Specific activation of the acetylcholine receptor subunit genes by MyoD family proteins

Acute high-intensity muscle contraction moderates AChR gene expression independent of rapamycin-sensitive mTORC1 pathway in rat skeletal muscle

The relationship between mechanistic target of rapamycin complex 1 (mTORC1) activation after resistance exercise and acetylcholine receptor (AChR) subunit gene expression remains largely unknown. Therefore, we aimed to investigate the effect of electrical stimulation-induced intense muscle contraction, which mimics acute resistance exercise, on the mRNA expression of AChR genes and the signalling pathways involved in neuromuscular junction (NMJ) maintenance, such as mTORC1 and muscle-specific kinase (MuSK). The gastrocnemius muscle of male adult Sprague-Dawley rats was isometrically exercised. Upon completion of muscle contraction, the rats were euthanized in the early (after 0, 1, 3, 6 or 24 h) and late (after 48 or 72 h) recovery phases and the gastrocnemius muscles were removed. Non-exercised control animals were euthanized in the basal state (control group). In the early recovery phase, Agrn gene expression increased whereas LRP4 decreased without any change in the protein and gene expression of AChR gene subunits. In the late recovery phase, Agrn, Musk, Chrnb1, Chrnd and Chrne gene expression were altered and agrin and MuSK protein expression increased. Moreover, mTORC1 and protein kinase B/Akt-histone deacetylase 4 (HDAC) were activated in the early phase but not in the late recovery phase. Furthermore, rapamycin, an inhibitor of mTORC1, did not disturb changes in AChR subunit gene expression after muscle contraction. However, rapamycin addition slightly increased AChR gene expression, while insulin did not impact it in rat L6 myotube. These results suggest that changes in the AChR subunits after muscle contraction are independent of the rapamycin-sensitive mTORC1 pathway.

Aberrant evoked calcium signaling and nAChR cluster morphology in a SOD1 D90A hiPSC-derived neuromuscular model

Familial amyotrophic lateral sclerosis (ALS) is a progressive neuromuscular disorder that is due to mutations in one of several target genes, including SOD1. So far, clinical records, rodent studies, and in vitro models have yielded arguments for either a primary motor neuron disease, or a pleiotropic pathogenesis of ALS. While mouse models lack the human origin, in vitro models using human induced pluripotent stem cells (hiPSC) have been recently developed for addressing ALS pathogenesis. In spite of improvements regarding the generation of muscle cells from hiPSC, the degree of maturation of muscle cells resulting from these protocols has remained limited. To fill these shortcomings, we here present a new protocol for an enhanced myotube differentiation from hiPSC with the option of further maturation upon coculture with hiPSC-derived motor neurons. The described model is the first to yield a combination of key myogenic maturation features that are consistent sarcomeric organization in association with complex nAChR clusters in myotubes derived from control hiPSC. In this model, myotubes derived from hiPSC carrying the SOD1 D90A mutation had reduced expression of myogenic markers, lack of sarcomeres, morphologically different nAChR clusters, and an altered nAChR-dependent Ca2+ response compared to control myotubes. Notably, trophic support provided by control hiPSC-derived motor neurons reduced nAChR cluster differences between control and SOD1 D90A myotubes. In summary, a novel hiPSC-derived neuromuscular model yields evidence for both muscle-intrinsic and nerve-dependent aspects of neuromuscular dysfunction in SOD1-based ALS.

Immune-mediated myogenesis and acetylcholine receptor clustering promote a slow disease progression in ALS mouse models

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is a heterogeneous disease in terms of onset and progression rate. This may account for therapeutic clinical trial failure. Transgenic SOD1G93A mice on C57 or 129Sv background have a slow and fast disease progression rate, mimicking the variability observed in patients. Based on evidence inferring the active influence of skeletal muscle on ALS pathogenesis, we explored whether dysregulation in hindlimb skeletal muscle reflects the phenotypic difference between the two mouse models.

METHODS

Ex vivo immunohistochemical, biochemical, and biomolecular methodologies, together with in vivo electrophysiology and in vitro approaches on primary cells, were used to afford a comparative and longitudinal analysis of gastrocnemius medialis between fast- and slow-progressing ALS mice.

