Target gene selectivity of the myogenic basic helix–loop–helix transcription factor myogenin in embryonic muscle

Exercise microdosing for skeletal muscle health applications to spaceflight

Findings from a recent 70-day bedrest investigation suggested intermittent exercise testing in the control group may have served as a partial countermeasure for skeletal muscle size, function, and fiber-type shifts. The purpose of the current study was to investigate the metabolic and skeletal muscle molecular responses to the testing protocols. Eight males (29 ± 2 yr) completed muscle power (6 × 4 s; peak muscle power: 1,369 ± 86 W) and V̇o2max (13 ± 1 min; 3.2 ± 0.2 L/min) tests on specially designed supine cycle ergometers during two separate trials. Blood catecholamines and lactate were measured pre-, immediately post-, and 4-h postexercise. Muscle homogenate and muscle fiber-type-specific [myosin heavy chain (MHC) I and MHC IIa] mRNA levels of exercise markers (myostatin, IκBα, myogenin, MuRF-1, ABRA, RRAD, Fn14, PDK4) and MHC I, IIa, and IIx were measured from vastus lateralis muscle biopsies obtained pre- and 4-h postexercise. The muscle power test altered (P ≤ 0.05) norepinephrine (+124%), epinephrine (+145%), lactate (+300%), and muscle homogenate mRNA (IκBα, myogenin, MuRF-1, RRAD, Fn14). The V̇o2max test altered (P ≤ 0.05) norepinephrine (+1,394%), epinephrine (+1,412%), lactate (+736%), and muscle homogenate mRNA (myostatin, IκBα, myogenin, MuRF-1, ABRA, RRAD, Fn14, PDK4). In general, both tests influenced MHC IIa muscle fibers more than MHC I with respect to the number of genes that responded and the magnitude of response. Both tests also influenced MHC mRNA expression in a muscle fiber-type-specific manner. These findings provide unique insights into the adaptive response of skeletal muscle to small doses of exercise and could help shape exercise dosing for astronauts and Earth-based individuals.NEW & NOTEWORTHY Declines in skeletal muscle health are a concern for astronauts on long-duration spaceflights. The current findings add to the growing body of exercise countermeasures data, suggesting that small doses of specific exercise can be beneficial for certain aspects of skeletal muscle health. This information can be used in conjunction with other components of existing exercise programs for astronauts and might translate to other areas focused on skeletal muscle health (e.g., sports medicine, rehabilitation, aging).

Establishment and Characterization of SV40 T-Antigen Immortalized Porcine Muscle Satellite Cell

Muscle satellite cells (MuSCs) are crucial for muscle development and regeneration. The primary pig MuSCs (pMuSCs) is an ideal in vitro cell model for studying the pig's muscle development and differentiation. However, the long-term in vitro culture of pMuSCs results in the gradual loss of their stemness, thereby limiting their application. To address this conundrum and maintain the normal function of pMuSCs during in vitro passaging, we generated an immortalized pMuSCs (SV40 T-pMuSCs) by stably expressing SV40 T-antigen (SV40 T) using a lentiviral-based vector system. The SV40 T-pMuSCs can be stably sub-cultured for over 40 generations in vitro. An evaluation of SV40 T-pMuSCs was conducted through immunofluorescence staining, quantitative real-time PCR, EdU assay, and SA-β-gal activity. Their proliferation capacity was similar to that of primary pMuSCs at passage 1, and while their differentiation potential was slightly decreased. SiRNA-mediated interference of SV40 T-antigen expression restored the differentiation capability of SV40 T-pMuSCs. Taken together, our results provide a valuable tool for studying pig skeletal muscle development and differentiation.

Muscle Progenitor Cell Fusion in the Maintenance of Skeletal Muscle

Skeletal muscle possesses a resident, multipotent stem cell population that is essential for its repair and maintenance throughout life. Here I highlight the role of this stem cell population in muscle repair and regeneration and review the genetic control of the process; the mechanistic steps of activation, migration, recognition, adhesion, and fusion of these cells; and discuss the novel recognition of the membrane signaling that coordinates myogenic cell-cell fusion, as well as the identification of a two-part fusogen system that facilitates it.

Lysine methylation signaling in skeletal muscle biology: from myogenesis to clinical insights

Lysine methylation signaling is well studied for its key roles in the regulation of transcription states through modifications on histone proteins. While histone lysine methylation has been extensively studied, recent discoveries of lysine methylation on thousands of non-histone proteins has broadened our appreciation for this small chemical modification in the regulation of protein function. In this review, we highlight the significance of histone and non-histone lysine methylation signaling in skeletal muscle biology, spanning development, maintenance, regeneration, and disease progression. Furthermore, we discuss potential future implications for its roles in skeletal muscle biology as well as clinical applications for the treatment of skeletal muscle-related diseases.

Postnatal skeletal muscle myogenesis governed by signal transduction networks: MAPKs and PI3K-Akt control multiple steps

Skeletal muscle myogenesis represents one of the most intensively and extensively examined systems of cell differentiation, tissue formation, and regeneration. Muscle regeneration provides an in vivo model system of postnatal myogenesis. It comprises multiple steps including muscle stem cell (or satellite cell) quiescence, activation, migration, myogenic determination, myoblast proliferation, myocyte differentiation, myofiber maturation, and hypertrophy. A variety of extracellular signaling and subsequent intracellular signal transduction pathways or networks govern the individual steps of postnatal myogenesis. Among them, MAPK pathways (the ERK, JNK, p38 MAPK, and ERK5 pathways) and PI3K-Akt signaling regulate multiple steps of myogenesis. Ca2+, cytokine, and Wnt signaling also participate in several myogenesis steps. These signaling pathways often control cell cycle regulatory proteins or the muscle-specific MyoD family and the MEF2 family of transcription factors. This article comprehensively reviews molecular mechanisms of the individual steps of postnatal skeletal muscle myogenesis by focusing on signal transduction pathways or networks. Nevertheless, no or only a partial signaling molecules or pathways have been identified in some responses during myogenesis. The elucidation of these unidentified signaling molecules and pathways leads to an extensive understanding of the molecular mechanisms of myogenesis.

CREB1 promotes proliferation and differentiation by mediating the transcription of CCNA2 and MYOG in bovine myoblasts

The cAMP response element binding protein 1 (CREB1) is an important nuclear transcription factor in eukaryotes. To explore the potential role of CREB1 on Qinchuan bovine skeletal myoblasts, we investigated the function of CREB1 on proliferation and differentiation. In this study, we found that CREB1 promoted cell proliferation by promoting DNA synthesis in S phase and cell division in G2 phase and promoted myogenic differentiation process in bovine myoblasts. Through dual luciferase experiments, we found that CREB1 can bind to the proximal promoter regions of CCNA2 and MyoG, indicating that CREB1 can play a positive regulatory role in the proliferation and differentiation of myoblasts by mediating the transcription of CCNA2 and MyoG. In addition, through downstream target gene analysis and transcriptome sequencing, we found that CREB1 plays a role in cell proliferation, myogenic differentiation, skeletal muscle repair and other related pathways.

