Requirement for serum response factor for skeletal muscle growth and maturation revealed by tissue-specific gene deletion in mice

Filamin A interacts with the coactivator MKL1 to promote the activity of the transcription factor SRF and cell migration

Megakaryoblastic leukemia 1 (MKL1) is a coactivator of serum response factor (SRF) that promotes the expression of genes associated with cell proliferation, motility, adhesion, and differentiation-processes that also involve dynamic cytoskeletal changes in the cell. MKL1 is inactive when bound to monomeric globular actin (G-actin), but signals that activate the small guanosine triphosphatase RhoA cause actin polymerization and MKL1 dissociation from G-actin. We found a new mechanism of MKL1 activation that is mediated through its binding to filamin A (FLNA), a protein that binds filamentous actin (F-actin). The interaction of FLNA and MKL1 was required for the expression of MKL1 target genes in primary fibroblasts, melanoma, mammary and hepatocellular carcinoma cells. We identified the regions of interaction between MKL1 and FLNA, and cells expressing an MKL1 mutant that was unable to bind FLNA exhibited impaired cell migration and reduced expression of MKL1-SRF target genes. Induction and repression of MKL1-SRF target genes correlated with increased or decreased MKL1-FLNA interaction, respectively. Lysophosphatidic acid-induced RhoA activation in primary human fibroblasts promoted the association of endogenous MKL1 with FLNA, whereas exposure to an actin polymerization inhibitor dissociated MKL1 from FLNA and decreased MKL1-SRF target gene expression in melanoma cells. Thus, FLNA functions as a positive cellular transducer linking actin polymerization to MKL1-SRF activity, counteracting the known repressive complex of MKL1 and monomeric G-actin.

A conserved MADS-box phosphorylation motif regulates differentiation and mitochondrial function in skeletal, cardiac, and smooth muscle cells

Exposure to metabolic disease during fetal development alters cellular differentiation and perturbs metabolic homeostasis, but the underlying molecular regulators of this phenomenon in muscle cells are not completely understood. To address this, we undertook a computational approach to identify cooperating partners of the myocyte enhancer factor-2 (MEF2) family of transcription factors, known regulators of muscle differentiation and metabolic function. We demonstrate that MEF2 and the serum response factor (SRF) collaboratively regulate the expression of numerous muscle-specific genes, including microRNA-133a (miR-133a). Using tandem mass spectrometry techniques, we identify a conserved phosphorylation motif within the MEF2 and SRF Mcm1 Agamous Deficiens SRF (MADS)-box that regulates miR-133a expression and mitochondrial function in response to a lipotoxic signal. Furthermore, reconstitution of MEF2 function by expression of a neutralizing mutation in this identified phosphorylation motif restores miR-133a expression and mitochondrial membrane potential during lipotoxicity. Mechanistically, we demonstrate that miR-133a regulates mitochondrial function through translational inhibition of a mitophagy and cell death modulating protein, called Nix. Finally, we show that rodents exposed to gestational diabetes during fetal development display muscle diacylglycerol accumulation, concurrent with insulin resistance, reduced miR-133a, and elevated Nix expression, as young adult rats. Given the diverse roles of miR-133a and Nix in regulating mitochondrial function, and proliferation in certain cancers, dysregulation of this genetic pathway may have broad implications involving insulin resistance, cardiovascular disease, and cancer biology.

Developmental regulation and evolution of muscle-specific microRNAs

MicroRNAs (miRs) are a group of small RNAs that play a major role in post-transcriptional regulation of gene expression. In animals, many of the miRs are expressed in a conserved spatiotemporal manner. Muscle tissues, the major cellular systems involved in the locomotion and physiological functions of animals, have been one of the main sites for verification of miR targets and analysis of their developmental functions. During the determination and differentiation of muscle cells, numerous miRs bind to and repress target mRNAs in a highly specific but redundant manner. Interspecific comparisons of the sequences and expression of miRs have suggested that miR regulation became increasingly important during the course of vertebrate evolution. However, the detailed molecular interactions that have led to the highly complex morphological structures still await investigation. In this review, we will summarize the recent findings on the functional and developmental characteristics of miRs that have played major roles in vertebrate myogenesis, and discuss how the evolution of miRs is related to the morphological complexity of the vertebrates.

Identification and expression of SRF targeted by miR-133a during early development of Paralichthys olivaceus

Serum response factor (SRF) is a MADS-box transcription factor that regulates the expression of genes involved in development, metabolism, cell proliferation, and differentiation. In the present study, we cloned the full-length SRF cDNA which includes the coding region of 1503 bp, a 573-bp 5'untranslated region (UTR) and a 400-bp 3'-UTR. The deduced 501 amino acid sequence of the SRF protein contained a MADS domain and NLS at the N terminus, similar to other organisms, and it also is highly phylogenetically conserved. SRF mRNA is ubiquitously expressed in various tissues, with the highest level in the kidneys, and it is also highly expressed during the embryonic and metamorphic stages. During metamorphosis, the SRF mRNA levels are down-regulated by exogenous thyroid hormone (TH) at 17 dph and by thiourea (TU) at 29, 36, and 41 dph, whereas SRF mRNA levels were significantly up-regulated by the added exogenous TH to the TU-treated larvae at 41 dph, which indicates that thyroid hormone is essential for expression of SRF mRNA, so, higher levels of TH did not result in changes of SRF mRNA levels, while TH deficiency or inhibited by the non-specific TU toxicity cause down-regulation of SRF mRNA, which indicated that TH can indirectly affect the SRF mRNA levels. Meanwhile, using a luciferase reporter assay, we verified that SRF is a common target gene of miR-133a which is a muscle-specific microRNA (miRNA), which indicated that SRF may be involved in the signaling pathway of miRNA that regulates muscle development.

Hedgehog/Patched-associated rhabdomyosarcoma formation from delta1-expressing mesodermal cells

Rhabdomyosarcoma (RMS) is the most common pediatric soft tissue sarcoma. In children, the 2 major RMS subtypes are alveolar and embryonal RMS. Aberrant Hedgehog/Patched1 (Hh/Ptch) signaling is a hallmark of embryonal RMS. We demonstrate that mice carrying a Ptch mutation in mesodermal Delta1-expressing cells develop embryonal-like RMS at a similar rate as mice harboring a Ptch mutation in the germline or the brachury-expressing mesoderm. The tumor incidence decreases dramatically when Ptch is mutated in Myf5- or Pax3-expressing cells. No RMS develop from Myogenin/Mef2c-expressing cells. This suggests that Hh/Ptch-associated RMS are derived from Delta1-positive, Myf5-negative, Myogenin-negative and Pax3-negative mesodermal progenitors that can undergo myogenic differentiation but lack stable lineage commitment. Additional preliminary genetic data and data on mesodermal progenitors further imply an interplay of Hh/Ptch and Delta/Notch signaling activity during RMS initiation. In contrast, Wnt signals supposedly suppress RMS formation because RMS multiplicity decreases after inactivation of the Wnt-inhibitor Wif1. Finally, our results strongly suggest that the tumor-initiating event determines the lineage of RMS origin.

Skeletal muscle interleukin-6 regulates metabolic factors in iWAT during HFD and exercise training

OBJECTIVE

To investigate the role of skeletal muscle (SkM) interleukin (IL)-6 in the regulation of adipose tissue metabolism.

METHODS

Muscle-specific IL-6 knockout (IL-6 MKO) and IL-6(loxP/loxP) (Floxed) mice were subjected to standard rodent diet (Chow), high-fat diet (HFD), or HFD in combination with exercise training (HFD ExTr) for 16 weeks.

RESULTS

Total fat mass increased (P < 0.05) in both genotypes with HFD. However, HFD IL-6 MKO mice had lower (P < 0.05) inguinal adipose tissue (iWAT) mass than HFD Floxed mice. Accordingly, iWAT glucose transporter 4 (GLUT4) protein content, 5'AMP activated protein kinase (AMPK)(Thr172) phosphorylation, and fatty acid synthase (FAS) mRNA content were lower (P < 0.05) in IL-6 MKO than Floxed mice on Chow. In addition, iWAT AMPK(Thr172) and hormone-sensitive lipase (HSL)(Ser565) phosphorylation as well as perilipin protein content was higher (P < 0.05) in HFD IL-6 MKO than HFD Floxed mice, and pyruvate dehydrogenase E1α (PDH-E1α) protein content was higher (P < 0.05) in HFD ExTr IL-6 MKO than HFD ExTr Floxed mice.