RESULTS

We reported that slow-progressing mice counteracted muscle denervation atrophy by increasing acetylcholine receptor clustering, enhancing evoked currents, and preserving compound muscle action potential. This matched with prompt and sustained myogenesis, likely triggered by an early inflammatory response switching the infiltrated macrophages towards a M2 pro-regenerative phenotype. Conversely, upon denervation, fast-progressing mice failed to promptly activate a compensatory muscle response, exhibiting a rapidly progressive deterioration of muscle force.

CONCLUSIONS

Our findings further pinpoint the pivotal role of skeletal muscle in ALS, providing new insights into underestimated disease mechanisms occurring at the periphery and providing useful (diagnostic, prognostic, and mechanistic) information to facilitate the translation of cost-effective therapeutic strategies from the laboratory to the clinic.

Xenopus SOX5 enhances myogenic transcription indirectly through transrepression

In anamniotes, somite compartimentalization in the lateral somitic domain leads simultaneously to myotome and dermomyotome formation. In the myotome, Xenopus Sox5 is co-expressed with Myod1 in the course of myogenic differentiation. Here, we studied the function of Sox5 using a Myod1-induced myogenic transcription assay in pluripotent cells of animal caps. We found that Sox5 enhances myogenic transcription of muscle markers Des, Actc1, Ckm and MyhE3. The use of chimeric transactivating or transrepressive Sox5 proteins indicates that Sox5 acts as a transrepressor and indirectly stimulates myogenic transcription except for the slow muscle-specific genes Myh7L, Myh7S, Myl2 and Tnnc1. We showed that this role is shared by Sox6, which is structurally similar to Sox5, both belonging to the SoxD subfamily of transcription factors. Moreover, Sox5 can antagonize the inhibitory function of Meox2 on myogenic differentiation. Meox2 which is a dermomyotome marker, represses myogenic transcription in Myod-induced myogenic transcription assay and in Nodal5-induced mesoderm from animal cap assay. The inhibitory function of Meox2 and the pro-myogenic function of Sox5 were confirmed during Xenopus normal development by the use of translation-blocking oligomorpholinos and dexamethasone inducible chimeric Sox5 and Meox2 proteins. We have therefore identified a new function for SoxD proteins in muscle cells, which can indirectly enhance myogenic transcription through transrepression, in addition to the previously identified function as a direct repressor of slow muscle-specific genes.

HCN2 Rescues brain defects by enforcing endogenous voltage pre-patterns

Endogenous bioelectrical signaling coordinates cell behaviors toward correct anatomical outcomes. Lack of a model explaining spatialized dynamics of bioelectric states has hindered the understanding of the etiology of some birth defects and the development of predictive interventions. Nicotine, a known neuroteratogen, induces serious defects in brain patterning and learning. Our bio-realistic computational model explains nicotine’s effects via the disruption of endogenous bioelectrical gradients and predicts that exogenous HCN2 ion channels would restore the endogenous bioelectric prepatterns necessary for brain patterning. Voltage mapping in vivo confirms these predictions, and exogenous expression of the HCN2 ion channel rescues nicotine-exposed embryos, resulting in normal brain morphology and molecular marker expression, with near-normal learning capacity. By combining molecular embryology, electrophysiology, and computational modeling, we delineate a biophysical mechanism of developmental brain damage and its functional rescue.

The authors have previously shown that membrane voltage can influence embryonic patterning during development. Here, the authors computationally model how nicotine disrupts Xenopus embryogenesis by perturbing voltage gradients, and rescue nicotine-inducted defects with HCN2 channel expression.

Comparison of neurotoxic potency between a novel chinbotulinumtoxinA with onabotulinumtoxinA, incobotulinumtoxinA and lanbotulinumtoxinA in rats

Four botulinumtoxin type A (BoNT/A) products, onabotulinumtoxinA (A/Ona), incobotulinumtoxinA (A/Inco), lanbotulinumtoxinA (A/Lan) and chinbotulinumtoxinA (A/Chin), are applied in the present study, among which A/Chin is newly produced. We aimed to compare the neurotoxic potency of these toxins by the gauge of muscle strength reduction. Furthermore, potential molecular and cellular mechanisms were also explored. According to our data, muscle strengths in the four toxin groups were all significantly decreased after injection for 1 week. A/Chin achieved the most obvious reduction in muscle strength as compared to the other three products at the dose of 0.5 U. However, there was no difference between the four toxins when increased to 2 U. As the toxins wore off, muscle strength recovered to basal level 12 weeks postinjection. We further measured the expression levels of key factors involved in neuromuscular junction stabilization and muscle genesis. Our results showed that nicotinic acetylcholine receptor, myogenic regulatory factors and muscle-specific receptor tyrosine kinase were all significantly upregulated upon BoNT/A treatment. Consistent with the result of muscle strength, A/Chin had the most obvious induction of gene expression. Moreover, we also found local inflammation response following BoNT/A injection. Owing to lack of complexing proteins, both A/Inco and A/Chin stimulated relatively lighter inflammation compared to that of A/Ona and A/Lan groups. In conclusion, our study provided evidence for the efficacy of the novel A/Chin and its similar functional mode to that of A/Ona, A/Inco and A/Lan. In addition, A/Chin has superiority in inducing muscle paralysis and inflammation stimulation, which may indicate faster onset and longer duration of this novel A/Chin.

