Improved success of myoblast transplantation in mdx mice by blocking the myostatin signal

Myostatin/Activin Receptor Ligands in Muscle and the Development Status of Attenuating Drugs

Muscle wasting disease indications are among the most debilitating and often deadly noncommunicable disease states. As a comorbidity, muscle wasting is associated with different neuromuscular diseases and myopathies, cancer, heart failure, chronic pulmonary and renal diseases, peripheral neuropathies, inflammatory disorders, and, of course, musculoskeletal injuries. Current treatment strategies are relatively ineffective and can at best only limit the rate of muscle degeneration. This includes nutritional supplementation and appetite stimulants as well as immunosuppressants capable of exacerbating muscle loss. Arguably, the most promising treatments in development attempt to disrupt myostatin and activin receptor signaling because these circulating factors are potent inhibitors of muscle growth and regulators of muscle progenitor cell differentiation. Indeed, several studies demonstrated the clinical potential of "inhibiting the inhibitors," increasing muscle cell protein synthesis, decreasing degradation, enhancing mitochondrial biogenesis, and preserving muscle function. Such changes can prevent muscle wasting in various disease animal models yet many drugs targeting this pathway failed during clinical trials, some from serious treatment-related adverse events and off-target interactions. More often, however, failures resulted from the inability to improve muscle function despite preserving muscle mass. Drugs still in development include antibodies and gene therapeutics, all with different targets and thus, safety, efficacy, and proposed use profiles. Each is unique in design and, if successful, could revolutionize the treatment of both acute and chronic muscle wasting. They could also be used in combination with other developing therapeutics for related muscle pathologies or even metabolic diseases.

Micro-dystrophin Gene Therapy Partially Enhances Exercise Capacity in Older Adult mdx Mice

Micro-dystrophin (μDys) gene therapeutics can improve striated muscle structure and function in different animal models of Duchenne muscular dystrophy. Most studies, however, used young mdx mice that lack a pronounced dystrophic phenotype, short treatment periods, and limited muscle function tests. We, therefore, determined the relative efficacy of two previously described μDys gene therapeutics (rAAV6:μDysH3 and rAAV6:μDys5) in 6-month-old mdx mice using a 6-month treatment regimen and forced exercise. Forelimb and hindlimb grip strength, metabolic rate (VO2 max), running efficiency (energy expenditure), and serum creatine kinase levels similarly improved in mdx mice treated with either vector. Both vectors produced nearly identical dose-responses in all assays. They also partially prevented the degenerative effects of repeated high-intensity exercise on muscle histology, although none of the metrics examined was restored to normal wild-type levels. Moreover, neither vector had any consistent effect on respiration while exercising. These data together suggest that, although μDys gene therapy can improve isolated and systemic muscle function, it may be only partially effective when dystrophinopathies are advanced or when muscle structure is significantly challenged, as with high-intensity exercise. This further suggests that restoring muscle function to near-normal levels will likely require ancillary or combinatorial treatments capable of enhancing muscle strength.

Follistatin N terminus differentially regulates muscle size and fat in vivo

Delivery of follistatin (FST) represents a promising strategy for both muscular dystrophies and diabetes, as FST is a robust antagonist of myostatin and activin, which are critical regulators of skeletal muscle and adipose tissues. FST is a multi-domain protein, and deciphering the function of different domains will facilitate novel designs for FST-based therapy. Our study aims to investigate the role of the N-terminal domain (ND) of FST in regulating muscle and fat mass in vivo. Different FST constructs were created and packaged into the adeno-associated viral vector (AAV). Overexpression of wild-type FST in normal mice greatly increased muscle mass while decreasing fat accumulation, whereas overexpression of an N terminus mutant or N terminus-deleted FST had no effect on muscle mass but moderately decreased fat mass. In contrast, FST-I-I containing the complete N terminus and double domain I without domain II and III had no effect on fat but increased skeletal muscle mass. The effects of different constructs on differentiated C2C12 myotubes were consistent with the in vivo finding. We hypothesized that ND was critical for myostatin blockade, mediating the increase in muscle mass, and was less pivotal for activin binding, which accounts for the decrease in the fat tissue. An in vitro TGF-beta1-responsive reporter assay revealed that FST-I-I and N terminus-mutated or -deleted FST showed differential responses to blockade of activin and myostatin. Our study provided direct in vivo evidence for a role of the ND of FST, shedding light on future potential molecular designs for FST-based gene therapy.