CONCLUSIONS

These findings indicate that SkM IL-6 affects iWAT mass through regulation of glucose uptake capacity as well as lipogenic and lipolytic factors.

The mammalian target of rapamycin signaling pathway regulates myocyte enhancer factor-2C phosphorylation levels through integrin-linked kinase in goat skeletal muscle satellite cells

Mammalian target of rapamycin (mTOR) signaling pathway plays a key role in muscle development and is involved in multiple intracellular signaling pathways. Myocyte enhancer factor-2 (MEF2) regulates muscle cell proliferation and differentiation. However, how the mTOR signaling pathway regulates MEF2 activity remains unclear. We isolated goat skeletal muscle satellite cells (gSSCs) as model cells to explore mTOR signaling pathway regulation of MEF2C. We inhibited mTOR activity in gSSCs with PP242 and found that MEF2C phosphorylation was decreased and that muscle creatine kinase (MCK) expression was suppressed. Subsequently, we detected integrin-linked kinase (ILK) using MEF2C coimmunoprecipitation; ILK and MEF2C were colocalized in the gSSCs. We found that inhibiting mTOR activity increased ILK phosphorylation levels and that inhibiting ILK activity with Cpd 22 and knocking down ILK with small interfering RNA increased MEF2C phosphorylation and MCK expression. In the presence of Cpd 22, mTOR activity inhibition did not affect MEF2C phosphorylation. Moreover, ILK dephosphorylated MEF2C in vitro. These results suggest that the mTOR signaling pathway regulates MEF2C positively and regulates ILK negatively and that ILK regulates MEF2C negatively. It appears that the mTOR signaling pathway regulates MEF2C through ILK, further regulating the expression of muscle-related genes in gSSCs.

Adipose triglyceride lipase deletion from adipocytes, but not skeletal myocytes, impairs acute exercise performance in mice

Adipose triglyceride lipase (ATGL) is the rate-limiting enzyme mediating triacylglycerol hydrolysis in virtually all cells, including adipocytes and skeletal myocytes, and hence, plays a critical role in mobilizing fatty acids. Global ATGL deficiency promotes skeletal myopathy and exercise intolerance in mice and humans, and yet the tissue-specific contributions to these phenotypes remain unknown. The goal of this study was to determine the relative contribution of ATGL-mediated triacylglycerol hydrolysis in adipocytes vs. skeletal myocytes to acute exercise performance. To achieve this goal, we generated murine models with adipocyte- and skeletal myocyte-specific targeted deletion of ATGL. We then subjected untrained mice to acute peak and submaximal exercise interventions and assessed exercise performance and energy substrate metabolism. Impaired ATGL-mediated lipolysis within adipocytes reduced peak and submaximal exercise performance, reduced peripheral energy substrate availability, shifted energy substrate preference toward carbohydrate oxidation, and decreased HSL Ser(660) phosphorylation and mitochondrial respiration within skeletal muscle. In contrast, impaired ATGL-mediated lipolysis within skeletal myocytes was not sufficient to reduce peak and submaximal exercise performance or peripheral energy substrate availability and instead tended to enhance metabolic flexibility during peak exercise. Furthermore, the expanded intramyocellular triacylglycerol pool in these mice was reduced following exercise in association with preserved HSL phosphorylation, suggesting that HSL may compensate for impaired ATGL action in skeletal muscle during exercise. These data suggest that adipocyte rather than skeletal myocyte ATGL-mediated lipolysis plays a greater role during acute exercise in part because of compensatory mechanisms that maintain lipolysis in muscle, but not adipose tissue, when ATGL is absent.

Myocyte enhancer factor 2C function in skeletal muscle is required for normal growth and glucose metabolism in mice

BACKGROUND

Skeletal muscle is the most abundant tissue in the body and is a major source of total energy expenditure in mammals. Skeletal muscle consists of fast and slow fiber types, which differ in their energy usage, contractile speed, and force generation. Although skeletal muscle plays a major role in whole body metabolism, the transcription factors controlling metabolic function in muscle remain incompletely understood. Members of the myocyte enhancer factor 2 (MEF2) family of transcription factors play crucial roles in skeletal muscle development and function. MEF2C is expressed in skeletal muscle during development and postnatally and is known to play roles in sarcomeric gene expression, fiber type control, and regulation of metabolic genes.

METHODS

We generated mice lacking Mef2c exclusively in skeletal muscle using a conditional knockout approach and conducted a detailed phenotypic analysis.

RESULTS

Mice lacking Mef2c in skeletal muscle on an outbred background are viable and grow to adulthood, but they are significantly smaller in overall body size compared to control mice and have significantly fewer slow fibers. When exercised in a voluntary wheel running assay, Mef2c skeletal muscle knockout mice aberrantly accumulate glycogen in their muscle, suggesting an impairment in normal glucose homeostasis. Consistent with this notion, Mef2c skeletal muscle knockout mice exhibit accelerated blood glucose clearance compared to control mice.

CONCLUSIONS

These findings demonstrate that MEF2C function in skeletal muscle is important for metabolic homeostasis and control of overall body size.

Srf: A key factor controlling skeletal muscle hypertrophy by enhancing the recruitment of muscle stem cells

Adult skeletal muscles adapt their fiber size to workload. We show that serum response factor (Srf) is required for satellite cell-mediated hypertrophic muscle growth. Deletion of Srf from myofibers, and not satellite cells, blunts overload-induced hypertrophy, and impairs satellite cell proliferation and recruitment to pre-existing fibers. We reveal a gene network in which Srf within myofibers modulates interleukin-6 and cyclooxygenase-2/interleukin-4 expressions and therefore exerts a paracrine control of satellite cell functions. In Srf-deleted muscles, in vivo overexpression of interleukin-6 is sufficient to restore satellite cell proliferation, but not satellite cell fusion and overall growth. In contrast, cyclooxygenase-2/interleukin-4 overexpression rescues satellite cell recruitment and muscle growth without affecting satellite cell proliferation, identifying altered fusion as the limiting cellular event. These findings unravel a role for Srf in the translation of mechanical cues applied to myofibers into paracrine signals, which in turn will modulate satellite cell functions and support muscle growth.

Overexpression of p49/STRAP alters cellular cytoskeletal structure and gross anatomy in mice

Background

The protein p49/STRAP (SRFBP1) is a transcription cofactor of serum response factor (SRF) which regulates cytoskeletal and muscle-specific genes.

Results

Two conserved domains were found in the p49/STRAP protein. The SRF-binding domain was at its N-terminus and was highly conserved among mammalian species, xenopus and zebrafish. A BUD22 domain was found at its C-terminus in three sequence databases. The BUD22 domain was conserved among mammalian p49/STRAP proteins, and yeast cellular morphogenesis proteins, which is involved in ribosome biogenesis that affects growth rate and cell size. The endogenous p49/SRAP protein was localized mainly in the nucleus but also widely distributed in the cytoplasm, and was in close proximity to the actin. Transfected GFP-p49/STRAP protein co-localized with nucleolin within the nucleolus. Overexpression of p49/STRAP reduced actin content in cultured cells and resulted in smaller cell size versus control cells. Increased expression of p49/STRAP in transgenic mice resulted in newborns with malformations, which included asymmetric abdominal and thoracic cavities, and substantial changes in cardiac morphology. p49/STRAP altered the expression of certain muscle-specific genes, including that of the SRF gene, which is a key regulator of cardiac genes at the developmental, structural and maintenance level and has two SRE binding sites.

Conclusions

Since p49/STRAP is a co-factor of SRF, our data suggest that p49/STRAP likely regulates cell size and morphology through SRF target genes. The function of its BUD22 domain warrants further investigation. The observed increase in p49/STRAP expression during cellular aging may contribute to observed morphological changes in senescence.

