IGF-1R Reduction Triggers Neuroprotective Signaling Pathways in Spinal Muscular Atrophy Mice.

Spinal muscular atrophy (SMA) is a neuromuscular disease characterized by the selective loss of spinal motor neurons due to the depletion of the survival of motor neuron (SMN) protein. No therapy is currently available for SMA, which represents the leading genetic cause of death in childhood. In the present study, we report that insulin-like growth factor-1 receptor (Igf-1r) gene expression is enhanced in the spinal cords of SMA-like mice. The reduction of expression, either at the physiological (through physical exercise) or genetic level, resulted in the following: (1) a significant improvement in lifespan and motor behavior, (2) a significant motor neuron protection, and (3) an increase in SMN expression in spinal cord and skeletal muscles through both transcriptional and posttranscriptional mechanisms. Furthermore, we have found that reducing IGF-1R expression is sufficient to restore intracellular signaling pathway activation profile lying downstream of IGF-1R, resulting in both the powerful activation of the neuroprotective AKT/CREB pathway and the inhibition of the ERK and JAK pathways. Therefore, reducing rather than enhancing the IGF-1 pathway could constitute a useful strategy to limit neurodegeneration in SMA. SIGNIFICANCE STATEMENT: Recent evidence of IGF-1 axis alteration in spinal muscular atrophy (SMA), a very severe neurodegenerative disease affecting specifically the motor neurons, have triggered a renewed interest in insulin-like growth factor-1 (IGF-1) pathway activation as a potential therapeutic approach for motor neuron diseases. The present study challenges this point of view and brings the alternative hypothesis that reducing rather than enhancing the IGF-1 signaling pathway exerts a neuroprotective effect in SMA. Furthermore, the present data substantiate a newly emerging concept that the modulation of IGF-1 receptor expression is a key event selectively determining the activation level of intracellular pathways that lie downstream of the receptor. This aspect should be considered when designing IGF-1-based treatments for neurodegenerative diseases.

A new role for the calcineurin/NFAT pathway in neonatal myosin heavy chain expression via the NFATc2/MyoD complex during mouse myogenesis.

The calcineurin/NFAT (nuclear factor of activated T-cells) signaling pathway is involved in the modulation of the adult muscle fiber type, but its role in the establishment of the muscle phenotype remains elusive. Here, we show that the NFAT member NFATc2 cooperates with the basic helix-loop-helix transcription factor MyoD to induce the expression of a specific myosin heavy chain (MHC) isoform, the neonatal one, during embryogenesis. We found this cooperation to be crucial, as Myod/Nfatc2 double-null mice die at birth, with a dramatic reduction of the major neonatal MHC isoform normally expressed at birth in skeletal muscles, such as limb and intercostal muscles, whereas its expression is unaffected in myofibers mutated for either factor alone. Using gel shift and chromatin immunoprecipitation assays, we identified NFATc2 bound to the neonatal Mhc gene, whereas NFATc1 and NFATc3 would preferentially bind the embryonic Mhc gene. We provide evidence that MyoD synergistically cooperates with NFATc2 at the neonatal Mhc promoter. Altogether, our findings demonstrate that the calcineurin/NFAT pathway plays a new role in establishing the early muscle fiber type in immature myofibers during embryogenesis.

Myogenic waves and myogenic programs during Xenopus embryonic myogenesis.

UNLABELLED: 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.

Mef2d acts upstream of muscle identity genes and couples lateral myogenesis to dermomyotome formation in Xenopus laevis.

Xenopus myotome is formed by a first medial and lateral myogenesis directly arising from the presomitic mesoderm followed by a second myogenic wave emanating from the dermomyotome. Here, by a series of gain and loss of function experiments, we showed that Mef2d, a member of the Mef2 family of MADS-box transcription factors, appeared as an upstream regulator of lateral myogenesis, and as an inducer of dermomyotome formation at the beginning of neurulation. In the lateral presomitic cells, we showed that Mef2d transactivates Myod expression which is necessary for lateral myogenesis. In the most lateral cells of the presomitic mesoderm, we showed that Mef2d and Paraxis (Tcf15), a member of the Twist family of transcription factors, were co-localized and activate directly the expression of Meox2, which acts upstream of Pax3 expression during dermomyotome formation. Cell tracing experiments confirm that the most lateral Meox2 expressing cells of the presomitic mesoderm correspond to the dermomyotome progenitors since they give rise to the most dorsal cells of the somitic mesoderm. Thus, Xenopus Mef2d couples lateral myogenesis to dermomyotome formation before somite segmentation. These results together with our previous works reveal striking similarities between dermomyotome and tendon formation in Xenopus: both develop in association with myogenic cells and both involve a gene transactivation pathway where one member of the Mef2 family, Mef2d or Mef2c, cooperates with a bHLH protein of the Twist family, Paraxis or Scx (Scleraxis) respectively. We propose that these shared characteristics in Xenopus laevis reflect the existence of a vertebrate ancestral mechanism which has coupled the development of the myogenic cells to the formation of associated tissues during somite compartmentalization.