Tributyrin, a Butyrate Pro-Drug, Primes Satellite Cells for Differentiation by Altering the Epigenetic Landscape

Satellite cells (SC) are a population of muscle resident stem cells that are responsible for postnatal muscle growth and repair. With investigation into the genomic regulation of SC fate, the role of the epigenome in governing SC myogenesis is becoming clearer. Histone deacetylase (HDAC) inhibitors have been demonstrated to be effective at enhancing the myogenic program of SC, but their role in altering the epigenetic landscape of SC remains undetermined. Our objective was to determine how an HDAC inhibitor, butyrate, promotes myogenic differentiation. SC from tributyrin treated neonatal piglets showed a decrease relative to SC from control animals in the expression of enhance of zeste homologue-2 (EZH2), a chromatin modifier, ex vivo. Chromatin Immunoprecipitation-Sequencing (ChIP-Seq) analysis of SC isolated from tributyrin treated pigs showed a global reduction of the tri-methylation of lysine 27 of histone H3 (H3K27me3) repressive chromatin mark. To determine if reductions in EZH2 was the primary mechanism through which butyrate affects SC behavior, SC were transfected with siRNA targeting EZH2, treated with 0.5 mM butyrate, or both. Treatment with butyrate reduced paired-box-7 (Pax7) and myogenic differentiation-1 (MyoD) gene expression, while siRNA caused reductions in EZH2 had no effect on their expression. EZH2 depletion did result in an increase in differentiating SC, but not in myotube hypertrophy. These results indicate that while EZH2 reduction may force myogenic differentiation, butyrate may operate through a parallel mechanism to enhance the myogenic program.

Hypoxia preconditioned bone marrow-derived mesenchymal stromal/stem cells enhance myoblast fusion and skeletal muscle regeneration

Background

The skeletal muscle reconstruction occurs thanks to unipotent stem cells, i.e., satellite cells. The satellite cells remain quiescent and localized between myofiber sarcolemma and basal lamina. They are activated in response to muscle injury, proliferate, differentiate into myoblasts, and recreate myofibers. The stem and progenitor cells support skeletal muscle regeneration, which could be disturbed by extensive damage, sarcopenia, cachexia, or genetic diseases like dystrophy. Many lines of evidence showed that the level of oxygen regulates the course of cell proliferation and differentiation.

Methods

In the present study, we analyzed hypoxia impact on human and pig bone marrow-derived mesenchymal stromal cell (MSC) and mouse myoblast proliferation, differentiation, and fusion. Moreover, the influence of the transplantation of human bone marrow-derived MSCs cultured under hypoxic conditions on skeletal muscle regeneration was studied.

Results

We showed that bone marrow-derived MSCs increased VEGF expression and improved myogenesis under hypoxic conditions in vitro. Transplantation of hypoxia preconditioned bone marrow-derived MSCs into injured muscles resulted in the improved cell engraftment and formation of new vessels.

Conclusions

We suggested that SDF-1 and VEGF secreted by hypoxia preconditioned bone marrow-derived MSCs played an essential role in cell engraftment and angiogenesis. Importantly, hypoxia preconditioned bone marrow-derived MSCs more efficiently engrafted injured muscles; however, they did not undergo myogenic differentiation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02530-3.

MiRNAs and Muscle Regeneration: Therapeutic Targets in Duchenne Muscular Dystrophy

microRNAs (miRNAs) are small non-coding RNAs required for the post-transcriptional control of gene expression. MicroRNAs play a critical role in modulating muscle regeneration and stem cell behavior. Muscle regeneration is affected in muscular dystrophies, and a critical point for the development of effective strategies for treating muscle disorders is optimizing approaches to target muscle stem cells in order to increase the ability to regenerate lost tissue. Within this framework, miRNAs are emerging as implicated in muscle stem cell response in neuromuscular disorders and new methodologies to regulate the expression of key microRNAs are coming up. In this review, we summarize recent advances highlighting the potential of miRNAs to be used in conjunction with gene replacement therapies, in order to improve muscle regeneration in the context of Duchenne Muscular Dystrophy (DMD).

Myogenin is required for assembly of the transcription machinery on muscle genes during skeletal muscle differentiation

Skeletal muscle gene expression is governed by the myogenic regulatory family (MRF) which includes MyoD (MYOD1) and myogenin (MYOG). MYOD1 and MYOG are known to regulate an overlapping set of muscle genes, but MYOD1 cannot compensate for the absence of MYOG in vivo. In vitro, late muscle genes have been shown to be bound by both factors, but require MYOG for activation. The molecular basis for this requirement was unclear. We show here that MYOG is required for the recruitment of TBP and RNAPII to muscle gene promoters, indicating that MYOG is essential in assembling the transcription machinery. Genes regulated by MYOD1 and MYOG include genes required for muscle fusion, myomaker and myomerger, and we show that myomaker is fully dependent on activation by MYOG. We also sought to determine the role of MYOD1 in MYOG dependent gene activation and unexpectedly found that MYOG is required to maintain Myod1 expression. However, we also found that exogenous MYOD1 was unable to compensate for the loss of Myog and activate muscle gene expression. Thus, our results show that MYOD1 and MYOG act in a feed forward loop to maintain each other’s expression and also show that it is MYOG, and not MYOD1, that is required to load TBP and activate gene expression on late muscle gene promoters bound by both factors.

Post-transcriptional regulation of MRTF-A by miRNAs during myogenic differentiation of myoblasts

The differentiation and regeneration of skeletal muscle from myoblasts to myotubes involves myogenic transcription factors, such as myocardin-related transcription factor A (MRTF-A) and serum response factor (SRF). In addition, post-transcriptional regulation by miRNAs is required during myogenesis. Here, we provide evidence for novel mechanisms regulating MRTF-A during myogenic differentiation. Endogenous MRTF-A protein abundance and activity decreased during C2C12 differentiation, which was attributable to miRNA-directed inhibition. Conversely, overexpression of MRTF-A impaired differentiation and myosin expression. Applying miRNA trapping by RNA affinity purification (miTRAP), we identified miRNAs which directly regulate MRTF-A via its 3′UTR, including miR-1a-3p, miR-206-3p, miR-24-3p and miR-486-5p. These miRNAs were upregulated during differentiation and specifically recruited to the 3′UTR of MRTF-A. Concomitantly, Ago2 recruitment to the MRTF-A 3′UTR was considerably increased, whereas Dicer1 depletion or 3′UTR deletion elevated MRTF-A and inhibited differentiation. MRTF-A protein expression was inhibited by ectopic miRNA expression in murine C2C12 and primary human myoblasts. 3′UTR reporter activity diminished upon differentiation or miRNA expression, whereas deletion of the predicted binding sites reversed these effects. Furthermore, TGF-β abolished MRTF-A reduction and decreased miR-486-5p expression. Our findings implicate miR-24-3p and miR-486-5p in the repression of MRTF-A and suggest a complex network of transcriptional and post-transcriptional mechanisms regulating myogenesis.

Longitudinal epitranscriptome profiling reveals the crucial role of N6-methyladenosine methylation in porcine prenatal skeletal muscle development

N⁶-methyladenosine (m⁶A) represents the most abundantly occurring mRNA modification and is involved in the regulation of skeletal muscle development. However, the status and function of m⁶A methylation in prenatal myogenesis remains unclear. In this study, we first demonstrated that knockdown of METTL14, an m⁶A methyltransferase, inhibited the differentiation and promoted the proliferation of C2C12 myoblast cells. Then, using a refined m⁶A-specific methylated RNA immunoprecipitation (RIP) with next generation sequencing (MeRIP-seq) method that is optimal for use with samples containing small amounts of RNA, we performed transcriptome-wide m⁶A profiling for six prenatal skeletal muscle developmental stages spanning two important waves of porcine myogenesis. The results revealed that, along with a continuous decrease in the mRNA expression of the m⁶A reader protein insulin-like growth factor 2 mRNA-binding protein 1 (IGF2BP1), the m⁶A methylome underwent highly dynamic changes across different development stages, with most of the affected genes being enriched in pathways related to skeletal muscle development. RNA immunoprecipitation confirmed that IGF2BP1 targets 76 genes involved in pathways associated with muscle development, including the key marker genes MYH2 and MyoG. Moreover, small interfering RNA (siRNA)-mediated knockdown of IGF2BP1 induced phenotypic changes in C2C12 myoblasts similar to those observed with knockdown of METTL14. In conclusion, we clarified the dynamics of m⁶A methylation and identified key genes involved in the regulatory network of porcine skeletal muscle development.