KLHL40 deficiency destabilizes thin filament proteins and promotes nemaline myopathy

Nemaline myopathy (NM) is a congenital myopathy that can result in lethal muscle dysfunction and is thought to be a disease of the sarcomere thin filament. Recently, several proteins of unknown function have been implicated in NM, but the mechanistic basis of their contribution to disease remains unresolved. Here, we demonstrated that loss of a muscle-specific protein, kelch-like family member 40 (KLHL40), results in a nemaline-like myopathy in mice that closely phenocopies muscle abnormalities observed in KLHL40-deficient patients. We determined that KLHL40 localizes to the sarcomere I band and A band and binds to nebulin (NEB), a protein frequently implicated in NM, as well as a putative thin filament protein, leiomodin 3 (LMOD3). KLHL40 belongs to the BTB-BACK-kelch (BBK) family of proteins, some of which have been shown to promote degradation of their substrates. In contrast, we found that KLHL40 promotes stability of NEB and LMOD3 and blocks LMOD3 ubiquitination. Accordingly, NEB and LMOD3 were reduced in skeletal muscle of both Klhl40-/- mice and KLHL40-deficient patients. Loss of sarcomere thin filament proteins is a frequent cause of NM; therefore, our data that KLHL40 stabilizes NEB and LMOD3 provide a potential basis for the development of NM in KLHL40-deficient patients.

A series of Cre-ER(T2) drivers for manipulation of the skeletal muscle lineage

We report the generation of five mouse strains with the tamoxifen-inducible Cre (Cre-ER(T) (2) ; CE) gene cassette knocked into the endogenous loci of Pax3, Myod1, Myog, Myf6, and Myl1, collectively as a resource for the skeletal muscle research community. We characterized these CE strains using the Cre reporter mice, R26R(L) (acZ) , during embryogenesis and show that they direct tightly controlled tamoxifen-inducible reporter expression within the expected cell lineage determined by each myogenic gene. We also examined a few selected adult skeletal muscle groups for tamoxifen-inducible reporter expression. None of these new CE alleles direct reporter expression in the cardiac muscle. All these alleles follow the same knock-in strategy by replacing the first exon of each gene with the CE cassette, rendering them null alleles of the endogenous gene. Advantages and disadvantages of this design are discussed. Although we describe potential immediate use of these strains, their utility likely extends beyond foreseeable questions in skeletal muscle biology.

MicroRNA-1 regulates chondrocyte phenotype by repressing histone deacetylase 4 during growth plate development

MicroRNAs (miRs) are noncoding RNAs (17-25 nt) that control translation and/or mRNA degradation. Using Northern blot analysis, we identified that miR-1 is specifically expressed in growth plate cartilage in addition to muscle tissue, but not in brain, intestine, liver, or lung. We obtained the first evidence that miR-1 is highly expressed in the hypertrophic zone of the growth plate, with an 8-fold increase compared with the proliferation zone; this location coincides with the Ihh and Col X expression regions in vivo. MiR-1 significantly induces chondrocyte proliferation and differentiation. We further identified histone deacetylase 4 (HDAC4) as a target of miR-1. HDAC4 negatively regulates chondrocyte hypertrophy by inhibiting Runx2, a critical transcription factor for chondrocyte hypertrophy. MiR-1 inhibits both endogenous HDAC4 protein by 2.2-fold and the activity of a reporter gene bearing the 3'-untranslated region (UTR) of HDAC4 by 3.3-fold. Conversely, knockdown of endogenous miR-1 relieves the repression of HDAC4. Deletion of the miR-1 binding site in HDAC4 3'-UTR or mutated miR-1 abolishes miR-1-mediated inhibition of the reporter gene activity. Overexpression of HDAC4 reverses miR-1 induction of chondrocyte differentiation markers Col X and Ihh. HDAC4 inhibits Runx2 promoter activity in a dosage-dependent manner. Thus, miR-1 plays an important role in the regulation of the chondrocyte phenotype during the growth plate development via direct targeting of HDAC4.

MKL1/2 and ELK4 co-regulate distinct serum response factor (SRF) transcription programs in macrophages

BACKGROUND

Serum response factor (SRF) is a widely expressed transcription factor involved in multiple regulatory programs. It is believed that SRF can toggle between disparate programs of gene expression through association with different cofactors. However, the direct evidence as to how these factors function on a genome-wide level is still lacking.

RESULTS

In the present study, I explored the functions of SRF and its representative cofactors, megakaryoblastic leukemia 1/2 (MKL1/2) and ETS-domain protein 4 (ELK4), during fungal infection challenge in macrophages. The knockdown study, combined with gene expression array analysis, revealed that MKL1/2 regulated SRF-dependent genes were related to actin cytoskeleton organization, while ELK4 regulated SRF-dependent genes were related to external stimulus responses. Subsequent chromatin immunoprecipitation coupled with massively parallel sequencing (ChIP-seq) suggested that many of these regulations were mediated directly in cis.

CONCLUSIONS

I conclude that SRF utilizes MKL1/2 to fulfill steady state cellular functions, including cytoskeletal organization, and utilizes ELK4 to facilitate acute responses to external infection. Together, these findings indicate that SRF, along with its two cofactors, are important players in both cellular homeostasis and stress responses in macrophages.

Age-related decreases of serum-response factor levels in human mesenchymal stem cells are involved in skeletal muscle differentiation and engraftment capacity

Skeletal muscle (SkM) comprise ∼40% of human body weight. Injury or damage to this important tissue can result in physical disability, and in severe cases is difficult for its endogenous stem cell-the satellite cell-to reverse effectively. Mesenchymal stem cells (MSC) are postnatal progenitor/stem cells that possess multilineage mesodermal differentiation capacity, including toward SkM. Adult bone marrow (BM) is the best-studied source of MSCs; however, aging also decreases BMMSC numbers and can adversely affect differentiation capacity. Therefore, we asked whether human sources of developmentally early stage mesenchymal stem cells (hDE-MSCs) isolated from embryonic stem cells, fetal bone, and term placenta could be cellular sources for SkM repair. Under standard muscle-inducing conditions, hDE-MPCs differentiate toward a SkM lineage rather than cardiomyocytic or smooth muscle lineages, as evidenced by increased expression of SkM-associated markers and in vitro myotube formation. In vivo transplantation revealed that SkM-differentiated hDE-MSCs can efficiently incorporate into host SkM tissue in a mouse model of SkM injury. In contrast, adult BMMSCs do not express SkM-associated genes after in vitro SkM differentiation nor engraft in vivo. Further investigation of possible factors responsible for this difference in SkM differentiation potential revealed that, compared with adult BMMSCs, hDE-MSCs expressed higher levels of serum response factor (SRF), a transcription factor critical for SkM lineage commitment. Moreover, knockdown of SRF in hDE-MSCs resulted in decreased expression of SkM-related genes after in vitro differentiation and decreased in vivo engraftment. Our results implicate SRF as a key factor in age-related SkM differentiation capacity of MSCs, and demonstrate that hDE-MSCs are possible candidates for SkM repair.

RNA-seq dependent transcriptional analysis unveils gene expression profile in the intestine of sea cucumber Apostichopus japonicus during aestivation

The seasonal marine, the sea cucumber Apostichopus japonicus (Selenka, 1867), cycles annually between periods of torpor when water temperature is above about 25°C in summer and active life when temperature is below about 18°C. This species is a good candidate model organism for studies of environmentally-induced aestivation in marine invertebrates. Previous studies have examined various aspects of aestivation of A. japonicus, however, knowledge of the molecular regulation underpinning these events is still fragmentary. In the present study, we constructed a global gene expression profile of the intestine tissue of A. japonicus using RNA-seq to identify transcriptional responses associated with transitions between different states: non-aestivation (NA), deep-aestivation (DA), and arousal from aestivation (AA). The analysis identified 1245 differentially expressed genes (DEGs) between DA vs. NA states, 1338 DEGs between AA vs. DA, and 1321 DEGs between AA vs. NA using the criteria |Log2Ratio|≥1 and FDR≤0.001. Of these, 25 of the most significant DEGs were verified by real-time PCR, showing trends in expression patterns that were almost in full concordance between the two techniques. GO analysis revealed that for DA vs. NA, 24 metabolic associated processes were highly enriched (corrected p value<0.05) whereas for AA vs. NA, 12 transport and metabolism associated processes were significantly enriched (corrected p value<0.05). Pathways associated with aestivation were also mined, and indicated that most DEGs were enriched in metabolic and signal transduction pathways in the deep aestivation stage. Two up pathways were significantly enriched at the arousal stage (ribosome and metabolism of xenobiotics by cytochrome P450 pathway). A set of key DEGs was identified that may play vital roles in aestivation; these involved metabolism, detoxification and tissue protection, and energy-expensive processes. Our work presents an overview of dynamic gene expression in torpor-arousal cycles during aestivation of A. japonicus and identifies a series of candidate genes and pathways for further research on the molecular mechanisms of aestivation.