Apoptosis-inducing factor regulates skeletal muscle progenitor cell number and muscle phenotype.

Apoptosis Inducing Factor (AIF) is a highly conserved, ubiquitous flavoprotein localized in the mitochondrial intermembrane space. In vivo, AIF provides protection against neuronal and cardiomyocyte apoptosis induced by oxidative stress. Conversely in vitro, AIF has been demonstrated to have a pro-apoptotic role upon induction of the mitochondrial death pathway, once AIF translocates to the nucleus where it facilitates chromatin condensation and large scale DNA fragmentation. Given that the aif hypomorphic harlequin (Hq) mutant mouse model displays severe sarcopenia, we examined skeletal muscle from the aif hypomorphic mice in more detail. Adult AIF-deficient skeletal myofibers display oxidative stress and a severe form of atrophy, associated with a loss of myonuclei and a fast to slow fiber type switch, both in "slow" muscles such as soleus, as well as in "fast" muscles such as extensor digitorum longus, most likely resulting from an increase of MEF2 activity. This fiber type switch was conserved in regenerated soleus and EDL muscles of Hq mice subjected to cardiotoxin injection. In addition, muscle regeneration in soleus and EDL muscles of Hq mice was severely delayed. Freshly cultured myofibers, soleus and EDL muscle sections from Hq mice displayed a decreased satellite cell pool, which could be rescued by pretreating aif hypomorphic mice with the manganese-salen free radical scavenger EUK-8. Satellite cell activation seems to be abnormally long in Hq primary culture compared to controls. However, AIF deficiency did not affect myoblast cell proliferation and differentiation. Thus, AIF protects skeletal muscles against oxidative stress-induced damage probably by protecting satellite cells against oxidative stress and maintaining skeletal muscle stem cell number and activation.

In vivo NMDA receptor activation accelerates motor unit maturation, protects spinal motor neurons, and enhances SMN2 gene expression in severe spinal muscular atrophy mice.

Spinal muscular atrophy (SMA), a lethal neurodegenerative disease that occurs in childhood, is caused by the misexpression of the survival of motor neuron (SMN) protein in motor neurons. It is still unclear whether activating motor units in SMA corrects the delay in the postnatal maturation of the motor unit resulting in an enhanced neuroprotection. In the present work, we demonstrate that an adequate NMDA receptor activation in a type 2 SMA mouse model significantly accelerated motor unit postnatal maturation, counteracted apoptosis in the spinal cord, and induced a marked increase of SMN expression resulting from a modification of SMN2 gene transcription pattern. These beneficial effects were dependent on the level of NMDA receptor activation since a treatment with high doses of NMDA led to an acceleration of the motor unit maturation but favored the apoptotic process and decreased SMN expression. In addition, these results suggest that the NMDA-induced acceleration of motor unit postnatal maturation occurred independently of SMN. The NMDA receptor activating treatment strongly extended the life span in two different mouse models of severe SMA. The analysis of the intracellular signaling cascade that lay downstream the activated NMDA receptor revealed an unexpected reactivation of the CaMKII/AKT/CREB (cAMP response element-binding protein) pathway that induced an enhanced SMN expression. Therefore, pharmacological activation of spinal NMDA receptors could constitute a useful strategy for both increasing SMN expression and limiting motor neuron death in SMA spinal cord.

Motoneuron survival is promoted by specific exercise in a mouse model of amyotrophic lateral sclerosis.

