upregulated in SCs isolated from mdxθ^−/− muscle compared with mdx derived SCs. Likewise, the expression of the Notch ligands Delta1 and Jagged1 was higher in mdxθ^−/− muscle compared with mdx. The expression level of Delta1 and Jagged1 was also higher in PKCθ^−/− muscle compared with WT muscle following acute injury. In addition, lack of PKCθ prolonged the survival and sustained the differentiation of transplanted myogenic progenitors. Overall, our results suggest that lack of PKCθ promotes muscle repair in dystrophic mice, supporting stem cells survival and maintenance through increased Delta-Notch signaling.

Pharmacological Inhibition of PKCθ Counteracts Muscle Disease in a Mouse Model of Duchenne Muscular Dystrophy

Inflammation plays a considerable role in the progression of Duchenne Muscular Dystrophy (DMD), a severe muscle disease caused by a mutation in the dystrophin gene. We previously showed that genetic ablation of Protein Kinase C θ (PKCθ) in mdx, the mouse model of DMD, improves muscle healing and regeneration, preventing massive inflammation. To establish whether pharmacological targeting of PKCθ in DMD can be proposed as a therapeutic option, in this study we treated young mdx mice with the PKCθ inhibitor Compound 20 (C20). We show that C20 treatment led to a significant reduction in muscle damage associated with reduced immune cells infiltration, reduced inflammatory pathways activation, and maintained muscle regeneration. Importantly, C20 treatment is efficient in recovering muscle performance in mdx mice, by preserving muscle integrity. Together, these results provide proof of principle that pharmacological inhibition of PKCθ in DMD can be considered an attractive strategy to modulate immune response and prevent the progression of the disease.

Research in context

Duchenne muscular dystrophy (DMD) is a severe muscle disease affecting 1:3500 male births. DMD is caused by a mutation in dystrophin gene, coding for a protein required for skeletal and cardiac muscle integrity. Lack of a functional dystrophin is primarily responsible for the muscle eccentric contraction-induced muscle damage, observed in dystrophic muscle. However, inflammation plays a considerable role in the progression of DMD. Glucocorticoids, which have anti-inflammatory properties, are being used to treat DMD with some success; however, long term treatment with these drugs induces muscle atrophy and wasting, outweighing their benefit. The identification of specific targets for anti-inflammatory therapies is one of the ongoing therapeutic options. Although blunting inflammation would not be a “cure” for the disease, the emerging clue is that multiple strategies, addressing different aspects of the pathology, which may eventually converge, may be successful. In this context, we previously showed that genetic ablation of Protein Kinase C θ (PKCθ), an enzyme known to be involved in immune response, in mdx, the mouse model of DMD, improves muscle healing and regeneration, preventing massive inflammation. To establish whether pharmacological targeting of PKCθ in DMD can be proposed as a therapeutic option, in this study we treated young mdx mice with the PKCθ inhibitor Compound 20 (C20). We show that C20 treatment led to a significant reduction in muscle damage associated with reduced immune cells infiltration, reduced inflammatory pathways activation, and maintained muscle regeneration. Importantly, C20 treatment is efficient in recovering muscle performance in mdx mice, by preserving muscle integrity. Together, these results provide proof of principle that pharmacological inhibition of PKCθ in DMD can be considered an attractive strategy to modulate immune response and prevent the progression of the disease.

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Immune-cell intrinsic PKCθ activity might play a hitherto unrecognized role of in the development of DMD.

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Mdx dystrophic mice were treated with the PKCθ inhibitor C20.

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C20 treatment prevents damage and inflammation in dystrophic muscle, while improving muscle regeneration.

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C20 treatment prevents drop in force and ameliorates fatigue resistance in dystrophic mice.

Targeting PKCθ in skeletal muscle and muscle diseases: good or bad?

Protein kinase Cθ (PKCθ) is a member of the novel calcium-independent PKC family, with a relatively selective tissue distribution. Most studies have focused on its unique role in T-lymphocyte activation and suggest that inhibition of PKCθ could represent a novel therapeutic approach in the treatment of chronic inflammation, autoimmunity and allograft rejection. However, considering that PKCθ is also expressed in other cell types, including skeletal muscle cells, it is important to understand its function in different tissues before proposing it as a molecular target for the treatment of immune-mediated diseases. A number of studies have highlighted the role of PKCθ in mediating several intracellular pathways, regulating muscle cell development, homoeostasis and remodelling, although a comprehensive picture is still lacking. Moreover, we recently showed that lack of PKCθ in a mouse model of Duchenne muscular dystrophy (DMD) ameliorates the progression of the disease. In the present article, we review new developments in our understanding of the involvement of PKCθ in intracellular mechanisms regulating skeletal muscle development, growth and maintenance under physiological conditions and recent advances showing a hitherto unrecognized role of PKCθ in promoting muscular dystrophy.

The Novel PKCθ from Benchtop to Clinic

The protein kinases C (PKCs) are a family of serine/threonine kinases involved in regulating multiple essential cellular processes such as survival, proliferation, and differentiation. Of particular interest is the novel, calcium-independent PKCθ which plays a central role in immune responses. PKCθ shares structural similarities with other PKC family members, mainly consisting of an N-terminal regulatory domain and a C-terminal catalytic domain tethered by a hinge region. This isozyme, however, is unique in that it translocates to the immunological synapse between a T cell and an antigen-presenting cell (APC) upon T cell receptor-peptide MHC recognition. Thereafter, PKCθ interacts physically and functionally with downstream effectors to mediate T cell activation and differentiation, subsequently leading to inflammation. PKCθ-specific perturbations have been identified in several diseases, most notably autoimmune disorders, and hence the modulation of its activity presents an attractive therapeutic intervention. To that end, many inhibitors of PKCs and PKCθ have been developed and tested in preclinical and clinical studies. And although selectivity remains a challenge, results are promising for the future development of effective PKCθ inhibitors that would greatly advance the treatment of several T-cell mediated diseases.

