Highly coordinated gene regulation in mouse skeletal muscle regeneration

Vascular endothelial growth factor stimulates skeletal muscle regeneration in vivo

Vascular endothelial growth factor (VEGF) is a major regulator of blood vessel formation during development and in the adult organism. Recent evidence indicates that this factor also plays an important role in sustaining the proliferation and differentiation of different cell types, including progenitor cells of different tissues, including bone marrow, bone, and the central nervous system. Here we show that the delivery of the 165-aa isoform of VEGF-A cDNA using an adeno-associated virus (AAV) vector exerts a powerful effect on skeletal muscle regeneration in vivo. Following ischemia-, glycerol-, or cardiotoxin-induced damage in mouse skeletal muscle, the delivery of AAV-VEGF markedly improved muscle fiber reconstitution with a dose-dependent effect. The expression of both VEGF receptor-1 (VEGFR-1) and VEGFR-2 was upregulated both in the satellite cells of the damaged muscles and during myotube formation in vitro; the VEGF effect was mediated by the VEGFR-2, since the transfer of PlGF, a VEGF family member interacting with the VEGFR-1, was ineffective. These results are consistent with the observation that VEGF promotes the growth of myogenic fibers and protects the myogenic cells from apoptosis in vitro and prompt a therapeutic use for VEGF gene transfer in a variety of muscular disorders.

Dual promoter structure of ZFP106: regulation by myogenin and nuclear respiratory factor-1

The WD40 repeats containing zinc finger protein 106 (ZFP106) is a conserved mammalian protein of unknown function. However, its cDNA shares an extended region of identity with the scr homology domain 3 binding protein 3 (Sh3bp3) cDNA encoding a protein implicated in the insulin signaling pathway. Asking, whether Zfp106 and Sh3bp3 are products of the same gene, we characterized the structures and transcriptional regulation of Zfp106 and its human homologue, ZFP106. A TATA-less, CpG island associated promoter (P1), was mapped by 5'-RACE to a region 19 kb upstream of the ZFP106 translation start site. P1 is active throughout development and at low levels in all adult tissues examined. A conserved cis-element in the proximal P1 region showed specific binding to nuclear respiratory factor-1 (NRF-1). Mutagenesis of this site and transfection of a dominant-negative NRF-1 both revealed the crucial role of NRF-1 in activation of P1. The broad tissue expression of P1 was in contrast to the high level of ZFP106 mRNA observed in striated muscle. This prompted additional 5'-RACE experiments that established a second, TATA box-containing promoter (P2) upstream of the third coding exon. P1 and P2 transcripts encode proteins with distinct N-terminal sequences, with Sh3bp3 corresponding to a rare, alternatively spliced P2 transcript. P2 initiated transcripts are specifically expressed in striated muscle and their level is strongly upregulated during myogenic, but not adipogenic differentiation. By deletion analysis, the region between nucleotides -296 to +96 was sufficient for robust P2 responsiveness to myogenic differentiation. This response is mediated by the additive effect of binding of myogenin to three critical E boxes within this region. In addition, transcriptional enhancer factor-1 family factors contribute to both basal and myogenesis induced P2 activity. In situ hybridization of mouse embryos confirmed predominant expression of Zfp106 in tissues with high developmental expression of either NRF-1 (brown fat and developing brain) or myogenin (striated muscle). Our results suggest distinct roles of tissue-specific ZFP106 isoforms in growth related metabolism and provide the foundation for further studies into the regulation and function of ZFP106.