The SMYD3 methyltransferase promotes myogenesis by activating the myogenin regulatory network

The coordinated expression of myogenic regulatory factors, including MyoD and myogenin, orchestrates the steps of skeletal muscle development, from myoblast proliferation and cell-cycle exit, to myoblast fusion and myotubes maturation. Yet, it remains unclear how key transcription factors and epigenetic enzymes cooperate to guide myogenic differentiation. Proteins of the SMYD (SET and MYND domain-containing) methyltransferase family participate in cardiac and skeletal myogenesis during development in zebrafish, Drosophila and mice. Here, we show that the mammalian SMYD3 methyltransferase coordinates skeletal muscle differentiation in vitro. Overexpression of SMYD3 in myoblasts promoted muscle differentiation and myoblasts fusion. Conversely, silencing of endogenous SMYD3 or its pharmacological inhibition impaired muscle differentiation. Genome-wide transcriptomic analysis of murine myoblasts, with silenced or overexpressed SMYD3, revealed that SMYD3 impacts skeletal muscle differentiation by targeting the key muscle regulatory factor myogenin. The role of SMYD3 in the regulation of skeletal muscle differentiation and myotube formation, partially via the myogenin transcriptional network, highlights the importance of methyltransferases in mammalian myogenesis.

The transcription elongation factor TCEA3 promotes the activity of the myogenic regulatory factors

The transcription elongation factor TFIIS is encoded by a three member gene family in vertebrates. Here we show that one member of this family, TCEA3, is upregulated during skeletal muscle differentiation and acts to promote gene activation by the myogenic regulatory family of transcription factors, which includes MyoD and myogenin. We show that myogenin is a direct regulator of Tcea3. Myogenin binds to the Tcea3 promoter and is required to recruit RNA polymerase II. TCEA3 can bind to both myogenin and MyoD and is co-recruited with the MRFs to promoters dependent on the MRFs. Depletion of myogenin inhibits the recruitment of TCEA3, suggesting that the interaction of TCEA3 with the MRFs serves to aid in recruitment to target promoters. Like TFIIS, we show that TCEA3 interacts with RNA polymerase II. TCEA3 travels with the elongating RNA polymerase II in the coding region of genes and depletions of TCEA3 inhibit the recruitment of RNA polymerase II to promoters. In proliferating cells, TCEA3 expressed at low levels and is present in both the nucleus and cytoplasm. However, upon differentiation, TCEA3 is upregulated and transported exclusively to the nucleus. Thus, our data show that TCEA3 is a required co-factor for MRF driven gene expression during myogenesis.

Bimodal antagonism of PKA signalling by ARHGAP36

Protein kinase A is a key mediator of cAMP signalling downstream of G-protein-coupled receptors, a signalling pathway conserved in all eukaryotes. cAMP binding to the regulatory subunits (PKAR) relieves their inhibition of the catalytic subunits (PKAC). Here we report that ARHGAP36 combines two distinct inhibitory mechanisms to antagonise PKA signalling. First, it blocks PKAC activity via a pseudosubstrate motif, akin to the mechanism employed by the protein kinase inhibitor proteins. Second, it targets PKAC for rapid ubiquitin-mediated lysosomal degradation, a pathway usually reserved for transmembrane receptors. ARHGAP36 thus dampens the sensitivity of cells to cAMP. We show that PKA inhibition by ARHGAP36 promotes derepression of the Hedgehog signalling pathway, thereby providing a simple rationale for the upregulation of ARHGAP36 in medulloblastoma. Our work reveals a new layer of PKA regulation that may play an important role in development and disease.

Protein kinase A (PKA) is a key mediator of cyclic AMP signalling. Here, Eccles et al. show that ARHGAP36 antagonizes PKA by acting as a kinase inhibitor and targeting the catalytic subunit for endolysosomal degradation, thus reducing sensitivity of cells to cAMP and promoting Hedgehog signalling.

Satellite Cells and Skeletal Muscle Regeneration

Skeletal muscles are essential for vital functions such as movement, postural support, breathing, and thermogenesis. Muscle tissue is largely composed of long, postmitotic multinucleated fibers. The life-long maintenance of muscle tissue is mediated by satellite cells, lying in close proximity to the muscle fibers. Muscle satellite cells are a heterogeneous population with a small subset of muscle stem cells, termed satellite stem cells. Under homeostatic conditions all satellite cells are poised for activation by stimuli such as physical trauma or growth signals. After activation, satellite stem cells undergo symmetric divisions to expand their number or asymmetric divisions to give rise to cohorts of committed satellite cells and thus progenitors. Myogenic progenitors proliferate, and eventually differentiate through fusion with each other or to damaged fibers to reconstitute fiber integrity and function. In the recent years, research has begun to unravel the intrinsic and extrinsic mechanisms controlling satellite cell behavior. Nonetheless, an understanding of the complex cellular and molecular interactions of satellite cells with their dynamic microenvironment remains a major challenge, especially in pathological conditions. The goal of this review is to comprehensively summarize the current knowledge on satellite cell characteristics, functions, and behavior in muscle regeneration and in pathological conditions.

Investigation of the Expression of Myogenic Transcription Factors, microRNAs and Muscle-Specific E3 Ubiquitin Ligases in the Medial Gastrocnemius and Soleus Muscles following Peripheral Nerve Injury

Despite surgical innovation, the sensory and motor outcome after a peripheral nerve injury remains incomplete. One contributing factor to the poor outcome is prolonged denervation of the target organ, leading to apoptosis of both mature myofibres and satellite cells with subsequent replacement of the muscle tissue with fibrotic scar and adipose tissue. In this study, we investigated the expression of myogenic transcription factors, muscle specific microRNAs and muscle-specific E3 ubiquitin ligases at several time points following denervation in two different muscles, the gastrocnemius (containing predominantly fast type fibres) and soleus (slow type) muscles, since these molecules may influence the degree of atrophy following denervation. Both muscles exhibited significant atrophy (compared with the contra-lateral sides) at 7 days following either a nerve transection or crush injury. In the crush model, the soleus muscle showed significantly increased muscle weights at days 14 and 28 which was not the case for the gastrocnemius muscle which continued to atrophy. There was a significantly more pronounced up-regulation of MyoD expression in the denervated soleus muscle compared with the gastrocnemius muscle. Conversely, myogenin was more markedly elevated in the gastrocnemius versus soleus muscles. The muscles also showed significantly contrasting transcriptional regulation of the microRNAs miR-1 and miR-206. MuRF1 and Atrogin-1 showed the highest levels of expression in the denervated gastrocnemius muscle. This study provides further insights regarding the intracellular regulatory molecules that generate and maintain distinct patterns of gene expression in different fibre types following peripheral nerve injury.

CEP2 attenuates myoblast differentiation but does not affect proliferation

CEP2 (CDC42EP2) is a member of the CDC42 subfamily that belongs to the Rho family. The Rho family plays an important role in a variety of cellular processes including skeletal myogenesis. Here, we find the expression of CEP2 increased significantly during C2C12 myogenesis. Overexpression of CEP2 could attenuate myoblast differentiation, while knockdown of CEP2 by siRNA results in enhancing myogenesis. Furthermore, we demonstrate for the first time that CEP2 attenuates myoblast differentiation via suppression of muscle regulatory factors (MRFs) rather than influencing myoblast proliferation. These results indicate that CEP2 acts as a repressor during myogenesis, which provides new insights into the role of CEP2 in muscle development.