Nuclear factor one transcription factors: Divergent functions in developmental versus adult stem cell populations

Nuclear factor one (NFI) transcription factors are a group of site-specific DNA-binding proteins that are emerging as critical regulators of stem cell biology. During development NFIs promote the production of differentiated progeny at the expense of stem cell fate, with Nfi null mice exhibiting defects such as severely delayed brain and lung maturation, skeletomuscular defects and renal abnormalities, phenotypes that are often consistent with patients with congenital Nfi mutations. Intriguingly, recent research suggests that in adult tissues NFI factors play a qualitatively different role than during development, with NFIs serving to promote the survival and maintenance of slow-cycling adult stem cell populations rather than their differentiation. Here we review the role of NFI factors in development, largely focusing on their role as promoters of stem cell differentiation, and attempt to reconcile this with the emerging role of NFIs in adult stem cell niches.

Protein kinase C theta (PKCθ) modulates the ClC-1 chloride channel activity and skeletal muscle phenotype: a biophysical and gene expression study in mouse models lacking the PKCθ

In skeletal muscle, the resting chloride conductance (gCl), due to the ClC-1 chloride channel, controls the sarcolemma electrical stability. Indeed, loss-of-function mutations in ClC-1 gene are responsible of myotonia congenita. The ClC-1 channel can be phosphorylated and inactivated by protein kinases C (PKC), but the relative contribution of each PKC isoforms is unknown. Here, we investigated on the role of PKCθ in the regulation of ClC-1 channel expression and activity in fast- and slow-twitch muscles of mouse models lacking PKCθ. Electrophysiological studies showed an increase of gCl in the PKCθ-null mice with respect to wild type. Muscle excitability was reduced accordingly. However, the expression of the ClC-1 channel, evaluated by qRT-PCR, was not modified in PKCθ-null muscles suggesting that PKCθ affects the ClC-1 activity. Pharmacological studies demonstrated that although PKCθ appreciably modulates gCl, other isoforms are still active and concur to this role. The modification of gCl in PKCθ-null muscles has caused adaptation of the expression of phenotype-specific genes, such as calcineurin and myocyte enhancer factor-2, supporting the role of PKCθ also in the settings of muscle phenotype. Importantly, the lack of PKCθ has prevented the aging-related reduction of gCl, suggesting that its modulation may represent a new strategy to contrast the aging process.

Suppression of protein kinase C theta contributes to enhanced myogenesis in vitro via IRS1 and ERK1/2 phosphorylation

Background

Differentiation and fusion of skeletal muscle myoblasts into multi-nucleated myotubes is required for neonatal development and regeneration in adult skeletal muscle. Herein, we report novel findings that protein kinase C theta (PKCθ) regulates myoblast differentiation via phosphorylation of insulin receptor substrate-1 and ERK1/2.

Results

In this study, PKCθ knockdown (PKCθ^shRNA) myotubes had reduced inhibitory insulin receptor substrate-1 ser1095 phosphorylation, enhanced myoblast differentiation and cell fusion, and increased rates of protein synthesis as determined by [³H] phenylalanine incorporation. Phosphorylation of insulin receptor substrate-1 ser632/635 and extracellular signal-regulated kinase1/2 (ERK1/2) was increased in PKCθ^shRNA cells, with no change in ERK5 phosphorylation, highlighting a PKCθ-regulated myogenic pathway. Inhibition of PI3-kinase prevented cell differentiation and fusion in control cells, which was attenuated in PKCθ^shRNA cells. Thus, with reduced PKCθ, differentiation and fusion occur in the absence of PI3-kinase activity. Inhibition of the ERK kinase, MEK1/2, impaired differentiation and cell fusion in control cells. Differentiation was preserved in PKCθ^shRNA cells treated with a MEK1/2 inhibitor, although cell fusion was blunted, indicating PKCθ regulates differentiation via IRS1 and ERK1/2, and this occurs independently of MEK1/2 activation.

Conclusion

Cellular signaling regulating the myogenic program and protein synthesis are complex and intertwined. These studies suggest that PKCθ regulates myogenic and protein synthetic signaling via the modulation of IRS1and ERK1/2 phosphorylation. Myotubes lacking PKCθ had increased rates of protein synthesis and enhanced myotube development despite reduced activation of the canonical anabolic-signaling pathway. Further investigation of PKCθ regulated signaling may reveal important interactions regulating skeletal muscle health in an insulin resistant state.

Intracellular signaling in ER stress-induced autophagy in skeletal muscle cells

Skeletal muscle remodeling in response to muscle disuse and unloading is known to be associated with so-called ER stress, which, in turn, activates autophagy and contributes to muscle atrophy. Different molecules are involved in ER stress-induced autophagy, among which PKCθ has recently been described. In this study, we dissected both in vitro and in vivo ER stress-induced autophagy pathways in muscle. Using C2C12 muscle cells in culture, we demonstrated that PKC activation induced autophagy in the absence of ER stress. We further demonstrated that PKCθ was strongly activated in cultured myoblasts and myotubes during ER stress induced by different stimuli, such as TG or TN treatment, and that it localized into Lc3-positive autophagic dots upon TG treatment. Neither Akt dephosphorylation nor Foxo or GSK3β activation was observed in these conditions. Moreover, PKCθ inhibition in myoblasts and myotubes prevented ER stress-induced Lc3 activation and autophagic dot formation, but not ER stress. In vivo, lack of PKCθ prevented both food deprivation- and immobilization-induced autophagy and muscle atrophy, irrespective of Akt pathway inhibition. Taken together, these results demonstrate that PKCθ functions as an ER stress sensor in skeletal muscle, required for ER-stress-dependent autophagy activation, and can be proposed as a novel molecular target to maintain muscle homeostasis in response to external stimuli, such as disuse and unloading, still allowing intracellular clearance.