Myoblast Therapy: From Bench to Bedside

Myoblasts are defined as stem cells containing skeletal muscle cell precursors. A decade of experimental work has revealed many properties of myoblasts, including the stability of resulting hybrid myofibers without immune suppression, the persistence of transgene expression, and the lack of tumorigenicity. Early phase clinical trials also showed that myoblast-based therapy is a promising approach for many intractable clinical conditions, including both muscle-related and non-muscle-related diseases. The potential application of myoblast therapy may be in the treatment of genetic muscle diseases, cardiomyocyte damaged heart diseases, and urinary incontinence. This review will provide an overview of myoblast biology, along with discussion of the potential application in clinical medicine. In addition, problems in current myoblast therapy and possible future improvements will be addressed.

Two Activin Type 2B Receptors from Sea Bream Function Similarly in vitro

Myostatin (MSTN) and activin are members of the transforming growth factor-β superfamily. Both signal through the activin type II receptors (ActRII and ActRIIB). In a previous report, we demonstrated that fish possess at least 2 genes for ActRIIB: ActRIIB-1 and ActRIIB-2, which differ in their amino acid sequence. We also showed that affinity-purified, fish-soluble ActRIIB-1 (extracellular domain; ECD), produced in the yeast Pichia pastoris, inhibited recombinant mouse/rat/human mature MSTN activity in vitro using a reporter gene assay in the mammalian A204 cell line. In the present study, we produced soluble ActRIIB-2a in P. pastoris, and showed that it is N-glycosylated, similar to soluble ActRIIB-1. Inhibition of MSTN and activin A activities by affinity-purified ActRIIB-2a was compared with that of soluble ActRIIB-1 using the CAGA-luciferase assay in A204 cells. The findings of this study provide evidence that both paralogs, which probably resulted from gene duplication, did not diversify in their functionality (neofunctionalization), but rather retained a similar function. Both ActRIIB isoforms are equally potent in the mammalian system, and both exhibited an inhibitory effect on mammalian MSTN and activin A. Moreover--in spite of the amino acid differences in ECD between the two paralogs--it appears that the residues important for ligand binding are conserved, and that they recognize the mammalian ligands activin A and MSTN to the same extent.

Concise Review: Epigenetic Regulation of Myogenesis in Health and Disease

This review describes the recent findings on epigenetic regulation in satellite stem cells and committed myoblasts. It also addresses the potential of epigenetic drugs, such as histone deacetylase inhibitors, and their molecular mechanism of action in muscle cells.

Skeletal muscle regeneration is initiated by satellite cells, a population of adult stem cells that reside in the muscle tissue. The ability of satellite cells to self-renew and to differentiate into the muscle lineage is under transcriptional and epigenetic control. Satellite cells are characterized by an open and permissive chromatin state. The transcription factor Pax7 is necessary for satellite cell function. Pax7 is a nodal factor regulating the expression of genes associated with satellite cell growth and proliferation, while preventing differentiation. Pax7 recruits chromatin modifiers to DNA to induce expression of specific target genes involved in myogenic commitment following asymmetric division of muscle stem cells. Emerging evidence suggests that replacement of canonical histones with histone variants is an important regulatory mechanism controlling the ability of satellite cells and myoblasts to differentiate. Differentiation into the muscle lineage is associated with a global gene repression characterized by a decrease in histone acetylation with an increase in repressive histone marks. However, genes important for differentiation are upregulated by the specific action of histone acetyltransferases and other chromatin modifiers, in combination with several transcription factors, including MyoD and Mef2. Treatment with histone deacetylase (HDAC) inhibitors enhances muscle regeneration and is considered as a therapeutic approach in the treatment of muscular dystrophy. This review describes the recent findings on epigenetic regulation in satellite stem cells and committed myoblasts. The potential of epigenetic drugs, such as HDAC inhibitors, as well as their molecular mechanism of action in muscle cells, will be addressed.