Antisense oligonucleotides and short interfering RNAs silencing the cyclin-dependent kinase inhibitor p21 improve proliferation of Duchenne muscular dystrophy patients' primary skeletal myoblasts

Increased levels of the cyclin-dependent kinase inhibitor p21 associated with decreased myoblast proliferation may be involved in the dystrophic process in Duchenne muscular dystrophy (DMD). Therefore we are interested to improve the proliferation of primary myoblasts of DMD patients by a reduction in p21 using either antisense oligonucleotides (ASO) or short interfering RNAs (siRNA). After transient transfection of myoblasts in cell culture proliferation was analyzed using a 5-bromo-2'-deoxyuridine assay comparing specific transfected cells with untransfected cells and cells transfected with scrambled ASO and luciferase siRNA, respectively. Four of five Dystrophin-deficient (Dys(-)) cell culture samples revealed an increase in proliferation between 7% and 18% compared to untransfected cells and between 8% and 36% compared to cells transfected with scrambled ASO. Transfection with siRNA was performed for selected samples to determine whether siRNA is more effective in gene silencing than ASO. The increase in proliferation using luciferase siRNA as reference was comparable to or less than ASO data using scrambled ASO as reference. Using untransfected cells as reference, the increase in proliferation was higher for siRNA than ASO (20-47% vs. 7-18%), but the data must be carefully interpreted with respect to nonspecific effects on gene expression by siRNA. Our findings of transient p21 gene silencing represent a basis for viral vector-mediated drug-inducible p21 shRNA expression in Dys(-) myoblasts which might enhance, prolong and regulate the proliferation effect.

Skeletal muscle adaptation in response to voluntary running in Ca2+/calmodulin-dependent protein kinase IV-deficient mice

Mammalian skeletal muscles undergo adaptation in response to alteration in functional demands by means of a variety of cellular signaling events. Previous experiments in transgenic mice showed that an active form of Ca2+/calmodulin-dependent protein kinase IV (CaMKIV) is capable of stimulating peroxisome proliferator-activated receptor γ-coactivator 1α (PGC-1α) gene expression, promoting fast-to-slow fiber type switching and augmenting mitochondrial biogenesis in skeletal muscle. However, a role for endogenous CaMKIV in skeletal muscle has not been investigated rigorously. We report that genetically modified mice devoid of CaMKIV have normal fiber type composition and mitochondrial enzyme expression in fast-twitch skeletal muscles and responded to long-term (4 wk) voluntary running with increased expression of myosin heavy chain type IIa, myoglobin, PGC-1α, and cytochrome c oxidase IV proteins in plantaris muscle in a manner similar to that of wild-type mice. Short-term motor nerve stimulation (2 h at 10 Hz) likewise increased PGC-1α mRNA expression in tibialis anterior muscles in both Camk4−/−and wild-type mice. In addition, we have confirmed that no detectable CaMKIV protein is expressed in murine skeletal muscle. Thus CaMKIV is not required for the maintenance of slow-twitch muscle phenotype and endurance training-induced mitochondrial biogenesis and IIb-to-IIa fiber type switching in murine skeletal muscle. Other protein kinases sharing substrates with constitutively active CaMKIV may function as endogenous mediators of activity-dependent changes in myofiber phenotype.

Large-scale gene expression analysis of human skeletal myoblast differentiation

To study pathways involved in human skeletal myogenesis, we profiled gene expression in human primary myoblast cells derived from three individuals using both oligonucleotide and cDNA microarrays. Following stringent statistical testing (false-positive rate 0.4%), we identified 146 genes differentially expressed over time. Interestingly, 86 of these genes have not been reported to be involved in myogenesis in mouse cell lines. This demonstrates the additional value of human primary cell cultures in the study of muscle differentiation. Many of the identified genes play a role in muscle regeneration, indicating the close relationship of this process with muscle development. In addition, we found overlap with expression profiling studies in muscle from Duchenne muscular dystrophy patients, confirming ongoing muscle regeneration in Duchenne muscular dystrophy. Further study of these genes can bring new insights into the process of muscle differentiation, and they are candidate genes for neuromuscular disorders with an as yet unidentified cause.