Growth of limb muscle is dependent on skeletal-derived Indian hedgehog

During embryogenesis, muscle and bone develop in close temporal and spatial proximity. We show that Indian Hedgehog, a bone-derived signaling molecule, participates in growth of skeletal muscle. In Ihh(-/-) embryos, skeletal muscle development appears abnormal at embryonic day 14.5 and at later ages through embryonic day 20.5, dramatic losses of hindlimb muscle occur. To further examine the role of Ihh in myogenesis, we manipulated Ihh expression in the developing chick hindlimb. Reduction of Ihh in chicken embryo hindlimbs reduced skeletal muscle mass similar to that seen in Ihh(-/-) mouse embryos. The reduction in muscle mass appears to be a direct effect of Ihh since ectopic expression of Ihh by RCAS retroviral infection of chicken embryo hindlimbs restores muscle mass. These effects are independent of bone length, and occur when Shh is not expressed, suggesting Ihh acts directly on fetal myoblasts to regulate secondary myogenesis. Loss of muscle mass in Ihh null mouse embryos is accompanied by a dramatic increase in myoblast apoptosis by a loss of p21 protein. Our data suggest that Ihh promotes fetal myoblast survival during their differentiation into secondary myofibers by maintaining p21 protein levels.

PKCθ signaling is required for myoblast fusion by regulating the expression of caveolin-3 and β1D integrin upstream focal adhesion kinase

Using both in vivo and in vitro protein kinase C (PKC) θ mutant models, we found that PKCθ, the PKC isoform predominantly expressed in skeletal muscle, is required for myoblast fusion and myofiber growth, by regulating focal adhesion kinase activity and, in turn, the expression of the pro-fusion genes caveolin-3 and β1D-integrin.

Fusion of mononucleated myoblasts to form multinucleated myofibers is an essential phase of skeletal myogenesis, which occurs during muscle development as well as during postnatal life for muscle growth, turnover, and regeneration. Many cell adhesion proteins, including integrins, have been shown to be important for myoblast fusion in vertebrates, and recently focal adhesion kinase (FAK), has been proposed as a key mediator of myoblast fusion. Here we focused on the possible role of PKCθ, the PKC isoform predominantly expressed in skeletal muscle, in myoblast fusion. We found that the expression of PKCθ is strongly up-regulated following freeze injury–induced muscle regeneration, as well as during in vitro differentiation of satellite cells (SCs; the muscle stem cells). Using both PKCθ knockout and muscle-specific PKCθ dominant-negative mutant mouse models, we observed delayed body and muscle fiber growth during the first weeks of postnatal life, when compared with wild-type (WT) mice. We also found that myofiber formation, during muscle regeneration after freeze injury, was markedly impaired in PKCθ mutant mice, as compared with WT. This phenotype was associated with reduced expression of the myogenic differentiation program executor, myogenin, but not with that of the SC marker Pax7. Indeed in vitro differentiation of primary muscle-derived SCs from PKCθ mutants resulted in the formation of thinner myotubes with reduced numbers of myonuclei and reduced fusion rate, when compared with WT cells. These effects were associated to reduced expression of the profusion genes caveolin-3 and β1D integrin and to reduced activation/phosphorylation of their up-stream regulator FAK. Indeed the exogenous expression of a constitutively active mutant form of PKCθ in muscle cells induced FAK phosphorylation. Moreover pharmacologically mediated full inhibition of FAK activity led to similar fusion defects in both WT and PKCθ-null myoblasts. We thus propose that PKCθ signaling regulates myoblast fusion by regulating, at least in part, FAK activity, essential for profusion gene expression.

Protein kinase Cθ is required for cardiomyocyte survival and cardiac remodeling

Protein kinase Cs (PKCs) constitute a family of serine/threonine kinases, which has distinguished and specific roles in regulating cardiac responses, including those associated with heart failure. We found that the PKCθ isoform is expressed at considerable levels in the cardiac muscle in mouse, and that it is rapidly activated after pressure overload. To investigate the role of PKCθ in cardiac remodeling, we used PKCθ^−/− mice. In vivo analyses of PKCθ^−/− hearts showed that the lack of PKCθ expression leads to left ventricular dilation and reduced function. Histological analyses showed a reduction in the number of cardiomyocytes, combined with hypertrophy of the remaining cardiomyocytes, cardiac fibrosis, myofibroblast hyper-proliferation and matrix deposition. We also observed p38 and JunK activation, known to promote cell death in response to stress, combined with upregulation of the fetal pattern of gene expression, considered to be a feature of the hemodynamically or metabolically stressed heart. In keeping with these observations, cultured PKCθ^−/− cardiomyocytes were less viable than wild-type cardiomyocytes, and, unlike wild-type cardiomyocytes, underwent programmed cell death upon stimulation with α1-adrenergic agonists and hypoxia. Taken together, these results show that PKCθ maintains the correct structure and function of the heart by preventing cardiomyocyte cell death in response to work demand and to neuro-hormonal signals, to which heart cells are continuously exposed.