Significance

This review summarizes recent findings concerning the epigenetic regulation of satellite cells in skeletal muscle.

Administration of a soluble activin type IIB receptor promotes the transplantation of human myoblasts in dystrophic mice

Duchenne muscular dystrophy (DMD) is a recessive disease caused by a dystrophin gene mutation. Myoblast transplantation permits the introduction of the dystrophin gene into dystrophic muscle fibers. However, this strategy has so far produced limited results. Modulation of transforming growth factor-β (TGF-β) superfamily signaling promotes skeletal muscle differentiation and growth and myogenic regeneration. We investigated the possibility that the combination of TGF-β superfamily signaling inhibition with myoblast transplantation might be an effective therapeutic approach in dystrophin-deficient patients. In vitro, blocking myostatin and other ligands with a soluble form of the extracellular domain of the activin IIB receptor (ActRIIB/Fc) upregulated the expression of myogenic differentiation factors and increased human myoblast fusion. In vivo, systemic inhibition of activin IIB receptor signaling by delivery of ActRIIB/Fc increased the success of the myoblast transplantation. This effect was further increased by forcing the mice to swim weekly to induce cycles of muscle degeneration and regeneration. Treatment of dystrophic mice with ActRIIB/Fc led to increased body weight, increased skeletal muscle mass, and improved myoblast transplantation. Thus, ActRIIB/Fc represents an effective therapeutic strategy for muscular dystrophies, and its effects are enhanced when combined with muscle exercise.

A high-content, high-throughput siRNA screen identifies cyclin D2 as a potent regulator of muscle progenitor cell fusion and a target to enhance muscle regeneration

Cell-mediated regenerative approaches using muscle progenitor cells hold promises for the treatment of many forms of muscle disorders. Their applicability in the clinic, however, is hindered by the low levels of regeneration obtained after transplantation and the large number of cells required to achieve an effect. To better understand the mechanisms that regulate the temporal switch of replicating muscle progenitor cells into terminally differentiated cells and to develop new strategies that could enhance muscle regeneration, we have developed and performed a high-throughput screening (HTS) capable of identifying genes that play active roles during myogenesis. Secondary and tertiary screens were used to confirm the effects of RNAi in vitro and in vivo and to select for candidate hits that significantly increase regeneration into skeletal muscles. Downregulation of cyclin D2 (CCND2) was shown to dramatically enhance myogenic differentiation of muscle progenitor cells and to induce a robust regeneration after cell transplantation into skeletal muscles of dystrophin-deficient mice. Protein interaction network and pathway analysis revealed that CCND2 directly interacts with the cyclin-dependent kinase Cdk4 to inhibit phosphorylation of the retinoblastoma protein (pRb), thus blocking the activation of the myogenic switch during fusion. These studies identify CCND2 as a new key regulator of terminal differentiation in muscle progenitor cells and open new possibilities for the treatment of many forms of muscle disorders characterized by impaired regeneration and loss of muscle mass.