Dormancy and quiescence of skeletal muscle stem cells

The skeletal muscle of vertebrates has a huge regenerative capacity. When destroyed after different types of injury, this organ can regenerate very quickly (less than 20 days following myotoxin injection in the mouse) ad integrum and repeatedly. The cell responsible for this regeneration is the so-called satellite cell, the muscle stem cell that lies on top of the muscle fibre, a giant, multinucleated cell that contains the contractile material. When injected in the muscle, satellite cells can efficiently differentiate into contractile muscle fibres. The satellite cell shows great therapeutic potential; and its regenerative capacity has triggered particular interest in the field of muscular degeneration. In this review we will focus on one particular property of the satellite cell: its quiescence and dormancy. Indeed adult satellite cells are quiescent; they lie between the basal lamina and the basement membrane of the muscle fibre, ready to proliferate, and fuse in order to regenerate myofibers upon injury. It has recently been shown that a subpopulation of satellite cells is able to enter dormancy in human and mice cadavers. Dormancy is defined by a low metabolic state, low mobility, and a long lag before division when plated in vitro, compared to quiescent cells. This definition is also based on current knowledge about long-term hematopoietic stem cells, a subpopulation of stem cells that are described as dormant based on the same criteria (rare division and low metabolism when compared to progeny which are dividing more often). In the first part of this review, we will provide a description of satellite cells which addresses their quiescent state. We will then focus on the uneven distribution of satellite cells in the muscle and describe evidence that suggests that their dormancy differs from one muscle to the next and that one should be cautious when making generalisations regarding this cellular state. In a second part, we will discuss the transition between active dividing cells in developing animals to quiescence. This mechanism could be used or amplified in the switch from quiescence to dormancy. In a third part, we will review the signals and dynamics that actively maintain the satellite cell quiescent. The in-depth understanding of these mechanisms is key to describing how dormancy relies on quiescent state of the cells. In a fourth part, we will deal with dormancy per se: how dormant satellite cells can be obtained, their characteristics, their metabolic profile, and their molecular signature as compared to quiescent cells. Here, we will highlight one of the most important recent findings: that quiescence is a prerequisite for the entry of the satellite cell into dormancy. Since dormancy is a newly discovered phenomenon, we will review the mechanisms responsible for quiescence and activation, as these two cellular states are better known and key to understanding satellite cell dormancy. This will allow us to describe dormancy and its prerequisites.

Actions of 1,25(OH)2-vitamin D3 on the cellular cycle depend on VDR and p38 MAPK in skeletal muscle cells

Previously, we have reported that 1,25(OH)2-vitamin D3 (1,25D) activates p38 MAPK (p38) in a vitamin D receptor (VDR)-dependent manner in proliferative C2C12 myoblast cells. It was also demonstrated that 1,25D promotes muscle cell proliferation and differentiation. However, we did not study these hormone actions in depth. In this study we have investigated whether the VDR and p38 participate in the signaling mechanism triggered by 1,25D. In C2C12 cells, the VDR was knocked down by a shRNA, and p38 was specifically inhibited using SB-203580. Results from cell cycle studies indicated that hormone stimulation prompts a peak of S-phase followed by an arrest in the G0/G1-phase, events which were dependent on VDR and p38. Moreover, 1,25D increases the expression of cyclin D3 and the cyclin-dependent kinase inhibitors, p21(Waf1/Cip1) and p27(Kip1), while cyclin D1 protein levels did not change during G0/G1 arrest. In all these events, p38 and VDR were required. At the same time, a 1,25D-dependent acute increase in myogenin expression was observed, indicating that the G0/G1 arrest of cells is a pro-differentiative event. Immunocytochemical assays revealed co-localization of VDR and cyclin D3, promoted by 1,25D in a p38-dependent manner. When cyclin D3 expression was silenced, VDR and myogenin levels were downregulated, indicating that cyclin D3 was required for 1,25D-induced VDR expression and the concomitant entrance into the differentiation process. In conclusion, the VDR and p38 are involved in control of the cellular cycle by 1,25D in skeletal muscle cells, providing key information on the mechanisms underlying hormone regulation of myogenesis.

Alternative splicing of MEF2C pre-mRNA controls its activity in normal myogenesis and promotes tumorigenicity in rhabdomyosarcoma cells

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children. Many cellular disruptions contribute to the progression of this pediatric cancer, including aberrant alternative splicing. The MEF2 family of transcription factors regulates many developmental programs, including myogenesis. MEF2 gene transcripts are subject to alternate splicing to generate protein isoforms with divergent functions. We found that MEF2Cα1 was the ubiquitously expressed isoform that exhibited no myogenic activity and that MEF2Cα2, the muscle-specific MEF2C isoform, was required for efficient differentiation. We showed that exon α in MEF2C was aberrantly alternatively spliced in RMS cells, with the ratio of α2/α1 highly down-regulated in RMS cells compared with normal myoblasts. Compared with MEF2Cα2, MEF2Cα1 interacted more strongly with and recruited HDAC5 to myogenic gene promoters to repress muscle-specific genes. Overexpression of the MEF2Cα2 isoform in RMS cells increased myogenic activity and promoted differentiation in RMS cells. We also identified a serine protein kinase, SRPK3, that was down-regulated in RMS cells and found that expression of SRPK3 promoted the splicing of the MEF2Cα2 isoform and induced differentiation. Restoration of either MEF2Cα2 or SPRK3 inhibited both proliferation and anchorage-independent growth of RMS cells. Together, our findings indicate that the alternative splicing of MEF2C plays an important role in normal myogenesis and RMS development. An improved understanding of alternative splicing events in RMS cells will potentially reveal novel therapeutic targets for RMS treatment.

The RNA-binding protein Rbm24 is transiently expressed in myoblasts and is required for myogenic differentiation during vertebrate development

RNA-binding proteins (RBP) contribute to gene regulation through post-transcriptional events. Despite the important roles demonstrated for several RBP in regulating skeletal myogenesis in vitro, very few RBP coding genes have been characterized during skeletal myogenesis in vertebrate embryo. In the present study we report that Rbm24, which encodes the RNA-binding motif protein 24, is required for skeletal muscle differentiation in vivo. We show that Rbm24 transcripts are expressed at all sites of skeletal muscle formation during embryogenesis of different vertebrates, including axial, limb and head muscles. Interestingly, we find that Rbm24 protein starts to accumulate in MyoD-positive myoblasts and is transiently expressed at the onset of muscle cell differentiation. It accumulates in myotomal and limb myogenic cells, but not in Pax3-positive progenitor cells. Rbm24 expression is under the direct regulation by MyoD, as demonstrated by in vivo chromatin immunoprecipitation assay. Using morpholino knockdown approach, we further show that Rbm24 is required for somitic myogenic progenitor cells to differentiate into muscle cells during chick somitic myogenesis. Altogether, these results highlight Rbm24 as a novel key regulator of the myogenic differentiation program during vertebrate development.

Postnatal muscle modification by myogenic factors modulates neuropathology and survival in an ALS mouse model

MyoD and myogenin are myogenic transcription factors preferentially expressed in adult fast and slow muscles, respectively. Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder in which motor neuron loss is accompanied by muscle denervation and paralysis. Studies suggest that muscle phenotype may influence ALS disease progression. Here we demonstrate that myogenin gene transfer into muscle supports spinal cord motor neuron survival and muscle endplate innervation in the G93A SOD1 fALS mice. On the other hand, MyoD gene transfer decreases survival and enhances motor neuron degeneration and muscle denervation. Although an increase in motor neuron count is associated with increased succinic dehydrogenase staining in the muscle, muscle overexpression of PGC-1α does not improve survival or motor function. Our study suggests that postnatal muscle modification influences disease progression and demonstrates that the muscle expression of myogenic and metabolic regulators differentially impact neuropathology associated with disease progression in the G93A SOD1 fALS mouse model.