Nfix regulates fetal-specific transcription in developing skeletal muscle

Skeletal myogenesis, like hematopoiesis, occurs in successive developmental stages that involve different cell populations and expression of different genes. We show here that the transcription factor nuclear factor one X (Nfix), whose expression is activated by Pax7 in fetal muscle, in turn activates the transcription of fetal specific genes such as MCK and beta-enolase while repressing embryonic genes such as slow myosin. In the case of the MCK promoter, Nfix forms a complex with PKC theta that binds, phosphorylates, and activates MEF2A. Premature expression of Nfix activates fetal and suppresses embryonic genes in embryonic muscle, whereas muscle-specific ablation of Nfix prevents fetal and maintains embryonic gene expression in the fetus. Therefore, Nfix acts as a transcriptional switch from embryonic to fetal myogenesis.

Cellular heterogeneity during vertebrate skeletal muscle development

Although skeletal muscles appear superficially alike at different anatomical locations, in reality there is considerably more diversity than previously anticipated. Heterogeneity is not only restricted to completely developed fibers, but is clearly apparent during development at the molecular, cellular and anatomical level. Multiple waves of muscle precursors with different features appear before birth and contribute to muscular diversification. Recent cell lineage and gene expression studies have expanded our knowledge on how skeletal muscle is formed and how its heterogeneity is generated. This review will present a comprehensive view of relevant findings in this field.

Intrinsic phenotypic diversity of embryonic and fetal myoblasts is revealed by genome-wide gene expression analysis on purified cells

Skeletal muscle development occurs asynchronously and it has been proposed to be dependent upon the generation of temporally distinct populations of myogenic cells. This long-held hypothesis has not been tested directly due to the inability to isolate and analyze purified populations of myoblasts derived from specific stages of prenatal development. Using a mouse strain with the GFP reporter gene targeted into the Myf5 locus, a cell-sorting method was developed for isolating embryonic and fetal myoblasts. The two types of myoblasts show an intrinsic difference in fusion ability, proliferation, differentiation and response to TGFbeta, TPA and BMP-4 in vitro. Microarray and quantitative PCR were used to identify differentially expressed genes both before and after differentiation, thus allowing a precise phenotypic analysis of the two populations. Embryonic and fetal myoblasts differ in the expression of a number of transcription factors and surface molecules, which may control different developmental programs. For example, only embryonic myoblasts express a Hox code along the antero-posterior axis, indicating that they possess direct positional information. Taken together, the data presented here demonstrate that embryonic and fetal myoblasts represent intrinsically different myogenic lineages and provide important information for the understanding of the molecular mechanisms governing skeletal muscle development.

Protein kinase C theta co-operates with calcineurin in the activation of slow muscle genes in cultured myogenic cells

Adult skeletal muscle fibers can be divided into fast and slow twitch subtypes on the basis of specific contractile and metabolic properties, and on distinctive patterns of muscle gene expression. The calcium, calmodulin-dependent protein phosphatase, calcineurin, stimulates slow fiber-specific genes (myoglobin (Mb), troponin I slow) in cultured skeletal muscle cells, as well as in transgenic mice, through the co-operation of peroxisome-proliferation-activator receptor gamma co-activator 1alpha (PGC1alpha) myocyte enhancer factor 2 (MEF2), and nuclear factor of activated T cells (NFAT) transcription factors. Specific protein kinase C isoforms have been shown to functionally co-operate with calcineurin in different cellular models. We investigated whether specific protein kinase C isoforms are involved in calcineurin-induced slow skeletal muscle gene expression. By pharmacological inhibition or exogenous expression of mutant forms, we show that protein kinase C theta (the protein kinase C isoform predominantly expressed in skeletal muscle) is required and co-operates with calcineurin in the activation of the Mb promoter, as well as in the induction of slow isoforms of myosin and troponin I expression, in cultured muscle cells. This co-operation acts primarily regulating MEF2 activity, as shown by using reporter gene expression driven by the Mb promoter mutated in the specific binding sites. MEF2 activity on the Mb promoter is known to be dependent on both PGC1alpha and inactivation of histone deacetylases (HDACs) activity. We show in this study that protein kinase C theta is required for, even though it does not co-operate in, PGC1alpha-dependent Mb activation. Importantly, protein kinase C theta regulates the HDAC5 nucleus/cytoplasm location. We conclude that protein kinase C theta ensures maximal activation of MEF2, by regulating both MEF2 transcriptional complex formation and HDACs nuclear export.

Satellite cells, myoblasts and other occasional myogenic progenitors: Possible origin, phenotypic features and role in muscle regeneration

In the vertebrate embryo, skeletal muscle originates from somites and is formed in discrete steps by different classes of progenitor cells. After myotome formation, embryonic myoblasts give rise to primary fibers in the embryo, while fetal myoblasts give rise to secondary fibers, initially smaller and surrounding primary fibers. Satellite cells appear underneath the newly formed basal lamina that develops around each fiber, and contribute to post-natal growth and regeneration of muscle fibers. Recently, different types of non somitic stem-progenitor cells have been shown to contribute to muscle regeneration. The origin of these different cell types and their possible lineage relationships with other myogenic cells as well as their possible role in muscle regeneration will be discussed. Finally, possible use of different myogenic cells in experimental protocols of cell therapy will be briefly outlined.