Neuromuscular junctions (NMJs): ultrastructural analysis and nicotinic acetylcholine receptor (nAChR) subunit mRNA expression in offspring subjected to protein restriction throughout pregnancy

Protein restriction during gestation can alter the skeletal muscle phenotype of offspring; however, little is known with regard to whether this also affects the neuromuscular junction (NMJ), as muscle phenotype maintenance depends upon NMJ functional integrity. This study aimed to evaluate the effects of a low protein (6%) intake by dams throughout gestation on male offspring NMJ morphology and nicotinic acetylcholine receptor (nAChR) α1, γ and ε subunit expression in the soleus (SOL) and extensor digitorum longus (EDL) muscles. Four groups of male Wistar offspring rats were studied. The offspring of dams fed low-protein (6% protein, LP) and normal protein (17% protein, NP) diets were evaluated at 30 and 120 days of age, and the SOL and EDL muscles were collected for analysis. Morphological studies using transmission electron microscopy revealed that only SOL NMJs were affected in 30-day-old offspring in the LP group compared with the NP group. SOL NMJs exhibited fewer synaptic folds, the postsynaptic membranes were smooth and myelin figures were also frequently observed in the terminal axons. With regard to the expression of mRNAs encoding nAChR subunits, only 30-day-old LP offspring EDL muscles exhibited reduced α, γ and ε subunit expression compared with the NP group. In conclusion, our results demonstrate that a low-protein diet (6%) imposed throughout pregnancy impairs the expression of mRNAs encoding the nAChR α, γ and ε subunits in EDL NMJs and promotes morphological changes in SOL NMJs of 30-day-old offspring, indicating specific differences among muscle types following long-term maternal protein restriction.

Making muscle: Morphogenetic movements and molecular mechanisms of myogenesis in Xenopus laevis

Xenopus laevis offers unprecedented access to the intricacies of muscle development. The large, robust embryos make it ideal for manipulations at both the tissue and molecular level. In particular, this model system can be used to fate map early muscle progenitors, visualize cell behaviors associated with somitogenesis, and examine the role of signaling pathways that underlie induction, specification, and differentiation of muscle. Several characteristics that are unique to X. laevis include myogenic waves with distinct gene expression profiles and the late formation of dermomyotome and sclerotome. Furthermore, myogenesis in the metamorphosing frog is biphasic, facilitating regeneration studies. In this review, we describe the morphogenetic movements that shape the somites and discuss signaling and transcriptional regulation during muscle development and regeneration. With recent advances in gene editing tools, X. laevis remains a premier model organism for dissecting the complex mechanisms underlying the specification, cell behaviors, and formation of the musculature system.

Key changes in denervated muscles and their impact on regeneration and reinnervation

The neuromuscular junction becomes progressively less receptive to regenerating axons if nerve repair is delayed for a long period of time. It is difficult to ascertain the denervated muscle's residual receptivity by time alone. Other sensitive markers that closely correlate with the extent of denervation should be found. After a denervated muscle develops a fibrillation potential, muscle fiber conduction velocity, muscle fiber diameter, muscle wet weight, and maximal isometric force all decrease; remodeling increases neuromuscular junction fragmentation and plantar area, and expression of myogenesis-related genes is initially up-regulated and then down-regulated. All these changes correlate with both the time course and degree of denervation. The nature and time course of these denervation changes in muscle are reviewed from the literature to explore their roles in assessing both the degree of detrimental changes and the potential success of a nerve repair. Fibrillation potential amplitude, muscle fiber conduction velocity, muscle fiber diameter, mRNA expression levels of myogenic regulatory factors and nicotinic acetylcholine receptor could all reflect the severity and length of denervation and the receptiveness of denervated muscle to regenerating axons, which could possibly offer an important clue for surgical choices and predict the outcomes of delayed nerve repair.