Di-(2-ethylhexyl)-phthalate reduces MyoD and myogenin expression and inhibits myogenic differentiation in C2C12 cells

The purpose of this study was to investigate the effects of di-(2-ethylhexyl) phthalate (DEHP) treatment on MyoD and myogenin expression and myotube formation in the murine C2C12 cells. Myogenic differentiation is principally regulated by activities of myogenic regulatory factors, such as MyoD and myogenin, leading the elongation and fusion of mononucleated myoblasts into multinucleated myotubes. In the present study, myogenic differentiation of C2C12 cells was induced by serum deprivation with medium containing vehicle or DEHP (10, 100, 1,000 μg/ml) for 5 days. Using 3-(4,5-dimethylthiazol-2-yl) 2,5-diphenyltetrazolium bromide (MTT) assay clearly demonstrated cell viability was not affected by DEHP at any given dose. At the dose of 1,000 μg/ml DEHP, the elongation of multinucleated myotubes, and the percent of nuclei incorporated into myosin heavy chain (MyHC)-stained myotubes were markedly reduced. In addition, immunoblotting revealed expression of muscle specific marker MyHC, as well as myogenic regulatory factors MyoD and myogenin, were reduced in DEHP-treated myotubes during myogenic differentiation. Taken together, the results showed that DEHP may impair myogenic differentiation through repression of myogenic regulatory factors, such as MyoD and myogenin, resulting in a reduction of MyHC expression. This in vitro study suggests that DEHP may be an environmental risk factor for myogenesis.

TBX2 blocks myogenesis and promotes proliferation in rhabdomyosarcoma cells

Rhabdomyosarcomas (RMSs) are the most frequent soft tissue sarcomas in children that share many features of developing skeletal muscle. We have discovered that a T-box family member, TBX2, is highly upregulated in tumor cells of both major RMS subtypes. TBX2 is a repressor that is often overexpressed in cancer cells and is thought to function in bypassing cell growth control, including repression of p14 and p21. The cell cycle regulator p21 is required for the terminal differentiation of skeletal muscle cells and is silenced in RMS cells. We have found that TBX2 interacts with the myogenic regulatory factors MyoD and myogenin and inhibits the activity of these factors. TBX2 is expressed in primary myoblasts and C2C12 cells, but is strongly downregulated upon differentiation. TBX2 recruits the histone deacetylase HDAC1 and is a potent inhibitor of the expression of muscle-specific genes and the cell cycle regulators, p21 and p14. TBX2 promotes the proliferation of RMS cells and either depletions of TBX2 or dominant negative TBX2 upregulate p21- and muscle-specific genes. Significantly, depletion or interference with TBX2 completely inhibits tumor growth in a xenograft assay, highlighting the oncogenic role of TBX2 in RMS cells. Thus, the data demonstrate that elevated expression of TBX2 contributes to the pathology of RMS cells by promoting proliferation and repressing differentiation-specific gene expression. These results show that deregulated TBX2 serves as an oncogene in RMS, suggesting that TBX2 may serve as a new diagnostic marker or therapeutic target for RMS tumors.

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.

Loss of MEF2D expression inhibits differentiation and contributes to oncogenesis in rhabdomyosarcoma cells

Background

Rhabdomyosarcoma (RMS) is a highly malignant pediatric cancer that is the most common form of soft tissue tumors in children. RMS cells have many features of skeletal muscle cells, yet do not differentiate. Thus, our studies have focused on the defects present in these cells that block myogenesis.

Methods

Protein and RNA analysis identified the loss of MEF2D in RMS cells. MEF2D was expressed in RD and RH30 cells by transient transfection and selection of stable cell lines, respectively, to demonstrate the rescue of muscle differentiation observed. A combination of techniques such as proliferation assays, scratch assays and soft agar assays were used with RH30 cells expressing MEF2D to demonstrate the loss of oncogenic growth in vitro and xenograft assays were used to confirm the loss of tumor growth in vivo.

Results

Here, we show that one member of the MEF2 family of proteins required for normal myogenesis, MEF2D, is largely absent in RMS cell lines representing both major subtypes of RMS as well as primary cells derived from an embryonal RMS model. We show that the down regulation of MEF2D is a major cause for the failure of RMS cells to differentiate. We find that MyoD and myogenin are bound with their dimerization partner, the E proteins, to the promoters of muscle specific genes in RMS cells. However, we cannot detect MEF2D binding at any promoter tested. We find that exogenous MEF2D expression can activate muscle specific luciferase constructs, up regulate p21 expression and increase muscle specific gene expression including the expression of myosin heavy chain, a marker for skeletal muscle differentiation. Restoring expression of MEF2D also inhibits proliferation, cell motility and anchorage independent growth in vitro. We have confirmed the inhibition of tumorigenicity by MEF2D in a tumor xenograft model, with a complete regression of tumor growth.

Conclusions

Our data indicate that the oncogenic properties of RMS cells can be partially attributed to the loss of MEF2D expression and that restoration of MEF2D may represent a useful therapeutic strategy to decrease tumorigenicity.

WITHDRAWN: VDR involvement in 1a,25-dihydroxyvitamin D3-action on cellular cycle in C2C12 skeletal muscle cells

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Myogenin recruits the histone chaperone facilitates chromatin transcription (FACT) to promote nucleosome disassembly at muscle-specific genes

Facilitates chromatin transcription (FACT) functions to reorganize nucleosomes by acting as a histone chaperone that destabilizes and restores nucleosomal structure. The FACT complex is composed of two subunits: SSRP1 and SPT16. We have discovered that myogenin interacts with the FACT complex. Transfection of FACT subunits with myogenin is highly stimulatory for endogenous muscle gene expression in 10T1/2 cells. We have also found that FACT subunits do not associate with differentiation-specific genes while C2C12 cells are proliferating but are recruited to muscle-specific genes as differentiation initiates and then dissociate as differentiation proceeds. The recruitment is dependent on myogenin, as knockdowns of myogenin show no recruitment of the FACT complex. These data suggest that FACT is involved in the early steps of gene activation through its histone chaperone activities that serve to open the chromatin structure and facilitate transcription. Consistent with this hypothesis, we find that nucleosomes are depleted at muscle-specific promoters upon differentiation and that this activity is dependent on the presence of FACT. Our results show that the FACT complex promotes myogenin-dependent transcription and suggest that FACT plays an important role in the establishment of the appropriate transcription profile in a differentiated muscle cell.

1α,25(OH)2D3-dependent modulation of Akt in proliferating and differentiating C2C12 skeletal muscle cells

We previously reported that 1α,25-dihydroxy-vitamin D(3) [1α,25(OH)(2)D(3)] induces non-transcriptional rapid responses through activation of Src and MAPKs in the skeletal muscle cell line C2C12. In the present study we investigated the modulation of Akt by the secosteroid hormone in C2C12 cells at proliferative stage (myoblasts) and at early differentiation stage. In proliferating cells, 1α,25(OH)(2)D(3) activates Akt by phosphorylation in Ser473 in a time-dependent manner (5-60 min). When these cells were pretreated with methyl-beta-cyclodextrin to disrupt caveolae microdomains, hormone-induced activation of Akt was suppressed. Similar results were obtained by siRNA silencing of caveolin-1 expression, further indicating that hormone effects on cell membrane caveolae are required for downstream signaling. PI3K and p38 MAPK, but not ERK1/2, participate in 1α,25(OH)(2)D(3) activation of Akt in myoblasts. The involvement of p38 MAPK in Akt phosphorylation by the hormone probably occurs through MAPK-activated protein kinase 2 (MK2), which is activated by the steroid. In addition, the participation of Src in Akt phosphorylation by 1α,25(OH)(2)D(3) was demonstrated using the inhibitor PP2 and antisense oligodeoxynucleotides that suppress Src expression. We also observed that PI3K participates in hormone-induced proliferation. During the early phase of C2C12 cell differentiation 1α,25(OH)(2)D(3) also increases Akt phosphorylation and activates Src. Of relevance, Src and PI3K are involved in Akt activation and in MHC and myogenin increased expression by 1α,25(OH)(2)D(3). Altogether, these data suggest that 1α,25(OH)(2)D(3) upregulates Akt through Src, PI(3)K, and p38 MAPK to stimulate myogenesis in C2C12 cells.