Transgenic mice with dominant negative PKC-theta in skeletal muscle: a new model of insulin resistance and obesity

Protein kinase C theta (PKC-theta) is the PKC isoform predominantly expressed in skeletal muscle, and it is supposed to mediate many signals necessary for muscle histogenesis and homeostasis, such as TGFbeta, nerve-dependent signals and insulin. To study the role of PKC-theta in these mechanisms we generated transgenic mice expressing a "kinase dead" mutant form of PKC-theta (PKC-thetaK/R), working as "dominant negative," specifically in skeletal muscle. These mice are viable and fertile, however, by the 6-7 months of age, they gain weight, mainly due to visceral fat deposition. Before the onset of obesity (4 months of age), they already show increased fasting and fed insulin levels and reduced insulin-sensitivity, as measured by ipITT, but normal glucose tolerance, as measured by ipGTT. After the 6-7 months of age, transgenic mice develop hyperinsulinemia in the fasting and fed state. The ipGTT revealed in the transgenic mice both hyperglycemia and hyperinsulinemia. At the molecular level, impaired activation of the IR/IRS/PI3K pathway and a significant decrease both in the levels and in insulin-stimulated activation of the serine/threonine kinase Akt were observed. Taken together these data demonstrate that over-expression of dominant negative PKC-theta in skeletal muscle causes obesity associated to insulin resistance, as demonstrated by defective receptor and post-receptorial activation of signaling cascade.

The block of ryanodine receptors selectively inhibits fetal myoblast differentiation

Differentiation and morphogenesis of skeletal muscle are complex and asynchronous events that involve various myogenic cell populations and extracellular signals. Embryonic and fetal skeletal myoblasts are responsible for the formation of primary and secondary fibers, respectively, although the mechanism that diversifies their fate is not fully understood. Calcium transients appear to be a signaling mechanism that is widely utilized in differentiation and embryogenesis. In mature skeletal muscle, calcium transients are generated mainly by ryanodine receptors (type 1 and type 3), which are involved in excitation-contraction coupling. However, it is not clear whether the activity of these receptors is important for contractile activity alone or whether it may also play a role in regulating the differentiation/developmental processes. To clarify this point, we first examined the expression of the receptors during development. The results show that the expression of both receptors appears as early as E13 during limb muscle development and parallels the expression of skeletal myosin. The expression and the activity of both receptors is maintained in vitro by all myogenic cell populations isolated from different stages of development, including somitic, embryonic and fetal myoblasts and satellite cells. Blocking ryanodine receptor activity by using ryanodine inhibits in vitro differentiation of fetal myoblasts (judged by the expression of sarcomeric myosin and formation of multinucleated myotubes) but not of somitic or embryonic and satellite muscle cells. This block is caused by the transcriptional inhibition of markers characteristic of terminal differentiation, rather than commitment, as the expression of muscle regulatory factors is not impaired by ryanodine treatment. Taken together, the data reported in this paper demonstrate that, although calcium transients represent a general mechanism for the control of differentiation and development, multiple calcium-dependent pathways may be relevant in different myogenic populations during development. Moreover, since fetal myoblasts are responsible for the formation of secondary fibers during development, and therefore for the building of the bulk of muscular mass, these results suggest that calcium release from ryanodine receptors plays a role in the histogenesis of mammalian skeletal muscle.

Polychlorobiphenyls inhibit skeletal muscle differentiation in culture

Polychlorinated biphenyls (PCBs) are ubiquitous and persistent pollutants whose role in developmental toxicity is of great concern. The observation that the offspring of PCB-exposed mothers (both in humans and rodents) display reduced body mass prompted us to investigate the effects of commercial mixtures of PCB congeners (Aroclor 1232, 1254, and 1262) on differentiation of both a myogenic cell line and primary myogenic cell cultures. The fusion of L6 myoblasts into multinucleated myotubes and the increase of creatine kinase (CK) activity were dose-dependently inhibited by Aroclor 1254 at concentrations (0.1-4 microg/ml) that caused no effect on cell density. Ultrastructural analysis demonstrated that Aroclor 1254 also prevented the accumulation of contractile filaments while inducing hypertrophy of the smooth endoplasmic reticulum and appearance of membrane-filled autophagosomes. Half-maximal inhibition (IC50) of CK activity accumulation occurred at 0.01 microg/ml for Aroclor 1262, 2 microg/ml for Aroclor 1254, and 8 microg/ml for Aroclor 1232. Aroclor-dependent inhibition of myogenic differentiation was also shown by the reduced expression and nuclear accumulation of beta-galactosidase in primary cultures of fetal myoblasts from transgenic mice expressing this reporter gene under the control of the myosin light chain promoter. These data show that skeletal muscle differentiation is specifically impaired by PCBs and may explain the reported depression of body mass growth in PCB-exposed offspring at birth. Furthermore, myogenic cell cultures are highly sensitive to PCBs and allow the detection of biological effects of environmental levels of these pollutants.

Altered primary myogenesis in NFATC3(-/-) mice leads to decreased muscle size in the adult

Signal transduction pathways involving calcineurin and its downstream effector NFAT have been implicated in regulating myogenesis. Several isoforms of NFAT exist that may differentially contribute to regulating skeletal muscle physiology. The purpose of this study was to determine the role of the NFATC3 isoform in skeletal muscle development. Adult mice lacking NFATC3 have reduced muscle mass compared to control mice. The smaller size of the muscles is not due to atrophy or blunted myofiber growth, but rather to a reduced number of myofibers. This reduction in myofiber number is not limited to a specific fiber type nor are the proportions of fiber types altered. The lower fiber number found in the adult NFATC3(-/-) mice is a consequence of impaired muscle development during embryogenesis. Immunohistochemical studies of E15 EDL muscles indicate that the total number of primary myofibers is decreased in NFATC3(-/-) embryos. At E17.5 no further decrease in primary myofiber number occurs; the size and organization of the myofibers are unaltered, and secondary myogenesis proceeds normally, suggesting a role for NFATC3 during early events in primary myogenesis. These results suggest a heretofore unknown role for the transcription factor NFAT in early skeletal muscle development.