The catalytic subunit of the system L1 amino acid transporter (slc7a5) facilitates nutrient signalling in mouse skeletal muscle

The System L1-type amino acid transporter mediates transport of large neutral amino acids (LNAA) in many mammalian cell-types. LNAA such as leucine are required for full activation of the mTOR-S6K signalling pathway promoting protein synthesis and cell growth. The SLC7A5 (LAT1) catalytic subunit of high-affinity System L1 functions as a glycoprotein-associated heterodimer with the multifunctional protein SLC3A2 (CD98). We generated a floxed Slc7a5 mouse strain which, when crossed with mice expressing Cre driven by a global promoter, produced Slc7a5 heterozygous knockout (Slc7a5+/−) animals with no overt phenotype, although homozygous global knockout of Slc7a5 was embryonically lethal. Muscle-specific (MCK Cre-mediated) Slc7a5 knockout (MS-Slc7a5-KO) mice were used to study the role of intracellular LNAA delivery by the SLC7A5 transporter for mTOR-S6K pathway activation in skeletal muscle. Activation of muscle mTOR-S6K (Thr389 phosphorylation) in vivo by intraperitoneal leucine injection was blunted in homozygous MS-Slc7a5-KO mice relative to wild-type animals. Dietary intake and growth rate were similar for MS-Slc7a5-KO mice and wild-type littermates fed for 10 weeks (to age 120 days) with diets containing 10%, 20% or 30% of protein. In MS-Slc7a5-KO mice, Leu and Ile concentrations in gastrocnemius muscle were reduced by ∼40% as dietary protein content was reduced from 30 to 10%. These changes were associated with >50% decrease in S6K Thr389 phosphorylation in muscles from MS-Slc7a5-KO mice, indicating reduced mTOR-S6K pathway activation, despite no significant differences in lean tissue mass between groups on the same diet. MS-Slc7a5-KO mice on 30% protein diet exhibited mild insulin resistance (e.g. reduced glucose clearance, larger gonadal adipose depots) relative to control animals. Thus, SLC7A5 modulates LNAA-dependent muscle mTOR-S6K signalling in mice, although it appears non-essential (or is sufficiently compensated by e.g. SLC7A8 (LAT2)) for maintenance of normal muscle mass.

Serum response factor: positive and negative regulation of an epithelial gene expression network in the destrin mutant cornea

Increased angiogenesis, inflammation, and proliferation are hallmarks of diseased tissues, and in vivo models of these disease phenotypes can provide insight into disease pathology. Dstn(corn1) mice, deficient for the actin depolymerizing factor destrin (DSTN), display an increase of serum response factor (SRF) that results in epithelial hyperproliferation, inflammation, and neovascularization in the cornea. Previous work demonstrated that conditional ablation of Srf from the corneal epithelium of Dstn(corn1) mice returns the cornea to a wild-type (WT) like state. This result implicated SRF as a major regulator of genes that contributes to abnormal phenotypes in Dstn(corn1) cornea. The purpose of this study is to identify gene networks that are affected by increased expression of Srf in the Dstn(corn1) cornea. Microarray analysis led to characterization of gene expression changes that occur when conditional knockout of Srf rescues mutant phenotypes in the cornea of Dstn(corn1) mice. Comparison of gene expression values from WT, Dstn(corn1) mutant, and Dstn(corn1) rescued cornea identified >400 differentially expressed genes that are downstream from SRF. Srf ablation had a significant effect on genes associated with epithelial cell-cell junctions and regulation of actin dynamics. The majority of genes affected by SRF are downregulated in the Dstn(corn1) mutant cornea, suggesting that increased SRF negatively affects transcription of SRF gene targets. ChIP-seq analysis on Dstn(corn1) mutant and WT tissue revealed that, despite being present in higher abundance, SRF binding is significantly decreased in the Dstn(corn1) mutant cornea. This study uses a unique model combining genetic and genomic approaches to identify genes that are regulated by SRF. These findings expand current understanding of the role of SRF in both normal and abnormal tissue homeostasis.

Serum miR-206 and other muscle-specific microRNAs as non-invasive biomarkers for Duchenne muscular dystrophy

Creatine kinase has been utilized as a diagnostic marker for Duchenne muscular dystrophy (DMD), but it correlates less well with the DMD pathological progression. In this study, we hypothesized that muscle-specific microRNAs (miR-1, -133, and -206) in serum may be useful for monitoring the DMD pathological progression, and explored the possibility of these miRNAs as potential non-invasive biomarkers for the disease. By using real-time quantitative reverse transcription-polymerase chain reaction in a randomized and controlled trial, we detected that miR-1, -133, and -206 were significantly over-expressed in the serum of 39 children with DMD (up to 3.20 ± 1.20, 2(-ΔΔCt) ): almost 2- to 4-fold enriched in comparison to samples from the healthy controls (less than 1.15 ± 0.34, 2(-ΔΔCt) ). To determine whether these miRNAs were related to the clinical features of children with DMD, we analyzed the associations compared to creatine kinase. There were very good inverse correlations between the levels of these miRNAs, especially miR-206, and functional performances: high levels corresponded to low muscle strength, muscle function, and quality of life. Moreover, by receiver operating characteristic curves analyses, we revealed that these miRNAs, especially miR-206, were able to discriminate DMD from controls. Thus, miR-206 and other muscle-specific miRNAs in serum are useful for monitoring the DMD pathological progression, and hence as potential non-invasive biomarkers for the disease. There has been a long-standing need for reliable, non-invasive biomarkers for Duchenne muscular dystrophy (DMD). We found that the levels of muscle-specific microRNAs, especially miR-206, in the serum of DMD were 2- to 4-fold higher than in the controls. High levels corresponded to low muscle strength, muscle function, and quality of life (QoL). These miRNAs were able to discriminate DMD from controls by receiver operating characteristic (ROC) curves analyses. Thus, miR-206 and other muscle-specific miRNAs are useful as non-invasive biomarkers for DMD.

Loss of Pgc-1α expression in aging mouse muscle potentiates glucose intolerance and systemic inflammation

Diabetes risk increases significantly with age and correlates with lower oxidative capacity in muscle. Decreased expression of peroxisome proliferator-activated receptor-γ coactivator-1α (Pgc-1α) and target gene pathways involved in mitochondrial oxidative phosphorylation are associated with muscle insulin resistance, but a causative role has not been established. We sought to determine whether a decline in Pgc-1α and oxidative gene expression occurs during aging and potentiates the development of age-associated insulin resistance. Muscle-specific Pgc-1α knockout (MKO) mice and wild-type littermate controls were aged for 2 yr. Genetic signatures of skeletal muscle (microarray and mRNA expression) and metabolic profiles (glucose homeostasis, mitochondrial metabolism, body composition, lipids, and indirect calorimetry) of mice were compared at 3, 12, and 24 mo of age. Microarray and gene set enrichment analysis highlighted decreased function of the electron transport chain as characteristic of both aging muscle and loss of Pgc-1α expression. Despite significant reductions in oxidative gene expression and succinate dehydrogenase activity, young mice lacking Pgc-1α in muscle had lower fasting glucose and insulin. Consistent with loss of oxidative capacity during aging, Pgc-1α and Pgc-1β expression were reduced in aged wild-type mouse muscle. Interestingly, the combination of age and loss of muscle Pgc-1α expression impaired glucose tolerance and led to increased fat mass, insulin resistance, and inflammatory markers in white adipose and liver tissues. Therefore, loss of Pgc-1α expression and decreased mitochondrial oxidative capacity contribute to worsening glucose tolerance and chronic systemic inflammation associated with aging.

Targeted CNS Delivery Using Human MiniPromoters and Demonstrated Compatibility with Adeno-Associated Viral Vectors

Critical for human gene therapy is the availability of small promoter tools to drive gene expression in a highly specific and reproducible manner. We tackled this challenge by developing human DNA MiniPromoters using computational biology and phylogenetic conservation. MiniPromoters were tested in mouse as single-copy knock-ins at the Hprt locus on the X Chromosome, and evaluated for lacZ reporter expression in CNS and non-CNS tissue. Eighteen novel MiniPromoters driving expression in mouse brain were identified, two MiniPromoters for driving pan-neuronal expression, and 17 MiniPromoters for the mouse eye. Key areas of therapeutic interest were represented in this set: the cerebral cortex, embryonic hypothalamus, spinal cord, bipolar and ganglion cells of the retina, and skeletal muscle. We also demonstrated that three retinal ganglion cell MiniPromoters exhibit similar cell-type specificity when delivered via adeno-associated virus (AAV) vectors intravitreally. We conclude that our methodology and characterization has resulted in desirable expression characteristics that are intrinsic to the MiniPromoter, not dictated by copy number effects or genomic location, and results in constructs predisposed to success in AAV. These MiniPromoters are immediately applicable for pre-clinical studies towards gene therapy in humans, and are publicly available to facilitate basic and clinical research, and human gene therapy.