Myostatin acts as an autocrine/paracrine negative regulator in myoblast differentiation from human induced pluripotent stem cells

Myostatin, also known as growth differentiation factor (GDF-8), regulates proliferation of muscle satellite cells, and suppresses differentiation of myoblasts into myotubes via down-regulation of key myogenic differentiation factors including MyoD. Recent advances in stem cell biology have enabled generation of myoblasts from pluripotent stem cells, but it remains to be clarified whether myostatin is also involved in regulation of artificial differentiation of myoblasts from pluripotent stem cells. Here we show that the human induced pluripotent stem (iPS) cell-derived cells that were induced to differentiate into myoblasts expressed myostatin and its receptor during the differentiation. An addition of recombinant human myostatin (rhMyostatin) suppressed induction of MyoD and Myo5a, resulting in significant suppression of myoblast differentiation. The rhMyostatin treatment also inhibited proliferation of the cells at a later phase of differentiation. RNAi-mediated silencing of myostatin promoted differentiation of human iPS-derived embryoid body (EB) cells into myoblasts. These results strongly suggest that myostatin plays an important role in regulation of myoblast differentiation from iPS cells of human origin. The present findings also have significant implications for potential regenerative medicine for muscular diseases.

Structural and functional characterizations of activin type 2B receptor (acvr2b) ortholog from the marine fish, gilthead sea bream, Sparus aurata: evidence for gene duplication of acvr2b in fish

Myostatin (MSTN), a negative regulator of muscle growth and a member of the transforming growth factor-β superfamily, can bind the two activin type 2 receptors (ACVR2). It has been previously shown that WT mice injected with ACVR2B extracellular domain (ACVR2B-ECD) had higher muscle mass. Likewise, fish larvae immersed in Pichia pastoris culture supernatant, containing goldfish Acvr2b-ECD, showed enhanced larval growth. However, it is not clear whether fish Mstn1 and Mstn2 signal through the same receptor and whether fish express more than one acvr2b gene. In the current study, three cDNAs encoding acvr2b (saacvr2b-1, saacvr2b-2a, and saacvr2b-2b) were cloned from gilthead sea bream. All three contain the short extracellular binding domain, a short transmembrane region, and a conserved catalytic domain of serine/threonine protein kinase. Bioinformatics analysis provided evidence for the existence of two acvr2b genes (acvr2b-1 and acvr2b-2) in several other fish species as well, probably as a result of gene or genome duplication. The two isoforms differ in their amino acid sequences. The direct inhibitory effect of Acvr2b-ECD on Mstn activity was tested in vitro. The saAcvr2b-1-ECD was expressed in the yeast P. pastoris. Evidence is provided for N-glycosylation of Acvr2b-1-ECD. The affinity-purified Acvr2b-1-ECD inhibited recombinant mouse/rat/human mature MSTN activity when determined in vitro using the CAGA-luciferase assay in A204 cells. A lower inhibitory activity was obtained when unprocessed purified, furin-digested, and activated saMstn1 was used. Results of this study demonstrate for the first time the existence of two acvr2b genes in fish. In addition, the study shows that bioactive fish Acvr2b-ECD can be produced from P. pastoris.

Macrophages improve survival, proliferation and migration of engrafted myogenic precursor cells into MDX skeletal muscle

Transplantation of muscle precursor cells is of therapeutic interest for focal skeletal muscular diseases. However, major limitations of cell transplantation are the poor survival, expansion and migration of the injected cells. The massive and early death of transplanted myoblasts is not fully understood although several mechanisms have been suggested. Various attempts have been made to improve their survival or migration. Taking into account that muscle regeneration is associated with the presence of macrophages, which are helpful in repairing the muscle by both cleansing the debris and deliver trophic cues to myoblasts in a sequential way, we attempted in the present work to improve myoblast transplantation by coinjecting macrophages. The present data showed that in the 5 days following the transplantation, macrophages efficiently improved: i) myoblast survival by limiting their massive death, ii) myoblast expansion within the tissue and iii) myoblast migration in the dystrophic muscle. This was confirmed by in vitro analyses showing that macrophages stimulated myoblast adhesion and migration. As a result, myoblast contribution to regenerating host myofibres was increased by macrophages one month after transplantation. Altogether, these data demonstrate that macrophages are beneficial during the early steps of myoblast transplantation into skeletal muscle, showing that coinjecting these stromal cells may be used as a helper to improve the efficiency of parenchymal cell engraftment.