Molecular mechanisms of treadmill therapy on neuromuscular atrophy induced via botulinum toxin A

Botulinum toxin A (BoNT-A) is a bacterial zinc-dependent endopeptidase that acts specifically on neuromuscular junctions. BoNT-A blocks the release of acetylcholine, thereby decreasing the ability of a spastic muscle to generate forceful contraction, which results in a temporal local weakness and the atrophy of targeted muscles. BoNT-A-induced temporal muscle weakness has been used to manage skeletal muscle spasticity, such as poststroke spasticity, cerebral palsy, and cervical dystonia. However, the combined effect of treadmill exercise and BoNT-A treatment is not well understood. We previously demonstrated that for rats, following BoNT-A injection in the gastrocnemius muscle, treadmill running improved the recovery of the sciatic functional index (SFI), muscle contraction strength, and compound muscle action potential (CMAP) amplitude and area. Treadmill training had no influence on gastrocnemius mass that received BoNT-A injection, but it improved the maximal contraction force of the gastrocnemius, and upregulation of GAP-43, IGF-1, Myo-D, Myf-5, myogenin, and acetylcholine receptor (AChR) subunits α and β was found following treadmill training. Taken together, these results suggest that the upregulation of genes associated with neurite and AChR regeneration following treadmill training may contribute to enhanced gastrocnemius strength recovery following BoNT-A injection.

Treadmill running upregulates the expression of acetylcholine receptor in rat gastrocnemius following botulinum toxin A injection

Treadmill running is a commonly used training method for patients with spasticity to improve functional performance. Botulinum toxin has been widely used therapeutically to reduce contraction force of spastic muscle. However, the effects of treadmill running in neuromuscular junction expression and motor unit physiology on muscle following botulinum toxin injection are not well established. To assess the effects of treadmill running on neuromuscular recovery of gastrocnemius following botulinum toxin A (BoNT-A) injection, we observed changes in gene expression. We hypothesized that the expression of acetylcholine receptor (AChR), myogenesis, and nerve plasticity could be enhanced. Twenty-four Sprague-Dawley rats received botulinum toxin injection in right gastrocnemius and were then randomly assigned into untrained control and treadmill running groups. The rats assigned to the treadmill running group were trained on a treadmill 3 times/week with a running speed of 15 m/min for 8 weeks. The duration of training was 20 min per session. Muscle strength and gene expression of AChR subunit (α, β, δ, γ, and ε), MyoD, Myf-5, MRF4, myogenin, p21, IGF-1, GAP43, were analyzed. Treadmill running had no influence on gastrocnemius mass, but improved the maximal contraction force of the gastrocnemius in the treadmill running group (p < 0.05). Upregulation of GAP-43, IGF-1, Myo-D, Myf-5, myogenin, and AChR subunits α and β were found following treadmill running. The expression of genes associated with neurite and AChR regeneration following treadmill exercise was upregulated, which may have contributed to enhanced recovery of gastrocnemius strength.

Myogenic waves and myogenic programs during Xenopus embryonic myogenesis

Although Xenopus is a key model organism in developmental biology, little is known about the myotome formation in this species. Here, we assessed the expression of myogenic regulatory factors of the Myod family (MRFs) during embryonic development and revealed distinct MRF programs.

RESULTS

The expression pattern of each MRF during embryonic development highlights three successive myogenic waves. We showed that a first median and lateral myogenesis initiates before dermomyotome formation: the median cell population expresses Myf5, Myod, and Mrf4, whereas the lateral one expresses Myod, moderate levels of Myogenin and Mrf4. The second wave of myoblasts arising from the dermomyotome is characterized by the full MRF program expression, with high levels of Myogenin. The third wave is revealed by Myf5 expression in the myotome and could contribute to the formation of plurinucleated fibers at larval stages. Furthermore, Myf5- or Myod-expressing anlagen are identified in craniofacial myogenesis.

CONCLUSIONS

The first median and lateral myogenesis and their associated MRF programs have probably disappeared in mammals. However, some aspects of Xenopus myogenesis have been conserved such as the development of somitic muscles by successive myogenic waves and the existence of Myf5-dependent and -independent lineages.