A novel ortholog of serum response factor (SRF) with immune defense function identified in Crassostrea hongkongensis

Serum response factor (SRF) function is essential for transcriptional regulation of numerous growth-factor-inducible genes and triggers proliferation, differentiation and apoptosis of the cells. In this report, the first mollusk serum response factor like homolog gene (designated ChSRF) was identified and characterized from the Hong Kong oyster, Crassostrea hongkongensis. The full-length cDNA of ChSRF was 1716 bp in length and encodes a putative protein of 434 amino acids respectively, and shares the MADS domain at the N-terminal. ChSRF is ubiquitously expressed in various tissues, with the highest expression level observed in muscle. Temporal expression of ChSRF following microbe infection shows that the expression of ChSRF in hemocytes increases from 3 to 24 h post-challenge. As a target gene of SRF, β-actin demonstrates a similar gene expression mode in constitutive tissue and pathogen infection. Furthermore, some protein profiles of ChSRF was revealed, fluorescence microscopy results show that ChSRF located in the nuclei of HeLa cells and over-expression of ChSRF activated the transcriptional activities of MAPK signal pathway in HEK293T cells. These results indicate that ChSRF maybe play an important role in signal transduction in the immunity and development response of oysters. Furthermore, we found that ChSRF could regulate the expression of β-actin gene, which indicate that ChSRF is a muscle differentiation regulator in the oyster and it will help us to improve aquaculture production.

Molecular and cellular regulation of skeletal myogenesis

Since the seminal discovery of the cell-fate regulator Myod, studies in skeletal myogenesis have inspired the search for cell-fate regulators of similar potential in other tissues and organs. It was perplexing that a similar transcription factor for other tissues was not found; however, it was later discovered that combinations of molecular regulators can divert somatic cell fates to other cell types. With the new era of reprogramming to induce pluripotent cells, the myogenesis paradigm can now be viewed under a different light. Here, we provide a short historical perspective and focus on how the regulation of skeletal myogenesis occurs distinctly in different scenarios and anatomical locations. In addition, some interesting features of this tissue underscore the importance of reconsidering the simple-minded view that a single stem cell population emerges after gastrulation to assure tissuegenesis. Notably, a self-renewing long-term Pax7+ myogenic stem cell population emerges during development only after a first wave of terminal differentiation occurs to establish a tissue anlagen in the mouse. How the future stem cell population is selected in this unusual scenario will be discussed. Recently, a wealth of information has emerged from epigenetic and genome-wide studies in myogenic cells. Although key transcription factors such as Pax3, Pax7, and Myod regulate only a small subset of genes, in some cases their genomic distribution and binding are considerably more promiscuous. This apparent nonspecificity can be reconciled in part by the permissivity of the cell for myogenic commitment, and also by new roles for some of these regulators as pioneer transcription factors acting on chromatin state.

Cytoglobin modulates myogenic progenitor cell viability and muscle regeneration

Mammalian skeletal muscle can remodel, repair, and regenerate itself by mobilizing satellite cells, a resident population of myogenic progenitor cells. Muscle injury and subsequent activation of myogenic progenitor cells is associated with oxidative stress. Cytoglobin is a hemoprotein expressed in response to oxidative stress in a variety of tissues, including striated muscle. In this study, we demonstrate that cytoglobin is up-regulated in activated myogenic progenitor cells, where it localizes to the nucleus and contributes to cell viability. siRNA-mediated depletion of cytoglobin from C2C12 myoblasts increased levels of reactive oxygen species and apoptotic cell death both at baseline and in response to stress stimuli. Conversely, overexpression of cytoglobin reduced reactive oxygen species levels, caspase activity, and cell death. Mice in which cytoglobin was knocked out specifically in skeletal muscle were generated to examine the role of cytoglobin in vivo. Myogenic progenitor cells isolated from these mice were severely deficient in their ability to form myotubes as compared with myogenic progenitor cells from wild-type littermates. Consistent with this finding, the capacity for muscle regeneration was severely impaired in mice deficient for skeletal-muscle cytoglobin. Collectively, these data demonstrate that cytoglobin serves an important role in muscle repair and regeneration.

Compound loss of muscleblind-like function in myotonic dystrophy

Myotonic dystrophy (DM) is a multi-systemic disease that impacts cardiac and skeletal muscle as well as the central nervous system (CNS). DM is unusual because it is an RNA-mediated disorder due to the expression of toxic microsatellite expansion RNAs that alter the activities of RNA processing factors, including the muscleblind-like (MBNL) proteins. While these mutant RNAs inhibit MBNL1 splicing activity in heart and skeletal muscles, Mbnl1 knockout mice fail to recapitulate the full-range of DM symptoms in these tissues. Here, we generate mouse Mbnl compound knockouts to test the hypothesis that Mbnl2 functionally compensates for Mbnl1 loss. Although Mbnl1^−/−; Mbnl2^−/− double knockouts (DKOs) are embryonic lethal, Mbnl1^−/−; Mbnl2^+/− mice are viable but develop cardinal features of DM muscle disease including reduced lifespan, heart conduction block, severe myotonia and progressive skeletal muscle weakness. Mbnl2 protein levels are elevated in Mbnl1^−/− knockouts where Mbnl2 targets Mbnl1-regulated exons. These findings support the hypothesis that compound loss of MBNL function is a critical event in DM pathogenesis and provide novel mouse models to investigate additional pathways disrupted in this RNA-mediated disease.

Skeletal muscle triacylglycerol hydrolysis does not influence metabolic complications of obesity

Intramyocellular triacylglycerol (IMTG) accumulation is highly associated with insulin resistance and metabolic complications of obesity (lipotoxicity), whereas comparable IMTG accumulation in endurance-trained athletes is associated with insulin sensitivity (the athlete's paradox). Despite these findings, it remains unclear whether changes in IMTG accumulation and metabolism per se influence muscle-specific and systemic metabolic homeostasis and insulin responsiveness. By mediating the rate-limiting step in triacylglycerol hydrolysis, adipose triglyceride lipase (ATGL) has been proposed to influence the storage/production of deleterious as well as essential lipid metabolites. However, the physiological relevance of ATGL-mediated triacylglycerol hydrolysis in skeletal muscle remains unknown. To determine the contribution of IMTG hydrolysis to tissue-specific and systemic metabolic phenotypes in the context of obesity, we generated mice with targeted deletion or transgenic overexpression of ATGL exclusively in skeletal muscle. Despite dramatic changes in IMTG content on both chow and high-fat diets, modulation of ATGL-mediated IMTG hydrolysis did not significantly influence systemic energy, lipid, or glucose homeostasis, nor did it influence insulin responsiveness or mitochondrial function. These data argue against a role for altered IMTG accumulation and lipolysis in muscle insulin resistance and metabolic complications of obesity.

De novo expression of connexin hemichannels in denervated fast skeletal muscles leads to atrophy

Denervation of skeletal muscles induces atrophy, preceded by changes in sarcolemma permeability of causes not yet completely understood. Here, we show that denervation-induced Evans blue dye uptake in vivo of fast, but not slow, myofibers was acutely inhibited by connexin (Cx) hemichannel/pannexin1 (Panx1) channel and purinergic ionotropic P2X7 receptor (P2X7R) blockers. Denervated myofibers showed up-regulation of Panx1 and de novo expression of Cx39, Cx43, and Cx45 hemichannels as well as P2X7Rs and transient receptor potential subfamily V, member 2, channels, all of which are permeable to small molecules. The sarcolemma of freshly isolated WT myofibers from denervated muscles also showed high hemichannel-mediated permeability that was slightly reduced by blockade of Panx1 channels or the lack of Panx1 expression, but was completely inhibited by Cx hemichannel or P2X7R blockers, as well as by degradation of extracellular ATP. However, inhibition of transient receptor potential subfamily V, member 2, channels had no significant effect on membrane permeability. Moreover, activation of the transcription factor NFκB and higher mRNA levels of proinflammatory cytokines (TNF-α and IL-1β) were found in denervated WT but not Cx43/Cx45-deficient muscles. The atrophy observed after 7 d of denervation was drastically reduced in Cx43/Cx45-deficient but not Panx1-deficient muscles. Therefore, expression of Cx hemichannels and P2X7R promotes a feed-forward mechanism activated by extracellular ATP, most likely released through hemichannels, that activates the inflammasome. Consequently, Cx hemichannels are potential targets for new therapeutic agents to prevent or reduce muscle atrophy induced by denervation of diverse etiologies.