Decorin promotes myogenic differentiation and mdx mice therapeutic effects after transplantation of rat adipose-derived stem cells

BACKGROUND AIMS

Adipose-derived stem cells (ADSC) have been considered as attractive candidates for the treatment of Duchenne muscular dystrophy (DMD), but the rate of ADSC myogenesis is very low. Myostatin (Mstn), a negative regulator of myogenesis, is known to be responsible for limiting skeletal muscle regeneration. Decorin could bind Mstn and deactivate it. Decorin has been shown to improve myogenic differentiation in mdx mice. We hypothesized that inhibition of Mstn by using decorin may ameliorate myogenic differentiation of ADSC.

METHODS

Rat ADSC were transfected with the lentivirus-containing green fluorescence protein (GFP) and human decorin gene. The transfected ADSC were induced by 5-azacytidine (5-AzaC). The rates of myogenic differentiation and adipogenesis were detected. The transfected ADSC were injected into mdx mice and the expression of Mstn and decorin detected by Western blot. Dystrophin was detected after transfected ADSC transplantation by immunofluorescence staining and Western blot. Serum creatine kinase (CK) and histologic changes were also evaluated.

RESULTS

The optimal multiplicity of infection of ADSC was 10. Decorin improved muscle mass. In accordance with the increased muscle mass, dystrophin expression increased. Following the level of decorin increase, the Mstn expression decreased. Furthermore, serum CK and histologic changes in centrally nucleated fiber (CNF) decreased.

CONCLUSIONS

Improved myogenic differentiation of ADSC was observed by using decorin. This process was probably the result of decorin inhibiting Mstn. A new method of DMD therapy combining Mstn inhibition (using decorin) and ADSC transplantation is probably feasible.

Aldehyde dehydrogenase activity promotes survival of human muscle precursor cells

Aldehyde dehydrogenases (ALDH) are a family of enzymes that efficiently detoxify aldehydic products generated by reactive oxygen species and might therefore participate in cell survival. Because ALDH activity has been used to identify normal and malignant cells with stem cell properties, we asked whether human myogenic precursor cells (myoblasts) could be identified and isolated based on their levels of ALDH activity. Human muscle explant-derived cells were incubated with ALDEFLUOR, a fluorescent substrate for ALDH, and we determined by flow cytometry the level of enzyme activity. We found that ALDH activity positively correlated with the myoblast-CD56⁺ fraction in those cells, but, we also observed heterogeneity of ALDH activity levels within CD56-purified myoblasts. Using lentiviral mediated expression of shRNA we demonstrated that ALDH activity was associated with expression of Aldh1a1 protein. Surprisingly, ALDH activity and Aldh1a1 expression levels were very low in mouse, rat, rabbit and non-human primate myoblasts. Using different approaches, from pharmacological inhibition of ALDH activity by diethylaminobenzaldehyde, an inhibitor of class I ALDH, to cell fractionation by flow cytometry using the ALDEFLUOR assay, we characterized human myoblasts expressing low or high levels of ALDH. We correlated high ALDH activity ex vivo to resistance to hydrogen peroxide (H₂O₂)-induced cytotoxic effect and in vivo to improved cell viability when human myoblasts were transplanted into host muscle of immune deficient scid mice. Therefore detection of ALDH activity, as a purification strategy, could allow non-toxic and efficient isolation of a fraction of human myoblasts resistant to cytotoxic damage.