Morphological aspects of neuromuscular junctions and gene expression of nicotinic acetylcholine receptors (nAChRs) in skeletal muscle of rats with heart failure

HF is syndrome initiated by a reduction in cardiac function and it is characterized by the activation of compensatory mechanisms. Muscular fatigue and dyspnoea are the more common symptoms in HF; these may be due in part to specific skeletal muscle myopathy characterized by reduced oxidative capacity, a shift from slow fatigue resistant type I to fast less fatigue resistant type II fibers and downregulation of myogenic regulatory factors (MRFs) gene expression that can regulate gene expression of nicotinic acetylcholine receptors (nAChRs). In chronic heart failure, skeletal muscle phenotypic changes could influence the maintenance of the neuromuscular junction morphology and nAChRs gene expression during this syndrome. Two groups of rats were studied: control (CT) and Heart Failure (HF), induced by a single intraperitoneal injection of monocrotaline (MCT). At the end of the experiment, HF was evaluated by clinical signs and animals were sacrificed. Soleus (SOL) muscles were removed and processed for morphological, morphometric and molecular NMJ analyses. Our major finding was an up-regulation in the gene expression of the alpha1 and epsilon subunits of nAChR and a spot pattern of nAChR in SOL skeletal muscle in this acute monocrotaline induced HF. Our results suggest a remodeling of nAChR alpha1 and epsilon subunit during heart failure and may provide valuable information for understanding the skeletal muscle myopathy that occurs during this syndrome.

The α1 subunit of nicotinic acetylcholine receptors in the inner ear: transcriptional regulation by ATOH1 and co-expression with the γ subunit in hair cells

Acetylcholine is a key neurotransmitter of the inner ear efferent system. In this study, we identify two novel nAChR subunits in the inner ear: α1 and γ, encoded by Chrna1 and Chrng, respectively. In situ hybridization shows that the messages of these two subunits are present in vestibular and cochlear hair cells during early development. Chrna1 and Chrng expression begin at embryonic stage E13.5 in the vestibular system and E17.5 in the organ of Corti. Chrna1 message continues through P7, whereas Chrng is undetectable at post-natal stage P6. The α1 and γ subunits are known as muscle-type nAChR subunits and are surprisingly expressed in hair cells which are sensory-neural cells. We also show that ATOH1/MATH1, a transcription factor essential for hair cell development, directly activates CHRNA1 transcription. Electrophoretic mobility-shift assays and supershift assays showed that ATOH1/E47 heterodimers selectively bind on two E boxes located in the proximal promoter of CHRNA1. Thus, Chrna1 could be the first transcriptional target of ATOH1 in the inner ear. Co-expression in Xenopus oocytes of the α1 subunit does not change the electrophysiological properties of the α9α10 receptor. We suggest that hair cells transiently express α1γ-containing nAChRs in addition to α9α10, and that these may have a role during development of the inner ear innervation.

The RNA-binding protein Seb4/RBM24 is a direct target of MyoD and is required for myogenesis during Xenopus early development

RNA-binding proteins play an important role to post-transcriptionally regulate gene expression. During early development they exhibit temporally and spatially regulated expression pattern. The expression of Xenopus laevis Seb4 gene, also known as RBM24 in other vertebrates, is restricted to the lateral and ventral mesoderm during gastrulation and then localized to the somitic mesoderm, in a similar pattern as XMyoD gene. Using a hormone-inducible form of MyoD to identify potential direct MyoD target genes, we find that Seb4 expression is directly regulated by MyoD at the gastrula stage. We further show that a 0.65kb X. tropicalis RBM24 regulatory region contains multiple E boxes (CANNTG), which are potential binding sites for MyoD and other bHLH proteins. By injecting a RBM24 reporter construct into the animal pole of X. laevis embryos, we find that this reporter gene is indeed specifically activated by MyoD and repressed by a dominant negative MyoD mutant. Knockdown of Seb4 produces similar effects as those obtained by the dominant negative MyoD mutant, indicating that it is required for the expression of myogenic genes and myogenesis in the embryo. In cultured ectodermal explants, although overexpression of Seb4 has no obvious effect on myogenesis, knockdown of Seb4 inhibits the expression of myogenic genes and myogenesis induced by MyoD. These results reveal that Seb4 is a target of MyoD during myogenesis and is required for myogenic gene expression.