An intragenic SRF-dependent regulatory motif directs cardiac-specific microRNA-1-1/133a-2 expression

Transcriptional regulation is essential for any gene expression including microRNA expression. MiR-1-1 and miR-133a-2 are essential microRNAs (miRs) involved in cardiac and skeletal muscle development and diseases. Early studies reveal two regulatory enhancers, an upstream and an intragenic, that direct the miR-1-1 and miR-133a-2 transcripts. In this study, we identify a unique serum response factor (SRF) binding motif within the enhancer through bioinformatic approaches. This motif is evolutionarily conserved and is present in a range of organisms from yeast, flies, to humans. We provide evidence to demonstrate that this regulatory motif is SRF-dependent in vitro by electrophoretic mobility shift assay, luciferase activity assay, and endogenous chromatin immunoprecipitation assay followed by DNA sequence confirmation, and in vivo by transgenic lacZ reporter mouse studies. Importantly, our transgenic mice indicate that this motif is indispensable for the expression of miR1-1/133a-2 in the heart, but not necessary in skeletal muscle, while the enhancer is sufficient for miR1-1/133a-2 gene expression in both tissues. The mutation of the motif alone completely abolishes miR-1-1/133a-2 gene expression in the animal heart, but not in the skeletal muscle. Our findings reveal an additional architecture of regulatory complex directing miR-1-1/133a-1 gene expression, and demonstrate how this intragenic enhancer differentially manages the expression of the two miRs in the heart and skeletal muscle, respectively.

Recurrent NCOA2 gene rearrangements in congenital/infantile spindle cell rhabdomyosarcoma

Spindle cell rhabdomyosarcoma (RMS) is a rare form of RMS with different clinical characteristics between children and adult patients. Its genetic hallmark remains unknown and it remains debatable if there is pathogenetic relationship between the spindle cell and the so-called sclerosing RMS. We studied two pediatric and one adult spindle cell RMS by next generation RNA sequencing and FusionSeq data analysis to detect novel fusions. An SRF-NCOA2 fusion was detected in a spindle cell RMS from the posterior neck in a 7-month-old child. The fusion matched the tumor karyotype and was confirmed by FISH and RT-PCR, which showed fusion of SRF exon 6 to NCOA2 exon 12. Additional 14 spindle cell (from 8 children and 6 adults) and 4 sclerosing (from 2 children and 2 adults) RMS were tested by FISH for the presence of abnormalities in NCOA2, SRF, as well as for PAX3 and NCOA1. NCOA2 rearrangements were found in two additional spindle cell RMS from a 3-month-old and a 4-week-old child. In the latter tumor, TEAD1 was identified by rapid amplification of cDNA ends (RACE) to be the NCOA2 gene fusion partner. None of the adult tumors were positive for NCOA2 rearrangement. Despite similar histomorphology in adults and young children, these results suggest that spindle cell RMS is a heterogeneous disease genetically as well as clinically. Our findings also support a relationship between NCOA2-rearranged spindle cell RMS occurring in young childhood and the so-called congenital RMS, which often displays rearrangements at 8q13 locus (NCOA2).

Disconnecting mitochondrial content from respiratory chain capacity in PGC-1-deficient skeletal muscle

The transcriptional coactivators PGC-1α and PGC-1β are widely thought to be required for mitochondrial biogenesis and fiber typing in skeletal muscle. Here, we show that mice lacking both PGC-1s in myocytes do indeed have profoundly deficient mitochondrial respiration but, surprisingly, have preserved mitochondrial content, isolated muscle contraction capacity, fiber-type composition, in-cage ambulation, and voluntary running capacity. Most of these findings are recapitulated in cell culture and, thus, are cell autonomous. Functional electron microscopy reveals normal cristae density with decreased cytochrome oxidase activity. These data lead to the following surprising conclusions: (1) PGC-1s are in fact dispensable for baseline muscle function, mitochondrial content, and fiber typing, (2) endurance fatigue at low workloads is not limited by muscle mitochondrial capacity, and (3) mitochondrial content and cristae density can be dissociated from respiratory capacity.

Mechanisms regulating skeletal muscle growth and atrophy

Skeletal muscle mass increases during postnatal development through a process of hypertrophy, i.e. enlargement of individual muscle fibers, and a similar process may be induced in adult skeletal muscle in response to contractile activity, such as strength exercise, and specific hormones, such as androgens and β-adrenergic agonists. Muscle hypertrophy occurs when the overall rates of protein synthesis exceed the rates of protein degradation. Two major signaling pathways control protein synthesis, the IGF1-Akt-mTOR pathway, acting as a positive regulator, and the myostatin-Smad2/3 pathway, acting as a negative regulator, and additional pathways have recently been identified. Proliferation and fusion of satellite cells, leading to an increase in the number of myonuclei, may also contribute to muscle growth during early but not late stages of postnatal development and in some forms of muscle hypertrophy in the adult. Muscle atrophy occurs when protein degradation rates exceed protein synthesis, and may be induced in adult skeletal muscle in a variety of conditions, including starvation, denervation, cancer cachexia, heart failure and aging. Two major protein degradation pathways, the proteasomal and the autophagic-lysosomal pathways, are activated during muscle atrophy and variably contribute to the loss of muscle mass. These pathways involve a variety of atrophy-related genes or atrogenes, which are controlled by specific transcription factors, such as FoxO3, which is negatively regulated by Akt, and NF-κB, which is activated by inflammatory cytokines.

A PGC-1α isoform induced by resistance training regulates skeletal muscle hypertrophy

PGC-1α is a transcriptional coactivator induced by exercise that gives muscle many of the best known adaptations to endurance-type exercise but has no effects on muscle strength or hypertrophy. We have identified a form of PGC-1α (PGC-1α4) that results from alternative promoter usage and splicing of the primary transcript. PGC-1α4 is highly expressed in exercised muscle but does not regulate most known PGC-1α targets such as the mitochondrial OXPHOS genes. Rather, it specifically induces IGF1 and represses myostatin, and expression of PGC-1α4 in vitro and in vivo induces robust skeletal muscle hypertrophy. Importantly, mice with skeletal muscle-specific transgenic expression of PGC-1α4 show increased muscle mass and strength and dramatic resistance to the muscle wasting of cancer cachexia. Expression of PGC-1α4 is preferentially induced in mouse and human muscle during resistance exercise. These studies identify a PGC-1α protein that regulates and coordinates factors involved in skeletal muscle hypertrophy.

Neuregulin1 signaling targets SRF and CREB and activates the muscle spindle-specific gene Egr3 through a composite SRF-CREB-binding site

Muscle spindles are sensory receptors embedded within muscle that detect changes in muscle length. Each spindle is composed of specialized muscle fibers, known as intrafusal muscle fibers, along with the endings of axons from sensory neurons that innervate these muscle fibers. Formation of muscle spindles requires neuregulin1 (NRG1), which is released by sensory axons, activating ErbB receptors in muscle cells that are contacted. In muscle cells, the transcription factor Egr3 is transcriptionally induced by NRG1, which in turn activates various target genes involved in forming the intrafusal fibers of muscle spindles. The signaling relay within the NRG1-ErbB pathway that acts to induce Egr3 is presumably critical for muscle spindle formation but for the most part has not been determined. In the current studies, we examined, using cultured muscle cells, transcriptional regulatory mechanisms by which Egr3 responds to NRG1. We identified a composite regulatory element for the Egr3 gene, consisting adjacent sites that bind cAMP response element binding protein (CREB) and serum response factor (SRF), with a role in NRG1 responsiveness. The SRF element also influences Egr3 basal expression in unstimulated myotubes, and in the absence of the SRF element, the CREB element influences basal expression. We show that NRG1 signaling, to target SRF, acts on the SRF coactivators myocardian-related transcription factor (MRTF)-A and MRTF-B, which are known to activate SRF-mediated transcription, by stimulating their translocation from the cytoplasm to the nucleus. CREB is phosphorylated, which is known to contribute to its activation, in response to NRG1. These results suggest that NRG1 induces expression of the muscle spindle-specific gene Egr3 by stimulating the transcriptional activity of CREB and SRF.