Several studies using transgenic mouse models of familial amyotrophic lateral sclerosis (ALS) have reported a life span increase in exercised animals, as long as animals are submitted to a moderate-intensity training protocol. However, the neuroprotective potential of exercise is still questionable. To gain further insight into the cellular basis of the exercise-induced effects in neuroprotection, we compared the efficiency of a swimming-based training, a high-frequency and -amplitude exercise that preferentially recruits the fast motor units, and of a moderate running-based training, that preferentially triggers the slow motor units, in an ALS mouse model. Surprisingly, we found that the swimming-induced benefits sustained the motor function and increased the ALS mouse life span by about 25 days. The magnitude of this beneficial effect is one of the highest among those induced by any therapeutic strategy in this disease. We have shown that, unlike running, swimming significantly delays spinal motoneuron death and, more specifically, the motoneurons of large soma area. Analysis of the muscular phenotype revealed a swimming-induced relative maintenance of the fast phenotype in fast-twitch muscles. Furthermore, the swimming programme preserved astrocyte and oligodendrocyte populations in ALS spinal cord. As a whole, these data are highly suggestive of a causal relationship not only linking motoneuron activation and protection, but also motoneuron protection and the maintenance of the motoneuron surrounding environment. Basically, exercise-induced neuroprotective mechanisms provide an example of the molecular adaptation of activated motoneurons.

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.

Exercise-induced activation of NMDA receptor promotes motor unit development and survival in a type 2 spinal muscular atrophy model mouse.

Spinal muscular atrophy (SMA) is an inborn neuromuscular disorder caused by low levels of survival motor neuron protein, and for which no efficient therapy exists. Here, we show that the slower rate of postnatal motor-unit maturation observed in type 2 SMA-like mice is correlated with the motor neuron death. Physical exercise delays motor neuron death and leads to an increase in the postnatal maturation rate of the motor-units. Furthermore, exercise is capable of specifically enhancing the expression of the gene encoding the major activating subunit of the NMDA receptor in motor neurons, namely the NR2A subunit, which is dramatically downregulated in the spinal cord of type 2 SMA-like mice. Accordingly, inhibiting NMDA-receptor activity abolishes the exercise-induced effects on muscle development, motor neuron protection and life span gain. Thus, restoring NMDA-receptor function could be a promising therapeutic approach to SMA treatment.

Exercise-induced modulation of calcineurin activity parallels the time course of myofibre transitions.

This study establishes a causal link between the limitation of myofibre transitions and modulation of calcineurin activity, during different exercise paradigms. We have designed a new swimming-based training protocol in order to draw a comparison between a high frequency and amplitude exercise (swimming) and low frequency and amplitude exercise (running). We initially analysed the time course of muscle adaptations to a 6- or 12-week swimming- or running-based training exercise program, on two muscles of the mouse calf, the slow-twitch soleus and the fast-twitch plantaris. The magnitude of exercise-induced muscle plasticity proved to be dependent on both the muscle type and the exercise paradigm. In contrast to the running-based training which generated a continuous increase of the slow phenotype throughout a 12-week training program, swimming induced transitions to a slower phenotype which ended after 6 weeks of training. We then compared the time course of the exercise-induced changes in calcineurin activity during muscle adaptation to training. Both exercises induced an initial activation followed by the inhibition of calcineurin. In the muscles of animals submitted to a 12-week swimming-based training, this inhibition was concomitant with the end of myofibre transition. Calcineurin inhibition was a consequence of the inhibition of its catalytic subunit gene expression on one hand, and of the expression increase of the modulatory calcineurin interacting proteins 1 gene (MCIP1), on the other. The present study provides the first experimental cues for an interpretation of muscle phenotypic variation control.

Sprouty gene expression is regulated by nerve and FGF6 during regeneration of mouse muscles.

Sprouty (Spry) proteins were identified as negative regulators of fibroblast growth factor (FGF) signaling in vertebrates and invertebrates. Given the importance of the FGFs in myogenesis, we performed cardiotoxin injury-induced regeneration experiments on soleus muscles of both, adult control and FGF6 ( - / - ) mutant mice and analyzed the accumulation of Spry (1, 2 and 4) transcripts using semi-quantitative and real-time RT-PCR assays and in situ hybridization. We also analyzed the effects of muscle denervation on the accumulation of Spry transcripts. The three Spry genes begin to be expressed as early as the first stages of muscle regeneration and are characterized by distinct expression patterns. Moreover, Spry gene expression was highly and differentially up-regulated, precociously by the lack of FGF6, and belatedly by muscle denervation strongly suggesting that the transient rise of Spry mRNA accumulation was associated to muscle differentiation. Rescue experiments supported the idea of a specific relationship between FGF6 and Spry 2, both being known for their particular involvement in myogenesis.