Blocking the myostatin signal with a dominant negative receptor improves the success of human myoblast transplantation in dystrophic mice

Duchenne muscular dystrophy (DMD) is a recessive disease caused by a dystrophin gene mutation. Myoblast transplantation permits to introduce the dystrophin gene in dystrophic muscle fibers. However, the success of this approach is reduced by the short duration of the regeneration following the transplantation, which reduces the number of hybrid fibers. Myostatin (MSTN) is a negative regulator of skeletal muscle development and responsible for limiting regeneration. It binds with high affinity to the activin type IIB receptor (ActRIIB). Our aim was to verify whether the success of the myoblast transplantation is enhanced by blocking the MSTN signal with expression of a dominant negative mutant of ActRIIB (dnActRIIB). In vitro, blocking MSTN activity with a lentivirus carrying dnActRIIB increased proliferation and fusion of human myoblasts because MSTN regulates the expression of several myogenic regulatory factors. In vivo, myoblasts infected with the dnActRIIB lentivirus were transplanted in immunodeficient dystrophic mice. Dystrophin immunostaining of tibialis anterior (TA) cross-sections of these mice 1 month post-transplantation revealed more human dystrophin-positive myofibers following the transplantation of dnActRIIB myoblasts than of control myoblasts. Thus, blocking the MSTN signal with dnActRIIB improved the success of myoblast transplantation by increasing the myoblast proliferation and fusion and changed the expression of myogenic regulatory factors.

Activin IIB receptor blockade attenuates dystrophic pathology in a mouse model of Duchenne muscular dystrophy

Modulation of transforming growth factor-β (TGF-β) signaling to promote muscle growth holds tremendous promise for the muscular dystrophies and other disorders involving the loss of functional muscle mass. Previous studies have focused on the TGF-β family member myostatin and demonstrated that inhibition of myostatin leads to muscle growth in normal and dystrophic mice. We describe a unique method of systemic inhibition of activin IIB receptor signaling via adeno-associated virus (AAV)-mediated gene transfer of a soluble form of the extracellular domain of the activin IIB receptor to the liver. Treatment of mdx mice with activin IIB receptor blockade led to increased skeletal muscle mass, increased force production in the extensor digitorum longus (EDL), and reduced serum creatine kinase. No effect on heart mass or function was observed. Our results indicate that activin IIB receptor blockade represents a novel and effective therapeutic strategy for the muscular dystrophies.

Growth Factor Coinjection Improves the Migration Potential of Monkey Myogenic Precursors without Affecting Cell Transplantation Success

Duchenne muscular dystrophy (DMD) is an inherited disease and a main target of myogenic cell transplantation (MT). After the failure of the first clinical trials with DMD patients, the poor migration of transplanted cells has been suspected to be a major problem for a more effective clinical application of MT. Previous investigations suggested that the quantity and dispersion of myofibers containing donor cell nuclei might be improved by increasing the migration of the transplanted cells outside the injection sites. Because the coinjection of motogenic factors with human myoblasts enhanced their intramuscular migration following MT in SCID mice, the present study aimed to investigate whether this approach was appropriate to increase MT success in muscles of nonhuman primates. In vitro studies indicated that IGF-1 or bFGF increased components of proteolytic systems involved in myoblast migration. In vitro and in vivo experiments also demonstrated that coinjection of bFGF or IGF-1 was able to improve monkey myogenic cell migration and invasion. Sixty hours after MT in skeletal muscle tissue, the migration distances reached by monkey myoblasts increased by nearly twofold when one of the growth factors was coinjected with the cells. However, long-term observations in adult monkeys suggest that promigratory treatments are not intrinsically sufficient to improve the success of MT. Even if short-term observations reveal that grafted cells are not always trapped inside the injection site and in spite of the fact that both factors enhanced transplanted cell migration, myofibers including grafted cell nuclei were still restrained to the injection trajectory without notable difference in their amount or their dispersion. The incapacity of transplanted cells to fuse with undamaged myofibers, which are located outside the injection sites, is a priority problem to solve in order to improve transplantation success and reduce the number of injections required for the treatment of DMD patients.