Development of the mammalian axial skeleton requires signaling through the Gα(i) subfamily of heterotrimeric G proteins

129/SvEv mice with a loss-of-function mutation in the heterotrimeric G protein α-subunit gene Gnai3 have fusions of ribs and lumbar vertebrae, indicating a requirement for Gα(i) (the "inhibitory" class of α-subunits) in somite derivatives. Mice with mutations of Gnai1 or Gnai2 have neither defect, but loss of both Gnai3 and one of the other two genes increases the number and severity of rib fusions without affecting the lumbar fusions. No myotome defects are observed in Gnai3/Gnai1 double-mutant embryos, and crosses with a conditional allele of Gnai2 indicate that Gα(i) is specifically required in cartilage precursors. Penetrance and expressivity of the rib fusion phenotype is altered in mice with a mixed C57BL/6 × 129/SvEv genetic background. These phenotypes reveal a previously unknown role for G protein-coupled signaling pathways in development of the axial skeleton.

Ezh2 maintains a key phase of muscle satellite cell expansion but does not regulate terminal differentiation

Tissue generation and repair requires a stepwise process of cell fate restriction to ensure that adult stem cells differentiate in a timely and appropriate manner. A crucial role has been implicated for Polycomb-group (PcG) proteins and the H3K27me3 repressive histone mark in coordinating the transcriptional programmes necessary for this process, but the targets and developmental timing for this repression remain unclear. To address these questions, we generated novel genome-wide maps of H3K27me3 and H3K4me3 in freshly isolated muscle stem cells. These data, together with the analysis of two conditional Ezh2-null mouse strains, identified a critical proliferation phase in which Ezh2 activity is essential. Mice lacking Ezh2 in satellite cells exhibited decreased muscle growth, severely impaired regeneration and reduced stem cell number, due to a profound failure of the proliferative progenitor population to expand. Surprisingly, deletion of Ezh2 after the onset of terminal differentiation did not impede muscle repair or homeostasis. Using these knockout models and the RNA-Seq and ChIP-Seq datasets, we show that Ezh2 does not regulate the muscle differentiation process in vivo. These results emphasise the lineage and cell-type-specific functions of Ezh2 and Polycomb repressive complex 2.

Regulation of mitochondrial oxidative metabolism by tumor suppressor FLCN

BACKGROUND

Birt-Hogg-Dubé (BHD) syndrome is a hereditary hamartoma syndrome that predisposes patients to develop hair follicle tumors, lung cysts, and kidney cancer. Genetic studies of BHD patients have uncovered the causative gene, FLCN, but its function is incompletely understood.

METHODS

Mice with conditional alleles of FLCN and/or peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PPARGC1A), a transcriptional coactivator that regulates mitochondrial biogenesis, were crossbred with mice harboring either muscle creatine kinase (CKM) -Cre or myogenin (MYOG) -Cre transgenes to knock out FLCN and/or PPARGC1A in muscle, or cadherin 16 (CDH16)- Cre transgenes to knock out FLCN and/or PPARGC1A in kidney. Real-time polymerase chain reaction, immunoblotting, electron microscopy, and metabolic profiling assay were performed to evaluate mitochondrial biogenesis and function in muscle. Immunoblotting, electron microscopy, and histological analysis were used to investigate expression and the pathological role of PPARGC1A in FLCN-deficient kidney. Real-time polymerase chain reaction, oxygen consumption measurement, and flow cytometry were carried out using a FLCN-null kidney cancer cell line. All statistical analyses were two-sided.

RESULTS

Muscle-targeted FLCN knockout mice underwent a pronounced metabolic shift toward oxidative phosphorylation, including increased mitochondrial biogenesis (FLCN ( f/f ) vs FLCN ( f/f ) /CKM-Cre: % mitochondrial area mean = 7.8% vs 17.8%; difference = 10.0%; 95% confidence interval = 5.7% to 14.3%; P < .001), and the observed increase in mitochondrial biogenesis was PPARGC1A dependent. Reconstitution of FLCN-null kidney cancer cells with wild-type FLCN suppressed mitochondrial metabolism and PPARGC1A expression. Kidney-targeted PPARGC1A inactivation partially rescued the enlarged kidney phenotype and abrogated the hyperplastic cells observed in the FLCN-deficient kidney.

CONCLUSION

FLCN deficiency and subsequent increased PPARGC1A expression result in increased mitochondrial function and oxidative metabolism as the source of cellular energy, which may give FLCN-null kidney cells a growth advantage and drive hyperplastic transformation.

The ubiquitin ligase Nedd4-1 participates in denervation-induced skeletal muscle atrophy in mice

Skeletal muscle atrophy is a consequence of muscle inactivity resulting from denervation, unloading and immobility. It accompanies many chronic disease states and also occurs as a pathophysiologic consequence of normal aging. In all these conditions, ubiquitin-dependent proteolysis is a key regulator of the loss of muscle mass, and ubiquitin ligases confer specificity to this process by interacting with, and linking ubiquitin moieties to target substrates through protein∶protein interaction domains. Our previous work suggested that the ubiquitin-protein ligase Nedd4-1 is a potential mediator of skeletal muscle atrophy associated with inactivity (denervation, unloading and immobility). Here we generated a novel tool, the Nedd4-1 skeletal muscle-specific knockout mouse (myo^Cre;Nedd4-1^flox/flox) and subjected it to a well validated model of denervation induced skeletal muscle atrophy. The absence of Nedd4-1 resulted in increased weights and cross-sectional area of type II fast twitch fibres of denervated gastrocnemius muscle compared with wild type littermates controls, at seven and fourteen days following tibial nerve transection. These effects are not mediated by the Nedd4-1 substrates MTMR4, FGFR1 and Notch-1. These results demonstrate that Nedd4-1 plays an important role in mediating denervation-induced skeletal muscle atrophy in vivo.

A mouse model of rhabdomyosarcoma originating from the adipocyte lineage

Rhabdomyosarcoma (RMS) is an aggressive skeletal muscle-lineage tumor composed of malignant myoblasts that fail to exit the cell cycle and are blocked from fusing into syncytial muscle. Rhabdomyosarcoma includes two histolopathologic subtypes: alveolar rhabdomyosarcoma, driven by the fusion protein PAX3-FOXO1 or PAX7-FOXO1, and embryonal rhabdomyosarcoma (ERMS), which is genetically heterogeneous. Here, we show that adipocyte-restricted activation of Sonic hedgehog signaling through expression of a constitutively active Smoothened allele in mice gives rise to aggressive skeletal muscle tumors that display the histologic and molecular characteristics of human ERMS with high penetrance. Our findings suggest that adipocyte progenitors can be a cell of origin for Sonic hedgehog-driven ERMS, showing that RMS can originate from nonskeletal muscle precursors.

In vitro culture and induced differentiation of sheep skeletal muscle satellite cells

Skeletal muscle satellite cells are adult muscle-derived stem cells receiving increasing attention. Sheep satellite cells have a greater similarity to human satellite cells with regard to metabolism, life span, proliferation and differentiation, than satellite cells of the rat and mouse. We have used 2-step enzymatic digestion and differential adhesion methods to isolate and purify sheep skeletal muscle satellite cells, identified the cells and induced differentiation to examine their pluripotency. The most efficient method for the isolation of sheep skeletal muscle satellite cells was the type I collagenase and trypsin 2-step digestion method, with the best conditions for in vitro culture being in medium containing 20% FBS+10% horse serum. Immunofluorescence staining showed that satellite cells expressed Desmin, α-Sarcomeric Actinin, MyoD1, Myf5 and PAX7. After myogenic induction, multinucleated myotubes formed, as indicated by the expression of MyoG and fast muscle myosin. After osteogenic induction, cells expressed Osteocalcin, with Alizarin Red and ALP (alkaline phosphatase) staining results both being positive. After adipogenic induction, cells expressed PPARγ2 (peroxisome-proliferator-activated receptor γ2) and clear lipid droplets were present around the cells, with Oil Red-O staining giving a positive result. In summary, a successful system has been established for the isolation, purification and identification of sheep skeletal muscle satellite cells.