Regular exercise prolongs survival in a type 2 spinal muscular atrophy model mouse.

Several studies indicate that physical exercise is likely to be neuroprotective, even in the case of neuromuscular disease. In the present work, we evaluated the efficiency of running-based training on type 2 spinal muscular atrophy (SMA)-like mice. The model used in this study is an SMN (survival motor neuron)-null mouse carrying one copy of a transgene of human SMN2. The running-induced benefits sustained the motor function and the life span of the type 2 SMA-like mice by 57.3%. We showed that the extent of neuronal death is reduced in the lumbar anterior horn of the spinal cord of running-trained mice in comparison with untrained animals. Notably, exercise enhanced motoneuron survival. We showed that the running-mediated neuroprotection is related to a change of the alternative splicing pattern of exon 7 in the SMN2 gene, leading to increased amounts of exon 7-containing transcripts in the spinal cord of trained mice. In addition, analysis at the level of two muscles from the calf, the slow-twitch soleus and the fast-twitch plantaris, showed an overall conserved muscle phenotype in running-trained animals. These data provide the first evidence for the beneficial effect of exercise in SMA and might lead to important therapeutic developments for human SMA patients.

Modulation of GABA receptor subunits in rat facial motoneurons after axotomy.

Facial nerve axotomy is a good model for studying neuronal plasticity and regeneration in the peripheral nervous system. In the present study, we investigated the effect of axotomy on the different subunits of GABA(A) and GABA(B) receptors of facial motoneurons. The facial nerve trunk was unilaterally sectioned and operated rats were sacrificed at 1, 3, 8, 30, and 60 days later. mRNAs coding for alpha1, beta2, and gamma2 of GABA(A) receptors and for GABA(1B) and GABA(B2) receptors were down-regulated by axotomy. This decrease began as soon as 1 or 3 days after axotomy, and the minimum was 8 days post-lesion; the mRNA levels remained lower than normal at day post-lesion 60. The abundance of mRNAs coding for the three other alpha2, beta1, and beta3 facial subunits of GABA(A) receptors and for the pre-synaptic GABA(B1A) subunit remained unchanged during the period 1-8 days post-lesion. Immunohistochemistry using specific antibodies against alpha1, gamma2 subunits of GABA(A) and against GABA(B2) subunits confirmed this down-regulation. Colchicine treatment and blockade of action potential by tetrodotoxin significantly decreased GABA(A)alpha1 immunoreactivity in the axotomized facial nucleus after 7 days. Finally, muscle destruction by cardiotoxin or facial palsy induced by botulinum toxin failed to change GABA(A)alpha1 subunit expression. Our data demonstrate that axotomy strongly reduced the amounts of alpha1, beta2, and gamma2 subunits of GABA(A) receptors and B(1B) and B(2) subunits of GABA(B) receptors in the axotomized facial motoneurons. The loss of GABA(A)alpha1 subunit was most probably induced by both the loss of trophic factors transported from the periphery and a positive injury signal. It also seems to be dependent on activity disruption.

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.

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

Whether the myogenic regulatory factors (MRFs) of the MyoD family can discriminate among the muscle gene targets for the proper and reproducible formation of skeletal muscle is a recurrent question. We have previously shown that, in Xenopus laevis, myogenin specifically transactivated muscle structural genes in vivo. In the present study, we used the Xenopus model to examine the role of XMyoD, XMyf5, and XMRF4 for the transactivation of the (nicotinic acetylcholine receptor) nAChR genes in vivo. During early Xenopus development, the expression patterns of nAChR subunit genes proved to be correlated with the expression patterns of the MRFs. We show that XMyf5 specifically induced the expression of the delta-subunit gene in cap animal assays and in endoderm cells of Xenopus embryos but was unable to activate the expression of the gamma-subunit gene. In embryos, overexpression of a dominant-negative XMyf5 variant led to the repression of delta-but not gamma-subunit gene expression. Conversely, XMyoD and XMRF4 activated gamma-subunit gene expression but were unable to activate delta-subunit gene expression. Finally, all MRFs induced expression of the alpha-subunit gene. These findings strengthen the concept that one MRF can specifically control a subset of muscle genes that cannot be activated by the other MRFs.