Protein kinase C signaling controls skeletal muscle fiber types

Slow myosin heavy chain 2 (MyHC2) gene expression in fetal avian skeletal muscle fibers is regulated by innervation and protein kinase C (PKC) activity. Fetal chick muscle fibers derived from the slow twitch medial adductor (MA) muscle express slow MyHC2 when innervated in vitro. The same pattern of slow MyHC2 regulation occurs in MA muscle fibers in which PKC activity is inhibited by staurosporine. To further test the function of PKC activity in the regulation of slow MyHC2 expression, wild-type and dominant-negative mutations of PKCalpha and PKCtheta were overexpressed in MA muscle fibers in vitro. Overexpression of wild-type PKCalpha and PKCtheta cDNAs resulted in increased PKC activities in muscle fibers and concomitant repression of slow MyHC2 expression under conditions that normally induced gene expression. Point mutations leading to single amino acid substitutions were generated in the ATP binding domains of PKCalpha and PKCtheta. Overexpression of CMVPKCalphaR368 and CMVPKCthetaR409 resulted in decreased PKC activities in transfected MA muscle fibers. Furthermore, transfection of CMVPKCalphaR368 and CMVPKCthetaR409 mutant constructs into MA muscle fibers did not repress the capacity of these fibers to express slow MyHC2 when cultured in medium containing staurosporine or when innervated. These results indicate that PKC activity represses slow MyHC2 expression and that PKC down-regulation, possibly in response to innervation, is required but not sufficient for slow MyHC2 expression.

Transforming growth factor-beta(1) regulation of surfactant protein B gene expression is mediated by protein kinase-dependent intracellular translocation of thyroid transcription factor-1 and hepatocyte nuclear factor 3

The transforming growth factor-beta (TGF beta) polypeptides control a variety of cellular processes including organogenesis and cellular proliferation and differentiation. In the developing lung, TGF beta(1) treatment inhibits airway branching and expression of the genes for surfactant proteins (SP). Many effects of TGF beta are mediated at the level of gene transcription but there is limited information regarding signaling pathways and target transcription factors. In this study with human pulmonary adenocarcinoma H441 cells, we investigated TGF beta(1) effects on SP-B, a protein which is essential for normal function of pulmonary surfactant. TGF beta(1) (10 ng/ml) reduced SP-B mRNA content in a time-dependent fashion, and transient transfection studies localized responsiveness to the region of the SP-B promoter (-112/-72 bp) containing binding sites for thyroid transcription factor-1 (TTF-1) and hepatocyte nuclear factor 3 (HNF3), transcription factors that are important enhancers of SP gene expression. Using electrophoretic mobility shift assay and immunofluorescence, we demonstrated rapid accumulation of these transcription factors in the cytoplasm and subsequent loss from the nucleus on TGF beta(1) treatment of both adenocarcinoma cells and cultured human fetal lung. TGF beta(1) treatment caused intracellular translocation of protein kinase C and effects of TGF beta(1) were mostly abrogated in the presence of the protein kinase inhibitor calphostin C. We conclude that TGF beta(1), acting via protein phosphorylation, blocks nuclear translocation of TTF-1 and HNF3 which results in down-regulation of the SP-B gene and presumably other pulmonary genes which are transactivated by these factors.

Loss of FGF receptor 1 signaling reduces skeletal muscle mass and disrupts myofiber organization in the developing limb

The identities of extracellular growth factors that regulate skeletal muscle development in vivo are largely unknown. We asked if FGFs, which act as repressors of myogenesis in culture, play a similar role in vivo by ectopically expressing in the developing limb a truncated FGF receptor 1 (dnFGFR1) that acts as a dominant negative mutant. Hind limbs and the adjacent somites of Hamburger and Hamilton (HH) stage 17 chickens were infected with a replication-competent RCAS virus encoding dnFGFR1. By ED5, the virus had spread extensively within the limb and the adjacent somites with little rostral or caudal expansion of the infection along the axial midline. Viral infection and mutant receptor expression were coincident as revealed by the distribution of a viral coat protein and an HA epitope tag present on the carboxy terminus of dnFGFR1. Within 48 h following injection of dnFGFR1, we could detect no obvious changes in skeletal muscle precursor cell migration into the hind limb as compared to control limbs infected with an empty RCAN virus. However, by 3 days following infection of RCAS-dnFGFR1 virus, the level of skeletal muscle-specific myosin heavy chain was decreased and the expression pattern altered, suggesting disruption of skeletal muscle development. Two striking muscular phenotypes were observed in dnFGFR1-expressing limbs, including an average loss of 30% in skeletal muscle wet weight and a 50% decrease in myofiber density. At all ages examined the loss of skeletal muscle mass was accompanied by a loss of myoblasts and an unexpected concomitant loss of fibroblasts. Consistent with these observations, explants of infected cells revealed a reduction in the number of myonuclei in myotubes. Although the myofiber density per unit area was decreased over 50% compared to controls there were no detectable effects on myofiber diameter. The loss in myofiber density was, however, accompanied by an increase in the space surrounding individual myofibers and a generalized loss of myofiber integrity. It is noteworthy that long-bone development was unaffected by RCAS-dnFGFR1 infection, suggesting that FGFR2 and FGFR3 signaling was not disrupted. Our data provide conclusive evidence that FGFR1 signaling is necessary to maintain myoblast number and plays a role in myofiber organization.