The Xenopus MEF2 gene family: evidence of a role for XMEF2C in larval tendon development

MEF2 transcription factors are well-established regulators of muscle development. In this report, we describe the cloning of multiple splicing isoforms of the XMEF2A and XMEF2C encoding genes, differentially expressed during Xenopus development. Using whole-mount in situ hybridization, we found that the accumulation of XMEF2C mRNA in the tadpole stages was restricted to intersomitic regions and to the peripheral edges of hypaxial and cranial muscle masses in contrast to XMEF2A and XMEF2D, characterized by a continuous muscle cell expression. The XMEF2C positive cells express the bHLH transcription factor, Xscleraxis, known as a specific marker for tendons. Gain of function experiments revealed that the use of a hormone-inducible XMEF2C construct is able to induce Xscleraxis expression. Furthermore, XMEF2C specifically cooperates with Xscleraxis to induce tenascin C and betaig-h3, two genes preferentially expressed in Xenopus larval tendons. These findings 1) highlight a previously unappreciated and specific role for XMEF2C in tendon development and 2) identify a novel gene transactivation pathway where MEF2C cooperates with the bHLH protein, Xscleraxis, to activate specific gene expression.

Dynamics of mutated GFAP aggregates revealed by real-time imaging of an astrocyte model of Alexander disease

Alexander disease (AxD) is a rare neurodegenerative disorder characterized by large cytoplasmic aggregates in astrocytes and myelin abnormalities and caused by dominant mutations in the gene encoding glial fibrillary acidic protein (GFAP), the main intermediate filament protein in astrocytes. We tested the effects of three mutations (R236H, R76H and L232P) associated with AxD in cells transiently expressing mutated GFAP fused to green fluorescent protein (GFP). Mutated GFAP-GFP expressed in astrocytes formed networks or aggregates similar to those found in the brains of patients with the disease. Time-lapse recordings of living astrocytes showed that aggregates of mutated GFAP-GFP may either disappear, associated with cell survival, or coalesce in a huge juxtanuclear structure associated with cell death. Immunolabeling of fixed cells suggested that this gathering of aggregates forms an aggresome-like structure. Proteasome inhibition and immunoprecipitation assays revealed mutated GFAP-GFP ubiquitination, suggesting a role of the ubiquitin-proteasome system in the disaggregation process. In astrocytes from wild-type-, GFAP-, and vimentin-deficient mice, mutated GFAP-GFP aggregated or formed a network, depending on qualitative and quantitative interactions with normal intermediate filament partners. Particularly, vimentin displayed an anti-aggregation effect on mutated GFAP. Our data indicate a dynamic and reversible aggregation of mutated GFAP, suggesting that therapeutic approaches may be possible.

How muscles recover from paresis and atrophy after intramuscular injection of botulinum toxin A: Study in juvenile rats

Botulinum toxin A (BoNT-A) is a potent biological toxin widely used for the management of skeletal muscle spasticity or dynamic joint contracture. Intramuscular injection of BoNT-A causes muscle denervation, paresis, and atrophy. This clinical effect of botulinum toxin A lasts 3 to 6 months, and injected muscle eventually regains muscle mass and recovers muscle function. The goal of the present study was to characterize the molecular and cellular mechanisms leading to neuromuscular junction (NMJ) regeneration and skeletal muscle functional recovery after BoNT-A injection. Fifty-six 1-month-old Sprague-Dawley rats were used. Botulinum toxin A was injected into the left gastrocnemius muscle at a dosage of 6 units/kg body weight. An equivalent volume of saline was injected into the right gastrocnemius muscle to serve as control. The gastrocnemius muscle samples were harvested from both hind limbs at 3 days, 7 days, 15 days, 30 days, 60 days, 90 days, 180 days, and 360 days after administration of toxin. In addition, the gastrocnemius muscles from 1-month-old rats with no injections were harvested to serve as uninjected control group. Muscle samples were processed and mRNA was extracted. Real-time polymerase chain reaction (PCR) and gene microarray technology were used to identify key molecules involved in NMJ stabilization and muscle functional recovery. More than 28,000 rat genes were analyzed and approximately 9000 genes are expressed in the rat gastrocnemius muscle. Seven days following BoNT-A injection, 105 genes were upregulated and 59 genes were downregulated. Key molecules involved in neuromuscular junction (NMJ) stabilization and muscle functional recovery were identified and their time course of gene expression following BoNT-A injection were characterized. This animal study demonstrates that following intramuscular injection of BoNT-A, there is a sequence of cellular events that eventually leads to NMJ stabilization, remodeling, and myogenesis and muscle functional recovery. This recovery process is divided into two stages (aneural and neural) and that the IGF-1 signaling pathway play a central role in the process.