Gene expression profiling in fetal rat lung during gestational perfluorooctane sulfonate exposure

Perfluorooctane sulfonate (PFOS) is a persistent environmental contaminant found in the tissues of humans and wildlife. It has been reported that gestational exposure to PFOS causes neonatal death of rats. However, the mechanism is still unclear. In this study, we investigated the effects of gestational PFOS exposure on the gene expression profiling of fetal rat lung at pseudoglandular stage. Adult Sprague Dawley dams were dosed orally from gestational day 12-18 with 0 (control), 5 mg/kg/day or 20 mg/kg/day PFOS. Animals were euthanized on day 18.5, fetal lung samples were collected for histochemical staining and RNA profiling analysis. PFOS did not cause apparent microscopic changes of fetal lungs. Gene expression profiling revealed that PFOS dose-dependently up-regulated the expression of 21 (5 mg/kg) and 43 (20 mg/kg) genes. These genes include five PPARα target genes (Acot1, Hmgcs2, Fabp4, Fabp1 and Myh7), and 4 of them are involved in lipid metabolism. The other genes were primarily included in the categories of cytoskeletal structure (Tpm1, Tnnt2, Actn3, Myoz2, Tmod1, and Mfap5), extracellular matrix (Ckm, Lum, Tnnc1, Art3, Dcn, Col17a1, Aspn, Ctsk, Itm2a, Spock2 and Orm1), transporting (Cox8h, Cox6a2 and Scnn1a) and secreted proteins (Scgb3a1, Nppb and Spp1). Our study demonstrates that in utero PFOS exposure resulted in the alteration of a set of genes which are involved in significant cytoskeletal, extracellular matrix remodeling, lipid metabolism and secreted proteins in the fetal rat lung.

Cigarette smoke-induced skeletal muscle atrophy is associated with up-regulation of USP-19 via p38 and ERK MAPKs

Ubiquitin-specific proteases (USPs) deubiquitinate ubiquitin-protein conjugates in the ubiquitin-proteasome system. Previous research shows that ubiquitin-specific protease-19 (USP-19) is up-regulated in mammalian skeletal muscle in some degradative conditions, such as including fasting, diabetes, dexamethasone treatment, and cancer, and its function is associated with muscle atrophy. However, it is still unclear whether USP-19 is involved in muscle atrophy induced by chronic obstructive pulmonary disease. Rats exposed to chronic cigarette smoke and L6 myotubes incubated with cigarette smoke extract (CSE) were studied here. Using western blot analysis and quantitative real-time polymerase chain reaction (qPCR), we observed over-expression of USP-19 and down-regulation of myosin heavy chain (MHC) in both models. Moreover, CSE exposure inhibited myogenic differentiation and myotube formation in L6 myotubes. To explore the mechanism underlying these effects, we investigated the levels of phosphorylated mitogen-activated protein kinases (MAPKs) and total MAPKs. Exposing myotubes to CSE resulted in the general activation of MAPKs such as p38, JNK, and ERK1/2. The ERK inhibitor PD98059 and the p38 inhibitor SB203580 significantly blocked the increase in USP-19 gene expression induced by CSE. Our findings suggest that USP-19 is associated with muscle atrophy in response to cigarette smoke and is a potential therapeutic target. CSE promotes myotube wasting in culture partly by inhibiting myogenic differentiation and acts via p38 and ERK MAPK to stimulate expression of USP-19 in vitro.

Gamma interferon modulates myogenesis through the major histocompatibility complex class II transactivator, CIITA

Gamma interferon (IFN-γ) is an inflammatory cytokine that has complex effects on myogenesis. Here, we show that the IFN-γ-induced inhibition of myogenesis is mediated by the major histocompatibility complex (MHC) class II transactivator, CIITA, which binds to myogenin and inhibits its activity. In IFN-γ-treated myoblasts, the inhibition of muscle-specific genes includes the expression of myogenin itself, while in myotubes, myogenin expression is unaffected. Thus, CIITA appears to act by both repressing the expression and inhibiting the activity of myogenin at different stages of myogenesis. Stimulation by IFN-γ in skeletal muscle cells induces CIITA expression as well as MHC class II gene expression. The IFN-γ-mediated repression is reversible, with myogenesis proceeding normally upon removal of IFN-γ. Through overexpression studies, we confirm that the expression of CIITA, independent of IFN-γ, is sufficient to inhibit myogenesis. Through knockdown studies, we also demonstrate that CIITA is necessary for the IFN-γ-mediated inhibition of myogenesis. Finally, we show that CIITA, which lacks DNA binding activity, is recruited to muscle-specific promoters coincident with reductions in RNA polymerase II recruitment. Thus, this work reveals how IFN-γ modulates myogenesis and demonstrates a key role for CIITA in this process.

Chromatin immunoprecipitation assay for tissue-specific genes using early-stage mouse embryos

Chromatin immunoprecipitation (ChIP) is a powerful tool to identify protein:chromatin interactions that occur in the context of living cells. This technique has been widely exploited in tissue culture cells, and to a lesser extent, in primary tissue. The application of ChIP to rodent embryonic tissue, especially at early times of development, is complicated by the limited amount of tissue and the heterogeneity of cell and tissue types in the embryo. Here we present a method to perform ChIP using a dissociated embryonic day 8.5 (E8.5) embryo. Sheared chromatin from a single E8.5 embryo can be divided into up to five aliquots, which allows the investigator sufficient material for controls and for investigation of specific protein:chromatin interactions. We have utilized this technique to begin to document protein:chromatin interactions during the specification of tissue-specific gene expression programs. The heterogeneity of cell types in an embryo necessarily restricts the application of this technique because the result is the detection of protein:chromatin interactions without distinguishing whether the interactions occur in all, a subset of, or a single cell type(s). However, examination of tissue-specific genes during or following the onset of tissue-specific gene expression is feasible for two reasons. First, immunoprecipitation of tissue specific factors necessarily isolates chromatin from the cell type where the factor is expressed. Second, immunoprecipitation of coactivators and histones containing post-translational modifications that are associated with gene activation should only be found at genes and gene regulatory sequences in the cell type where the gene is being or has been activated. The technique should be applicable to the study of most tissue-specific gene activation events. In the example described below, we utilized E8.5 and E9.5 mouse embryos to examine factor binding at a skeletal muscle specific gene promoter. Somites, which are the precursor tissues from which the skeletal muscles of the trunk and limbs will form, are present at E8.5-9.5. Myogenin is a regulatory factor required for skeletal muscle differentiation. The data demonstrate that myogenin is associated with its own promoter in E8.5 and E9.5 embryos. Because myogenin is only expressed in somites at this stage of development, the data indicate that myogenin interactions with its own promoter have already occurred in skeletal muscle precursor cells in E8.5 embryos.

Human neurotrophin receptor p75NTR defines differentiation-oriented skeletal muscle precursor cells: implications for muscle regeneration

Satellite cells are resident stem cells of adult skeletal muscle that have roles in tissue repair. Although several efforts have led to the functional characterization of distinct myogenic populations in animal models, the translation of these findings to humans has been limited. Here, we analyzed the expression and function of the neurotrophin receptor p75NTR in human skeletal muscle precursor cells. We combined histological investigations of muscle biopsies with molecular and cellular analyses of primary muscle precursor cells. p75NTR is expressed by most satellite cells in vivo and is a marker for regenerating fibers in inflamed and dystrophic muscle. p75NTR mRNA and protein are also detectable in primary myoblasts, and these levels increase transiently when cell differentiation is triggered. Transcriptome analyses of p75NTR high versus p75NTR low muscle cells showed that p75NTR is the prototype marker for a precursor cell population that has a broad transcriptional repertoire associated with muscle development and maturation. Several in vitro experiments, including receptor blockade and gene silencing in myoblasts, proved that p75NTR specifically regulates myogenesis and dystrophin expression. Taken together, the results indicate that p75NTR is a novel marker of human differentiation-prone muscle precursor cells that is involved in myogenesis in vivo and in vitro.