Rat embryonic myoblasts are restricted to forming primary fibres while later myogenic populations are pluripotent

Three populations of myoblasts, embryonic, foetal and adult, appear sequentially during myogenesis. The present study uses retroviruses to mark myoblasts clones in vivo from these populations. Myoblasts labelled at E15 (embryonic) contributed to primary fibres only. The majority of marked primary fibres were slow but a small number of clones contained marked primaries which were no longer slow at E19. Myoblasts labelled at E17 (foetal) fused with both primary and secondary fibres and most clones contained both fast and slow fibres. Similarly, adult myoblasts marked at P0 fused with all fibre types. These results indicate that embryonic myoblasts are restricted to producing only primary fibres which are initially slow but which can convert to being fast. Clones of foetal and adult myoblasts fuse with both primary and secondary fibres which may be either fast or slow.

Muscle regeneration and fiber-type transformation during distraction osteogenesis

The successful outcome of distraction osteogenesis depends in part on the adequate adaptation of the surrounding soft tissue. We characterized the adaptation of the tibialis anterior during distraction osteogenesis at different rates (0.7 and 1.4 mm/day) and amounts (15 and 30%) of lengthening. We documented the increased expression of neonatal and slow myosin heavy chain in the tibialis anterior of skeletally immature rabbits. There was neither expression of neonatal myosin heavy chain in the experimental soleus or in the slow muscle fibers of the tibialis anterior nor increased expression of slow myosin heavy chain in the soleus or gastrocnemius. The increased amount of neonatal myosin heavy chain was concentrated in the distal half of the muscle, whereas the increase in the number of fibers that were labeled with antibodies to slow myosin occurred to the same extent throughout the tibialis anterior. Electrophysiological methods showed that the tibialis anterior was functionally intact during and after distraction osteogenesis. We concluded that in the tibialis anterior of young, skeletally immature animals, distraction osteogenesis seems to induce a recapitulation of the developmental process without leading to functional changes. In addition, during distraction osteogenesis, a fiber-type transformation occurs similar to that observed in models of muscle overloading.

Seeking muscle stem cells

Skeletal muscle development requires the formation of myoblasts that can fuse with each other to form multinucleate myofibers. Distinct primary and secondary, slow and fast, populations of myofibers form by the time of birth. At embryonic, fetal, and perinatal stages of development, temporally distinct lineages of myogenic cells arise and contribute to the formation of these multiple types of myofibers. In addition, spatially distinct lineages of myogenic cells arise and form the anterior head muscles, limb (hypaxial) muscles, and dorsal (epaxial) muscles. There is strong evidence that myoblasts are produced from muscle stem cells, which are self-renewing cells that do not themselves terminally differentiate but produce progeny that are capable of becoming myoblasts and myofibers. Muscle stem cells, which may be multipotent, appear to be distinguishable from myoblasts by a number of indirect and direct criteria. Muscle stem cells arise either in unsegmented paraxial mesoderm (anterior head muscle progenitors) or in segmented mesoderm of the somites (epaxial and hypaxial muscle progenitors). These initial stages of myogenesis are regulated by positive and negative signals, including Wnt, BMP, and Shh family members, from nearby notochord, neural tube, ectoderm, and lateral mesoderm tissues. The formation of skeletal muscles, therefore, depends on the generation of spatially and temporally distinct lineages of myogenic cells. Myogenic cell lineages begin with muscle stem cells which produce the myoblasts that fuse to form myofibers.

The four populations of myoblasts involved in human limb muscle formation are present from the onset of primary myotube formation

To understand how and when myogenic precursor cells become committed to their particular developmental programs, we have analysed the different populations of myoblasts which grow out from explants of muscle tissue isolated from human limb buds from the beginning of primary fibre formation throughout subsequent development and post-natal growth. Four phenotypically distinct types of myoblasts were identified on the basis of their expression of desmin, myogenin and myosin heavy chain isoforms (MyHC), and after 5 and 20 divisions, cells were cloned. All four types of myoblasts were present at the beginning of primary myogenesis. Each respective phenotype was stably heritable through cloning and subsequent proliferation. The type 1 clones correspond to a novel class of myoblasts never described during human development, that biochemically differentiates, but does not fuse. Type 2 clones are composed of small myotubes expressing only embryonic MyHC. Type 3 clones are composed of thin and long myotubes expressing both embryonic and fetal MyHCs. The type 4 clones are composed of myotubes that have a phenotype very similar to human satellite cells. Contrasting with others species, no other population of myoblasts appear during fetal development and only the relative number of these four types changes.

Inhibition of myogenesis by transforming growth factor beta is density-dependent and related to the translocation of transcription factor MEF2 to the cytoplasm

Transforming growth factor beta (TGF-beta) was found to inhibit differentiation of myogenic cells only when they were grown to high density. Inhibition also occurred when myogenic cells were cocultured with other types of mesenchymal cells but not when they were cocultured with epithelial cells. It is therefore possible that some density-dependent signaling mediates the intracellular response to TGF-beta. Within 30 min of treatment, TGF-beta induced translocation of MEF2, but not MyoD, myogenin, or p21, to the cytoplasm of myogenic cells grown to high density. Translocation was reversible on withdrawal of TGF-beta. By using immune electron microscopy and Western blot analysis on subcellular fractions, MEF2 was shown to be tightly associated with cytoskeleton membrane components. To test whether MEF2 export from the nucleus was causally related to the inhibitory action of TGF-beta, we transfected C2C12 myoblasts with MEF2C containing the nuclear localization signal of simian virus 40 large T antigen (nlsSV40). Myogenic cells expressing the chimerical MEF2C/nlsSV40, but not wild-type MEF2C, retained this transcription factor in the nucleus and were resistant to the inhibitory action of TGF-beta. We propose a mechanism in which the inhibition of myogenesis by TGF-beta is mediated through MEF2 localization to the cytoplasm, thus preventing it from participating in an active transcriptional complex.