Spatio-temporal expression of MRF4 transcripts and protein duringXenopus laevis embryogenesis

Whereas there have been extensive studies of the expression of XMyf5 and XMyoD during Xenopus embryogenesis, nothing is known about the spatio-temporal accumulation of XMRF4 transcripts and protein. In this report, we describe the cloning and characterization of two full-length MRF4 cDNAs and of their proximal promoters in Xenopus laevis. The comparison of the relative transcript levels of the XMRF4-a and -b genes in developing and adult muscles is highly suggestive of specific functions for the corresponding XMRF4 proteins. Whole-mount embryo in situ hybridization revealed the first XMRF4 transcripts in the more differentiated anterior myocytes of the embryo when the myosin heavy chain E3 mRNA begins to be detectable. XMRF4 mRNA accumulation later extended posteriorly but was never detected in the posterior unsegmented mesoderm, in contrast to XMyoD and XMyf-5. Whole-mount embryo immunohistochemistry revealed that XMRF4 protein accumulated in somite nuclei slightly after XMRF4 transcripts.

Analysis of mRNAs that are enriched in the post-synaptic domain of the neuromuscular junction

The identity of synaptically-enriched genes was investigated by comparing the abundance of various mRNAs in the synaptic and extra-synaptic regions of the same muscle fibers. The mRNAs for several known synaptic proteins were significantly elevated in the synaptic region when measured by real-time PCR. The synaptic mRNAs were then further analyzed using microarrays and real-time PCR to identify putative regulators of the neuromuscular junction (NMJ). MRF4 was the only member of the MyoD family that was concentrated at the mature NMJ, suggesting that it may have a unique role in the maintenance of post-synaptic specialization. Three potential regulators of the NMJ were identified and confirmed by real-time PCR: glia maturation factor gamma was concentrated at the NMJ whereas Unr protein and protein tyrosine phosphatase were repressed synaptically. The identification of synaptically-repressed genes may indicate that synaptic specialization is created by a combination of positive and negative signals.

Resting and load-induced levels of myogenic gene transcripts differ between older adults with demonstrable sarcopenia and young men and women

Regenerative capacity appears to be impaired in sarcopenic muscle. As local growth factors and myogenic regulatory factors (MRFs) modulate repair/regeneration responses after overload, we hypothesized that resistance loading (RL)-induced expression of MRFs and muscle IGF-I-related genes would be blunted in older (O) males (M) and females (F) with demonstrable sarcopenia vs. young (Y) adults. Y (20-35 yr, 10 YF, 10 YM) and O (60-75 yr, 9 OF, 9 OM) underwent vastus lateralis biopsy before and 24 h after knee extensor RL. Sarcopenia was assessed by cross-sectional area of type I, IIa, and IIx myofibers. Transcript levels were assessed by relative RT-PCR and analyzed by age x gender x load repeated-measures ANOVA. O were sarcopenic based on type II atrophy with smaller type IIa (P < 0.05) and IIx (P < 0.001) myofibers. Within-gender cross-sectional area differences were more marked in F (OF < YF: IIa 21%, IIx 42%). Load effects (P < 0.05) were seen for four of seven mRNAs as IGF-IEa (34%), myogenin (53%), and MyoD (20%) increased, and myf-6 declined 10%. Increased IGF-IEa was driven by O (48%) and/or M (43%). An age x gender x load interaction was found for MyoD (P < 0.05). An age x load interaction for type 1 IGF receptor (P < 0.05) was driven by a small increase in O (16%, P < 0.05). A gender x load interaction (P < 0.05) was noted for IGF binding protein-4. Age effects (P < 0.05) resulted from higher MyoD (54%), myf-5 (21%), and IGF binding protein-4 (17%) in O and were primarily localized to F at baseline (OF > YF; MyoD 94%, myf-5 47%, P < 0.05). We conclude that RL acutely increases mRNA expression of IGF-IEa and myogenin, which may promote growth/regeneration in both Y and O. Higher resting levels of MRFs in OF vs. YF suggest elevated basal regenerative activity in sarcopenic muscle of OF.