Antiretroviral protease inhibitors prevent l6 muscle cell fusion by reducing calpain activity

The antiretroviral protease inhibitors indinavir (IDV) and ritonavir (RTV) are used in highly active antiretroviral therapies (HAART). Side effects from long-term HAART therapy include loss of muscle mass. Myoblasts when cultured in media low in growth factors withdraw from the cell cycle, express muscle-specific differentiation inducers and proteins, and fuse to form myotubes. The neutral protease, calpain, is required for myotube formation and RTV decreased calpain activity in vitro. We found lower calpain activity, but not protein, in homogenates of RTV-treated L6 cells than in control cultures. Importantly, L6 and C2C12 myoblasts did not form myotubes when cultured with 10 or 20 microM IDV or RTV. Control and drug-related L6 myoblasts showed identical decreases in proliferating cell nuclear antigens expression indicating proliferation arrest. Similarly, muscle differentiation inducers MyoD and myogenin and their downstream target, myosin heavy chain, were expressed at similar levels in control and drug-treated cells. Thus, whereas muscle differentiation was unaffected by protease inhibitors, calpain activity was reduced and myotube formation prevented. We conclude that RTV and IDV reduced myotube formation by reducing calpain activity. Our data suggest that protease inhibitors included in HAART might be directly involved in muscle wasting by reducing muscle remodeling.

E2F-1 Regulates the Expression of a Subset of Target Genes during Skeletal Myoblast Hypertrophy

Cellular hypertrophy, or growth without division, is an adaptive response to various physiological and pathological stimuli in postmitotic muscle. We demonstrated previously that angiotensin II stimulates hypertrophy in C2C12 myoblasts by transient activation of the cyclin-dependent kinase 4 complex, subsequent phosphorylation of retinoblastoma protein, release of histone deacetylase 1 from the retinoblastoma protein inhibitory complex, and partial activation of the transcription factor E2F-1. These observations led us to propose a model in which partial inactivation of the retinoblastoma protein complex leads to the derepression of a subset of E2F-1 targets necessary for cell growth without division during hypertrophy. We now present data that support this model and suggest the mechanism by which E2F-1 regulates hypertrophy. We examined expression profiles of angiotensin II-stimulated myoblasts and identified a subset of E2F-1 target genes that are specifically regulated during the hypertrophic response. We showed that the expression of E2F-1 targets involved in G1/S transit, DNA replication, and mitosis is not altered during the hypertrophic response, while the expression of E2F-1-regulated genes controlling early G1 progression, cytoskeletal organization, protein synthesis, mitochondrial function, and programmed cell death is up-regulated. Furthermore, we demonstrated that activation of cytochrome c oxidase genes occurs during the development of hypertrophy and that cytochrome c oxidase IV is a direct transcriptional target of E2F-1. These studies demonstrated that E2F-1 activity at specific promoters is dependent on physiological circumstances and that E2F-1 should be considered a potential target in the treatment of pathologic hypertrophy.

Real-time imaging of peroxisome proliferator-activated receptor-γ coactivator-1α promoter activity in skeletal muscles of living mice

In response to sustained increase in contractile activity, mammalian skeletal muscle undergoes adaptation with enhanced mitochondrial biogenesis and fiber type switching. The peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) was recently identified as a key regulator for these adaptive processes. To investigate the sequence elements in the PGC-1α gene that are responsible for activity-dependent transcriptional activation, we have established a unique system to analyze promoter activity in skeletal muscle of living mice. Expression of PGC-1α-firefly luciferase reporter gene in mouse tibialis anterior muscle transfected by electric pulse-mediated gene transfer was assessed repeatedly in the same muscle by using optical bioluminescence imaging analysis before and after low-frequency (10 Hz) motor nerve stimulation. Nerve stimulation (2 h) resulted in a transient 3-fold increase ( P < 0.05) in PGC-1α promoter activity along with a 1.6-fold increase ( P < 0.05) in endogenous PGC-1α mRNA. Mutation of two consensus myocyte enhancer factor 2 (MEF2) binding sites (−2901 and −1539) or a cAMP response element (CRE) (−222) completely abolished nerve stimulation-induced increase in PGC-1α promoter activity. These findings provide direct evidence that contractile activity-induced PGC-1α promoter activity in skeletal muscle is dependent on the MEF2 and the CRE sequence elements. The experimental methods used in the present study have general applicability to studies of gene regulation in muscle.