Sequential association of myogenic regulatory factors and E proteins at muscle-specific genes

Background

Gene expression in skeletal muscle is controlled by a family of basic helix-loop-helix transcription factors known as the myogenic regulatory factors (MRFs). The MRFs work in conjunction with E proteins to regulate gene expression during myogenesis. However, the precise mechanism by which the MRFs activate gene expression is unclear. In this work, we sought to define the binding profiles of MRFs and E proteins on muscle-specific genes throughout a time course of differentiation.

Results

We performed chromatin immunoprecipitation (ChIP) assays for myogenin, MyoD, Myf5 and E proteins over a time course of C₂C₁₂ differentiation, resulting in several surprising findings. The pattern of recruitment is specific to each promoter tested. The recruitment of E proteins often coincides with the arrival of the MRFs, but the binding profile does not entirely overlap with the MRF binding profiles. We found that E12/E47 is bound to certain promoters during proliferation, but every gene tested is preferentially bound by HEB during differentiation. We also show that MyoD, myogenin and Myf5 have transient roles on each of these promoters during muscle differentiation. We also found that RNA polymerase II occupancy correlates with the transcription profile of these promoters. ChIP sequencing assays confirmed that MyoD, myogenin and Myf5 co-occupy promoters.

Conclusions

Our data reveal the sequential association of MyoD, myogenin, Myf5 and HEB on muscle-specific promoters. These data suggest that each of the MRFs, including Myf5, contribute to gene expression at each of the geness analyzed here.. The dynamic binding profiles observed suggest that MRFs and E proteins are recruited independently to promoters.

Transcriptional analysis of the titin cap gene

Mutations in titin cap (Tcap), also known as telethonin, cause limb-girdle muscular dystrophy type 2G (LGMD2G). Tcap is one of the titin interacting Z-disc proteins involved in the regulation and development of normal sarcomeric structure. Given the essential role of Tcap in establishing and maintaining normal skeletal muscle architecture, we were interested in determining the regulatory elements required for expression of this gene in myoblasts. We have defined a highly conserved 421 bp promoter proximal promoter fragment that contains two E boxes and multiple putative Mef2 binding sequences. This promoter can be activated by MyoD and myogenin in NIH3T3 fibroblast cells, and maintains the differentiated cell-specific expression pattern of the endogenous Tcap in C2C12 cells. We find that while both E boxes are required for full activation by MyoD or myogenin in NIH3T3 cells, the promoter proximal E box has a greater contribution to activation of this promoter in C2C12 cells and to activation by MyoD in NIH3T3 cells. Together, the data suggest an important role for MyoD in activating Tcap expression through the promoter proximal E box. We also show that myogenin is required for normal expression in vivo and physically binds to the Tcap promoter during embryogenesis.

Muscle transcriptome profiling in divergent phenotype swine breeds during growth using microarray and RT-PCR tools

Using an array consisting of 10 665 70-mer oligonucleotide probes, the longissimus dorsi muscle tissue expression during growth in nine pigs belonging to Casertana (CT), an autochthonous breed characterized by slow growth and a massive accumulation of backfat, was compared with that of two cosmopolitan breeds, Large White (LW) and a crossbreed (CB; Duroc × Landrace × Large White). The results were validated by real-time PCR. All animals were of the same age and were raised under the same environmental conditions. Muscle tissues were collected at 3, 6, 9 and 11 months of age, and a total of 173 genes showed significant differential expression between CT and the cosmopolitan genetic types at 3 months of age. Time series cluster analysis indicated that the CT breed had a different pattern of gene expression compared with that of the LW and the CB. Four of the eight clusters highlighted the gene differences between CT and the other two breeds, which were further supported by statistical analyses: clusters 4 and 5 contained a total of 71 genes that were underexpressed at 3 months of age, and cluster 3 and cluster 7 included 28 and 42 genes respectively that were overexpressed at 3 months of age. As expected, differentially expressed genes belonged to the category of genes coding for contractile fibres and transcription factors involved in muscle development and differentiation. These findings highlight muscle expression genes during pig growth and are useful to understand the genetic meaning of the different developmental rates.

Excitation-transcription coupling in skeletal muscle: the molecular pathways of exercise

Muscle fibres have different properties with respect to force, contraction speed, endurance, oxidative/glycolytic capacity etc. Although adult muscle fibres are normally post-mitotic with little turnover of cells, the physiological properties of the pre-existing fibres can be changed in the adult animal upon changes in usage such as after exercise. The signal to change is mainly conveyed by alterations in the patterns of nerve-evoked electrical activity, and is to a large extent due to switches in the expression of genes. Thus, an excitation-transcription coupling must exist. It is suggested that changes in nerve-evoked muscle activity lead to a variety of activity correlates such as increases in free intracellular Ca²⁺ levels caused by influx across the cell membrane and/or release from the sarcoplasmatic reticulum, concentrations of metabolites such as lipids and ADP, hypoxia and mechanical stress. Such correlates are detected by sensors such as protein kinase C (PKC), calmodulin, AMP-activated kinase (AMPK), peroxisome proliferator-activated receptor δ (PPARδ), and oxygen dependent prolyl hydroxylases that trigger intracellular signaling cascades. These complex cascades involve several transcription factors such as nuclear factor of activated T-cells (NFAT), myocyte enhancer factor 2 (MEF2), myogenic differentiation factor (myoD), myogenin, PPARδ, and sine oculis homeobox 1/eyes absent 1 (Six1/Eya1). These factors might act indirectly by inducing gene products that act back on the cascade, or as ultimate transcription factors binding to and transactivating/repressing genes for the fast and slow isoforms of various contractile proteins and of metabolic enzymes. The determination of size and force is even more complex as this involves not only intracellular signaling within the muscle fibres, but also muscle stem cells called satellite cells. Intercellular signaling substances such as myostatin and insulin-like growth factor 1 (IGF-1) seem to act in a paracrine fashion. Induction of hypertrophy is accompanied by the satellite cells fusing to myofibres and thereby increasing the capacity for protein synthesis. These extra nuclei seem to remain part of the fibre even during subsequent atrophy as a form of muscle memory facilitating retraining. In addition to changes in myonuclear number during hypertrophy, changes in muscle fibre size seem to be caused by alterations in transcription, translation (per nucleus) and protein degradation.

Arsenic inhibits myogenic differentiation and muscle regeneration

BACKGROUND

The incidence of low birth weights is increased in offspring of women who are exposed to high concentrations of arsenic in drinking water compared with other women. We hypothesized that effects of arsenic on birth weight may be related to effects on myogenic differentiation.

OBJECTIVE

We investigated the effects of arsenic trioxide (As2O3) on the myogenic differentiation of myoblasts in vitro and muscle regeneration in vivo.

METHODS

C2C12 myoblasts and primary mouse and human myoblasts were cultured in differentiation media with or without As2O3 (0.1-0.5 microM) for 4 days. Myogenic differentiation was assessed by myogenin and myosin heavy chain expression and multinucleated myotube formation in vitro; skeletal muscle regeneration was tested using an in vivo mouse model with experimental glycerol myopathy.

RESULTS

A submicromolar concentration of As2O3 dose-dependently inhibited myogenic differentiation without apparent effects on cell viability. As2O3 significantly and dose-dependently decreased phosphorylation of Akt and p70s6k proteins during myogenic differentiation. As2O3-induced inhibition in myotube formation and muscle-specific protein expression was reversed by transfection with the constitutively active form of Akt. Sections of soleus muscles stained with hematoxylin and eosin showed typical changes of injury and regeneration after local glycerol injection in mice. Regeneration of glycerol-injured soleus muscles, myogenin expression, and Akt phosphorylation were suppressed in muscles isolated from As2O3-treated mice compared with untreated mice.