Pituitary expression of protein kinase C isotypes during early development

Protein kinase C (PKC) is a critical regulator of signal transduction and cell function in many tissues, including pituitary. Although PKC influences pituitary hormone secretion in adults, its role in determining characteristic perinatal patterns of hormone secretion and synthesis is not known, and the expression of major PKC isotypes in perinatal pituitary is poorly defined. We therefore determined the developmental, cell-specific expression of the major PKC isotypes, using Western analysis and double label immunohistochemistry, in pituitaries of perinatal and mature rats. Expression of specific PKC isotypes was strikingly age-dependent. Pituitary expression of PKC alpha was particularly high in neonates and declined significantly with age, with levels in adult rats approximately half those of neonates as assessed by Western analysis. Similarly, immunohistochemistry indicated that PKC alpha was less abundant in adult than in neonatal pituitaries; the most intensely staining cells of both age groups were identified as somatotrophs and gonadotrophs. In contrast to PKC alpha, pituitary expression of PKC epsilon increased approximately two-fold with advancing age as assessed by Western analysis; this age-dependent pattern was confirmed by immunohistochemistry. Perinatal pituitaries expressed PKC epsilon in some somatotrophs and in all gonadotrophs, whereas PKC epsilon expression was limited to gonadotrophs in the mature pituitary. Pituitary expression of PKC betaII, delta, and zeta did not differ with age, and PKC gamma was not detected in pituitaries of any age group. These results indicate that expression of PKC isotypes within the pituitary is developmentally regulated in a cell-specific and isotype-specific manner, and are consistent with the concept that PKC contributes to the regulation of pituitary function during early development.

Growth and metabolism of fetal and maternal muscles of adolescent sheep on adequate or high feed intake: possible role of protein kinase C-alpha in fetal muscle growth

From days 4-104 of pregnancy, adolescent sheep, weighing 43.7 (SE 0.87) kg were offered a complete diet at two different intakes (approximately 5 or 15 kg/week) designed to meet slightly, or well above, maternal maintenance requirements. The fetal and maternal muscles were taken on day 104 of pregnancy and analysed for total DNA, RNA and protein. Ewes offered a high intake to promote rapid maternal weight gain, weighed more (76.5 (SE 4.5) v 50.0 (SE 1.7) kg) and had muscles with a greater fresh weight, whilst their fetuses had smaller muscles, than those fed at a lower intake. Plantaris muscle of the ewes fed at the high intake contained more RNA and protein; again the opposite situation was found in the fetal muscle. On the higher maternal intakes, the DNA, RNA and protein contents of the fetal plantaris muscle were less than in fetuses of ewes fed at the lower intake. To investigate the possible mechanisms involved in this decrease in fetal muscle mass, cytosolic and membrane-associated muscle proteins were subjected to Western immunoblotting with antibodies to nine isoforms of protein kinase C (PKC), a family of enzymes known to play an important role in cell growth. Five PKC isoforms (alpha, epsilon, theta, mu, zeta) were identified in fetal muscle. One of these, PKC-alpha was located predominantly in the cytosolic compartment in the smaller fetuses of the ewes fed at a high plane of nutrition, but was present to a greater extent in the membranes of the more rapidly growing fetuses of the ewes fed at the lower intake. This was the only isoform to demonstrate nutritionally related changes in it subcellular compartmentation suggesting that it may mediate some aspects of the change in fetal growth rate.

An activated protein kinase C alpha gives a differentiation signal for hematopoietic progenitor cells and mimicks macrophage colony-stimulating factor-stimulated signaling events

Highly enriched, bipotent, hematopoietic granulocyte macrophage colony-forming cells (GM-CFC) require cytokines for their survival, proliferation, and development. GM-CFC will form neutrophils in the presence of the cytokines stem cell factor and granulocyte colony-stimulating factor, whereas macrophage colony-stimulating factor leads to macrophage formation. Previously, we have shown that the commitment to the macrophage lineage is associated with lipid hydrolysis and translocation of protein kinase C alpha (PKCalpha) to the nucleus. Here we have transfected freshly prepared GM-CFC with a constitutively activated form of PKCalpha, namely PKAC, in which the regulatory domain has been truncated. Greater than 95% of the transfected cells showed over a twofold increase in PKCalpha expression with the protein being located primarily within the nucleus. The expression of PKAC caused macrophage development even in the presence of stimuli that normally promote only neutrophilic development. Thus, M-CSF-stimulated translocation of PKCalpha to the nucleus is a signal associated with macrophage development in primary mammalian hematopoietic progenitor cells, and this signal can be mimicked by ectopic PKAC, which is also expressed in the nucleus.

Establishing myogenic identity during somitogenesis

Over the past year, interest has focused on identifying signalling molecules--including Wnts, Sonic hedgehog, BMP-4, and noggin--that divert somitic mesodermal cells into the muscle lineage, either by induction or derepression. New mouse mutants have also provided insights into somite formation and differentiation, as well as pointing to novel differences between head, trunk, and limb myogenic programmes. In addition, recent genetic, embryological, and molecular studies have shed new light on somite formation and the establishment of muscle progenitor cells.