Transient up-regulation of biglycan during skeletal muscle regeneration: delayed fiber growth along with decorin increase in biglycan-deficient mice

The onset and progression of skeletal muscle regeneration are controlled by a complex set of interactions between muscle precursor cells and their environment. Decorin is the main proteoglycan present in the extracellular matrix (ECM) of adult muscle while biglycan expression is lower, but both are increased in mdx mice dystrophic muscle. Both of these small leucine-rich proteoglycans (SLRPs) can bind other matrix proteins and to the three TGF-beta isoforms, acting as modulators of their biological activity. We evaluated biglycan and decorin expression in skeletal muscle during barium chloride-induced skeletal muscle regeneration in mice. A transient and dramatic up-regulation of biglycan was associated with newly formed myotubes, whereas decorin presented only minor variations. Studies both in vitro and in intact developing newborn mice showed that biglycan expression is initially high and then decreases during skeletal muscle differentiation and maturation. To further evaluate the role of biglycan during the regenerative process, skeletal muscle regeneration was studied in biglycan-null mice. Skeletal muscle maintains its regenerative capacity in the absence of biglycan, but a delay in regenerated fiber growth and a decreased expression of embryonic myosin were observed despite to normal expression of MyoD and myogenin. Transient up-regulation of decorin during muscle regeneration in these mice may possibly obscure further roles of SLRPs in this process.

Transcriptional profile of postmortem skeletal muscle

Autopsy specimens are often used in molecular biological studies of disease pathophysiology. However, few analyses have focused specifically on postmortem changes in skeletal muscles, and almost all of those investigate protein or metabolic changes. Although some structural and enzymatic changes have been described, the sequence of transcriptional events associated with these remains unclear. We analyzed a series of new and preexisting human skeletal muscle data sets on ≃12,500 genes and expressed sequence tags (ESTs) generated by the Affymetrix U95Av2 GeneChips from seven autopsy and seven surgical specimens. Remarkably, postmortem specimens (up to 46 h) revealed a significant and prominent upregulation of transcripts involved with protein biosynthesis. Additional upregulated transcripts are associated with cellular responses to oxidative stress, hypoxia, and ischemia; however, only a subset of genes in these pathways was affected. Overexpression was also seen for apoptosis-related, cell cycle regulation/arrest-related, and signal transduction-related genes. No major gene expression differences were seen between autopsy specimens with <20-h and 34- to 46-h postmortem intervals or between pediatric and adult cases. These data demonstrate that, likely in response to hypoxia and oxidative stress, skeletal muscle undergoes a highly active transcriptional, and possibly, translational phase during the initial 46-h postmortem interval. Knowledge of these changes is important for proper interpretation of gene expression studies utilizing autopsy specimens.

An Unusual Peptide Deformylase Features in the Human Mitochondrial N-terminal Methionine Excision Pathway

Dedicated machinery for N-terminal methionine excision (NME) was recently identified in plant organelles and shown to be essential in plastids. We report here the existence of mitochondrial NME in mammals, as shown by the identification of cDNAs encoding specific peptide deformylases (PDFs) and new methionine aminopeptidases (MAP1D). We cloned the two full-length human cDNAs and showed that the N-terminal domains of the encoded enzymes were specifically involved in targeting to mitochondria. In contrast to mitochondrial MAP1D, the human PDF sequence differed from that of known PDFs in several key features. We characterized the human PDF fully in vivo and in vitro. Comparison of the processed human enzyme with the plant mitochondrial PDF1A, to which it is phylogenetically related, showed that the human enzyme had an extra N-terminal domain involved in both mitochondrial targeting and enzyme stability. Mammalian PDFs also display non-random substitutions in the conserved motifs important for activity. Human PDF site-directed mutagenesis variants were studied and compared with the corresponding plant PDF1A variants. We found that amino acid substitutions in human PDF specifically altered its catalytic site, resulting in an enzyme intermediate between bacterial PDF1Bs and plant PDF1As. Because (i) human PDF was found to be active both in vitro and in vivo, (ii) the entire machinery is conserved and expressed in most animals, (iii) the mitochondrial genome expresses substrates for these enzymes, and (iv) mRNA synthesis is regulated, we conclude that animal mitochondria have a functional NME machinery that can be regulated.