CONCLUSION

Our results suggest that As2O3 inhibits myogenic differentiation by inhibiting Akt-regulated signaling.

MyoD expression profile and developmental differences of leg and breast muscle in Peking duck (Anas platyrhynchos Domestica) during embryonic to neonatal stages

In order to investigate the developmental differences between the duck breast muscle and leg muscle tissues during the embryonic stage to neonatal stages, as well as the expression profile of MyoD between the two muscle tissues, the morphologic characteristics in the two muscle tissues during duck embryo stages at E14, E18, E22, E27 and D7 were compared through the muscle paraffin sections. The coding domain sequence of duck MyoD gene was cloned, and then the expression of MyoD in duck leg muscle and breast muscle during embryo stage on E10, E14, E18, E22, E27 and D7 was detected using qRT-PCR method. Results showed that the developmental status of the duck breast muscle in embryonic phrases lag behind that of leg muscle. The CDS of duck MyoD gene consists of 894 nucleotides, and showed relatively high similarity with the gene of other species. The MyoD mRNA expressed in both kinds of muscle tissues and the expression profile had a similar trend, although the expression level of MyoD in the breast muscle was significantly higher than that in the leg muscle at each developmental stages (p<0.05). Results suggested that MyoD might have potential functions in controlling muscle fiber phenotype during the secondary myogenesis of muscle development. These fundamental works may provide some valuable clues for knowing the roles of MyoD in the myogenesis and the muscle fiber type differentiation in birds.

Analysis of epigenetic alterations to chromatin during development

Each cell within a multicellular organism has distinguishable characteristics established by its unique patterns of gene expression. This individual identity is determined by the expression of genes in a time and place-dependent manner, and it is becoming increasingly clear that chromatin plays a fundamental role in the control of gene transcription in multicellular organisms. Therefore, understanding the regulation of chromatin and how the distinct identity of a cell is passed to daughter cells during development is paramount. Techniques with which to study chromatin have advanced rapidly over the past decade. Development of high throughput techniques and their proper applications has provided us essential tools to understand the regulation of epigenetic phenomena and its effect on gene expression. Understanding the changes that occur in chromatin during the course of development will not only contribute to our knowledge of normal gene expression, but will also add to our knowledge of how gene expression goes awry during disease. This review opens with an introduction to some of the key premises of epigenetic regulation of gene expression. A discussion of experimental techniques with which one can study epigenetic alterations to chromatin during development follows, emphasizing recent breakthroughs in this area. We then present examples of epigenetic mechanisms exploited in the control of developmental cell fate and regulation of tissue-specific gene expression. Finally, we discuss some of the frontiers and challenges in this area of research.

Ectopic calcification is caused by elevated levels of serum inorganic phosphate in mdx mice

Ectopic calcification occurs in the skeletal muscle of mdx mice, a dystrophin-deficient animal model of Duchenne muscular dystrophy. The purpose of this study was to clarify the mechanism of the calcification. The calcified deposits were identified as hydroxyapatite, a crystallized form of calcium phosphate, and the serum inorganic phosphate (Pi) level in the mdx mice was approximately 1.4 times higher than that in the normal B10 mice, suggesting that Pi plays a critical role in the ectopic calcification. When C2C12 mouse myoblasts were cultured under high-Pi conditions, myogenic differentiation was retarded while the expression of osteogenic markers such as osteocalcin and Runx2 were upregulated. This was followed by the generation of calcium deposition. Moreover, ectopic calcification reduced to an undetectable level in most of the mdx mice fed a Pi-reduced diet. We therefore conclude that the Pi-induced osteogenesis of muscle cells is responsible for ectopic calcification in the skeletal muscle of mdx mice.

Myogenin regulates a distinct genetic program in adult muscle stem cells

In contrast to the detailed understanding we have for the regulation of skeletal muscle gene expression in embryos, similar insights into postnatal muscle growth and regeneration are largely inferential or do not directly address gene regulatory mechanisms. Muscle stem cells (satellite cells) are chiefly responsible for providing new muscle during postnatal and adult life. The purpose of this study was to determine the role that the myogenic basic helix-loop-helix regulatory factor myogenin has in postnatal muscle growth and adult muscle stem cell gene expression. We found that myogenin is absolutely required for skeletal muscle development and survival until birth, but it is dispensable for postnatal life. However, Myog deletion after birth led to reduced body size implying a role for myogenin in regulating body homeostasis. Despite a lack of skeletal muscle defects in Myog-deleted mice during postnatal life and the efficient differentiation of cultured Myog-deleted adult muscle stem cells, the loss of myogenin profoundly altered the pattern of gene expression in cultured muscle stem cells and adult skeletal muscle. Remarkably, these changes in gene expression were distinct from those found in Myog-null embryonic skeletal muscle, indicating that myogenin has separate functions during postnatal life.

Molecular characterization and expression patterns of AMP deaminase1 (AMPD1) in porcine skeletal muscle

AMPD1 is the muscle-specific form of the AMPD multigene families in mammals and plays an important role in the purine nucleotide cycle and energy metabolism in skeletal muscle. In this study, we cloned and characterized AMPD1 from Sus scrofa muscle. The promoter of porcine AMPD1 contained several putative muscle-specific transcription factor binding sites (E box, myogenin, MEF2, Spl-CTF/NF-l), one RORalpha2 binding motif and NF-kappaB site. The deduced amino acid sequence of porcine AMPD1 contains an AMP deaminase signature sequence (SLSTDDP). RT-PCR analyses showed that AMPD1 was expressed specifically in skeletal muscle. Expression of AMPD1 was up-regulated during the muscle development and was higher in Yorkshire than in Meishan pigs. AMPD1 gene was expressed at higher levels in longissimus dorsi and bicepsfemoris muscles compared with soleus and masseter muscles in both Yorkshire and Meishan pigs. Moreover, we found that a single nucleotide polymorphism (SNP, T/C(426)) in exon12 of the AMPD1 gene was significantly associated with loin muscle area trait (p<0.01), loin muscle height (p<0.01) and average backfat thickness (p<0.05). This result suggests that the AMPD1 gene might be a candidate gene of meat production trait and provides useful information for further studies on its roles in porcine skeletal muscle.

Integration of embryonic and fetal skeletal myogenic programs at the myosin light chain 1f/3f locus

The genetic control of skeletal muscle differentiation at the onset of myogenesis in the embryo is relatively well understood compared to the formation of muscle during the fetal period giving rise to the bulk of skeletal muscle fibers at birth. The Mlc1f/3f (Myl1) locus encodes two alkali myosin light chains, Mlc1f and Mlc3f, from two promoters that are differentially regulated during development. The Mlc1f promoter is active in embryonic, fetal and adult fast skeletal muscle whereas the Mlc3f promoter is upregulated during fetal development and remains on in adult fast skeletal muscle. Two enhancer elements have been identified at the mammalian Mlc1f/3f locus, a 3' element active at all developmental stages and an intronic enhancer activated during fetal development. Here, using transgenesis, we demonstrate that these enhancers act combinatorially to confer the spatial, temporal and quantitative expression profile of the endogenous Mlc3f promoter. Using double reporter transgenes we demonstrate that each enhancer can activate both Mlc1f and Mlc3f promoters in vivo, revealing enhancer sharing rather than exclusive enhancer-promoter interactions. Finally, we demonstrate that the fetal activated enhancer contains critical E-box myogenic regulatory factor binding sites and that enhancer activation is impaired in vivo in the absence of myogenin but not in the absence of innervation. Together our observations provide insights into the regulation of fetal myogenesis and the mechanisms by which temporally distinct genetic programs are integrated at a single locus.