Serum response factor regulates bone formation via IGF-1 and Runx2 signals

Serum response factor (SRF) plays vital roles in numerous cellular processes; however, the physiological function of SRF in skeletal tissue remains unknown. In several organ systems, SRF regulates the expression of insulin-like growth factor-1 (IGF-1), which is crucial for normal development of mineralized skeleton and bone remodeling throughout life. Here, we show that conditional deletion of SRF in osteoblasts by osteocalcin-Cre generated viable mice with normal body size and body weight. Compared with normal siblings, osteoblast-specific SRF-deficient adult mice exhibited a marked decrease in bone mineral density and bone formation rate. Deletion of SRF in primary mouse calvarial osteoblasts reduced cell differentiation and mineralization in vitro. This was accompanied by a decrease in IGF-1 expression and secretion. Addition of IGF-1 in the culture media enhanced osteoblast differentiation in control cells and partially restored the mineralization defect of SRF-deficient cells, supporting an important role of SRF in regulating IGF-1 and IGF-1-mediated osteoblast differentiation. IGF-1-induced Akt activation was inhibited in SRF-deficient calvarial cells and enhanced in the SRF overexpressed cells. In addition, SRF deficiency decreased the transcriptional activity of Runx2, the key transcription factor for osteogenesis. Overexpression of SRF induced Runx2 transactivity in control cells and restored Runx2 transactivity in the SRF-deficient cells. Taken together, we conclude that SRF is important for IGF-1-induced osteoblast differentiation and mineralization via regulating IGF-1 expression and Runx2 transactivity.

PGC-1α is dispensable for exercise-induced mitochondrial biogenesis in skeletal muscle

Exercise confers numerous health benefits, many of which are thought to stem from exercise-induced mitochondrial biogenesis (EIMB) in skeletal muscle. The transcriptional coactivator PGC-1α, a potent regulator of metabolism in numerous tissues, is widely believed to be required for EIMB. We show here that this is not the case. Mice engineered to lack PGC-1α specifically in skeletal muscle (Myo-PGC-1αKO mice) retained intact EIMB. The exercise capacity of these mice was comparable to littermate controls. Induction of metabolic genes after 2 weeks of in-cage voluntary wheel running was intact. Electron microscopy revealed no gross abnormalities in mitochondria, and the mitochondrial biogenic response to endurance exercise was as robust in Myo-PGC-1αKO mice as in wildtype mice. The induction of enzymatic activity of the electron transport chain by exercise was likewise unperturbed in Myo-PGC-1αKO mice. These data demonstrate that PGC-1α is dispensable for exercise-induced mitochondrial biogenesis in skeletal muscle, in sharp contrast to the prevalent assumption in the field.

STARS is essential to maintain cardiac development and function in vivo via a SRF pathway

Background

STARS (STriated muscle Activator of Rho Signaling) is a sarcomeric protein expressed early in cardiac development that acts as an acute stress sensor for pathological remodeling. However the role of STARS in cardiac development and function is incompletely understood. Here, we investigated the role of STARS in heart development and function in the zebrafish model and in vitro.

Methodology and Principal Findings

Expression of zebrafish STARS (zSTARS) first occurs in the somites by the 16 somite stage [17 hours post fertilization (hpf)]. zSTARS is expressed in both chambers of the heart by 48 hpf, and also in the developing brain, jaw structures and pectoral fins. Morpholino-induced knockdown of zSTARS alters atrial and ventricular dimensions and decreases ventricular fractional shortening (measured by high-speed video microscopy), with pericardial edema and decreased or absent circulation [abnormal cardiac phenotypes in 126/164 (77%) of morpholino-injected embryos vs. 0/152 (0%) of control morpholino embryos]. Co-injection of zsrf (serum response factor) mRNA rescues the cardiac phenotype of zSTARS knockdown, resulting in improved fractional shortening and ventricular end-diastolic dimensions. Ectopic over-expression of STARS in vitro activates the STARS proximal promoter, which contains a conserved SRF site. Chromatin immunoprecipitation demonstrates that SRF binds to this site in vivo and the SRF inhibitor CCG-1423 completely blocks STARS proximal reporter activity in H9c2 cells.

Conclusions/Significance

This study demonstrates for the first time that STARS deficiency severely disrupts cardiac development and function in vivo and revealed a novel STARS-SRF feed-forward autoregulatory loop that could play an essential role in STARS regulation and cardiac function.

Defective mitochondrial morphology and bioenergetic function in mice lacking the transcription factor Yin Yang 1 in skeletal muscle

The formation, distribution, and maintenance of functional mitochondria are achieved through dynamic processes that depend strictly on the transcription of nuclear genes encoding mitochondrial proteins. A large number of these mitochondrial genes contain binding sites for the transcription factor Yin Yang 1 (YY1) in their proximal promoters, but the physiological relevance is unknown. We report here that skeletal-muscle-specific YY1 knockout (YY1mKO) mice have severely defective mitochondrial morphology and oxidative function associated with exercise intolerance, signs of mitochondrial myopathy, and short stature. Gene set enrichment analysis (GSEA) revealed that the top pathways downregulated in YY1mKO mice were assigned to key metabolic and regulatory mitochondrial genes. This analysis was consistent with a profound decrease in the level of mitochondrial proteins and oxidative phosphorylation (OXPHOS) bioenergetic function in these mice. In contrast to the finding for wild-type mice, inactivation of the mammalian target of rapamycin (mTOR) did not suppress mitochondrial genes in YY1mKO mice. Mechanistically, mTOR-dependent phosphorylation of YY1 resulted in a strong interaction between YY1 and the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator 1α (PGC1α), a major regulator of mitochondrial function. These results underscore the important role of YY1 in the maintenance of mitochondrial function and explain how its inactivation might contribute to exercise intolerance and mitochondrial myopathies.

STIM1-Ca(2+) signaling is required for the hypertrophic growth of skeletal muscle in mice

Immediately after birth, skeletal muscle must undergo an enormous period of growth and differentiation that is coordinated by several intertwined growth signaling pathways. How these pathways are integrated remains unclear but is likely to involve skeletal muscle contractile activity and calcium (Ca(2+)) signaling. Here, we show that Ca(2+) signaling governed by stromal interaction molecule 1 (STIM1) plays a central role in the integration of signaling and, therefore, muscle growth and differentiation. Conditional deletion of STIM1 from the skeletal muscle of mice (mSTIM1(-/-) mice) leads to profound growth delay, reduced myonuclear proliferation, and perinatal lethality. We show that muscle fibers of neonatal mSTIM1(-/-) mice cannot support the activity-dependent Ca(2+) transients evoked by tonic neurostimulation, even though excitation contraction coupling (ECC) remains unperturbed. In addition, disruption of tonic Ca(2+) signaling in muscle fibers attenuates downstream muscle growth signaling, such as that of calcineurin, mitogen-activated protein (MAP) kinases, extracellular signal-regulated kinase 1 and 2 (ERK1/2), and AKT. Based on our findings, we propose a model wherein STIM1-mediated store-operated calcium entry (SOCE) governs the Ca(2+) signaling required for cellular processes that are necessary for neonatal muscle growth and differentiation.

Repression of cardiac hypertrophy by KLF15: underlying mechanisms and therapeutic implications

The Kruppel-like factor (KLF) family of transcription factors regulates diverse cell biological processes including proliferation, differentiation, survival and growth. Previous studies have shown that KLF15 inhibits cardiac hypertrophy by repressing the activity of pivotal cardiac transcription factors such as GATA4, MEF2 and myocardin. We set out this study to characterize the interaction of KLF15 with putative other transcription factors. We first show that KLF15 interacts with myocardin-related transcription factors (MRTFs) and strongly represses the transcriptional activity of MRTF-A and MRTF-B. Second, we identified a region within the C-terminal zinc fingers of KLF15 that contains the nuclear localization signal. Third, we investigated whether overexpression of KLF15 in the heart would have therapeutic potential. Using recombinant adeno-associated viruses (rAAV) we have overexpressed KLF15 specifically in the mouse heart and provide the first evidence that elevation of cardiac KLF15 levels prevents the development of cardiac hypertrophy in a model of Angiotensin II induced hypertrophy.