A selective role for MRF4 in innervated adult skeletal muscle: Na(V) 1.4 Na+ channel expression is reduced in MRF4-null mice

The factors that regulate transcription and spatial expression of the adult skeletal muscle Na+ channel, Na(V) 1.4, are poorly understood. Here we tested the role of the transcription factor MRF4, one of four basic helix-loop-helix (bHLH) factors expressed in skeletal muscle, in regulation of the Na(V) 1.4 Na+ channel. Overexpression of MRF4 in C2C12 muscle cells dramatically elevated Na(V) 1.4 reporter gene expression, indicating that MRF4 is more efficacious than the other bHLH factors expressed at high levels endogenously in these cells. In vivo, MRF4 protein was found both in extrajunctional and subsynaptic muscle nuclei. To test the importance of MRF4 in Na(V) 1.4 gene regulation in vivo, we examined Na+ channel expression in MRF4-null mice using several techniques, including Western blotting, immunocytochemistry, and electrophysiological recording. By all methods, we found that expression of the Na(V) 1.4 Na+ channel was substantially reduced in MRF4-null mice, both in the surface membrane and at neuromuscular junctions. In contrast, expression of the acetylcholine receptor, and in particular its alpha subunit, was unchanged, indicating that MRF4 regulation of Na+ channel expression was selective. Expression of the bHLH factors myf-5, MyoD, and myogenin was increased in MRF4-null mice, but these factors were not able to fully maintain Na(V) 1.4 Na+ channel expression either in the extrajunctional membrane or at the synapse. Thus, MRF4 appears to play a novel and selective role in adult muscle.

Myogenic regulatory factors: Redundant or specific functions? Lessons fromXenopus

The discovery, in the late 1980s, of the MyoD gene family of muscle transcription factors has proved to be a milestone in understanding the molecular events controlling the specification and differentiation of the muscle lineage. From gene knock-out mice experiments progressively emerged the idea that each myogenic regulatory factor (MRF) has evolved a specialized as well as a redundant role in muscle differentiation. To date, MyoD serves as a paradigm for the MRF mode of function. The features of gene regulation by MyoD support a model in which subprograms of gene expression are achieved by the combination of promoter-specific regulation of MyoD binding and MyoD-mediated binding of various ancillary proteins. This binding likely includes site-specific chromatin reorganization by means of direct or indirect interaction with remodeling enzymes. In this cascade of molecular events leading to the proper and reproducible activation of muscle gene expression, the role and mode of function of other MRFs still remains largely unclear. Recent in vivo findings using the Xenopus embryo model strongly support the concept that a single MRF can specifically control a subset of muscle genes and, thus, can be substituted by other MRFs albeit with dramatically lower efficiency. The topic of this review is to summarize the molecular data accounting for a redundant and/or specific involvement of each member of the MyoD family in myogenesis in the light of recent studies on the Xenopus model.

IGF-II is up-regulated and myofibres are hypertrophied in regenerating soleus of mice lacking FGF6

Important functions in myogenesis have been proposed for FGF6, a member of the fibroblast growth factor family accumulating almost exclusively in the myogenic lineage. However, the use of FGF6(-/-) mutant mice gave contradictory results and the role of FGF6 during myogenesis remains largely unclear. Using FGF6(-/-) mice, we first analysed the morphology of the regenerated soleus following cardiotoxin injection and showed hypertrophied myofibres in soleus of the mutant mice as compared to wild-type mice. Secondly, to examine the function of the IGF family in the hypertrophy process, we used semiquantitative and real-time RT-PCR assays and Western blots to monitor the expression of the insulin-like growth factors (IGF-I and IGF-II), their receptors [type I IGF receptor (IGF1R) and IGF-II receptor (IGF2R)], and of a binding protein IGFBP-5 in regenerating soleus muscles of FGF6(-/-) knockout mice vs. wild-type mice. In the mutant, both IGF-II and IGF2R, but not IGF-I and IGF1R, were strongly up-regulated, whereas IGFBP5 was down-regulated, strongly suggesting that, in the absence of FGF6, the mechanisms leading to myofibre hypertrophy were mediated specifically by an IGF-II/IGF2R signalling pathway distinct from the classic mechanism involving IGF-I and IGF1R previously described for skeletal muscle hypertrophy. The potential regulating role of IGFBP5 on IGF-II expression is also discussed. This report shows for the first time a specific role for FGF6 in the regulation of myofibre size during a process of in vivo myogenesis.