Clinical, agricultural, and evolutionary biology of myostatin: a comparative review

The discovery of myostatin and our introduction to the "Mighty Mouse" over a decade ago spurred both basic and applied research and impacted popular culture as well. The myostatin-null genotype produces "double muscling" in mice and livestock and was recently described in a child. The field's rapid growth is by no means surprising considering the potential benefits of enhancing muscle growth in clinical and agricultural settings. Indeed, several recent studies suggest that blocking myostatin's inhibitory effects could improve the clinical treatment of several muscle growth disorders, whereas comparative studies suggest that these actions are at least partly conserved. Thus, neutralizing myostatin's effects could also have agricultural significance. Extrapolating between studies that use different vertebrate models, particularly fish and mammals, is somewhat confusing because whole genome duplication events have resulted in the production and retention of up to four unique myostatin genes in some fish species. Such comparisons, however, suggest that myostatin's actions may not be limited to skeletal muscle per se, but may additionally influence other tissues including cardiac muscle, adipocytes, and the brain. Thus, therapeutic intervention in the clinic or on the farm must consider the potential of alternative side effects that could impact these or other tissues. In addition, the presence of multiple and actively diversifying myostatin genes in most fish species provides a unique opportunity to study adaptive molecular evolution. It may also provide insight into myostatin's nonmuscle actions as results from these and other comparative studies gain visibility in biomedical fields.

Inhibiting myostatin with follistatin improves the success of myoblast transplantation in dystrophic mice

Duchenne muscular dystrophy is a recessive disease due to a mutation in the dystrophin gene. Myoblast transplantation permits to introduce the dystrophin gene in dystrophic muscle fibers. However, the success of this approach is reduced by the short duration of the regeneration following the transplantation, which reduces the number of hybrid fibers. Our aim was to verify whether the success of the myoblast transplantation is enhanced by blocking the myostatin signal with an antagonist, follistatin. Three different approaches were studied to overexpress follistatin in the muscles of mdx mice transplanted with myoblasts. First, transgenic follistatin/mdx mice were generated; second, a follistatin plasmid was electroporated in mdx muscles, and finally, follistatin was induced in mdx mice muscles by a treatment with a histone deacetylase inhibitor. The three approaches improved the success of the myoblast transplantation. Moreover, fiber hypertrophy was also observed in all muscles, demonstrating that myostatin inhibition by follistatin is a good method to improve myoblast transplantation and muscle function. Myostatin inhibition by follistatin in combination with myoblast transplantation is thus a promising novel therapeutic approach for the treatment of muscle wasting in diseases such as Duchenne muscular dystrophy.

Myostatin is a procachectic growth factor during postnatal myogenesis

PURPOSE OF REVIEW

To describe the most relevant recent observations concerning the molecular mechanisms behind myostatin-induced muscle wasting.

RECENT FINDINGS

The main theme of this review is to summarize the biology and function of myostatin. Myostatin is a secreted growth factor that negatively regulates muscle growth. While inactivation of myostatin leads to muscle growth in vivo, excess levels of myostatin induces cachectic-like muscle wasting. Molecular analyses reveal that excess levels of myostatin induce Atrogin-1 expression by reducing Akt phosphorylation and thereby increasing FoxO1 activity. Recent findings have further speculated that myostatin may also play a role in cardiac cachexia.

SUMMARY

As myostatin is a potent inducer of muscle wasting, antagonists to myostatin have been speculated to have great therapeutic value in alleviating muscle wasting. Indeed, myostatin peptide antagonists and antibodies have shown great promise in containing muscle loss in animal models of muscle wasting. Given the beneficial effects of myostatin antagonists in animal models, clinical trials are underway with myostatin antibodies, peptibodies and soluble receptor. Therefore, this review article on the role of myostatin in muscle wasting is highly relevant to current themes in muscle biology.

Load-mediated downregulation of myostatin mRNA is not sufficient to promote myofiber hypertrophy in humans: a cluster analysis

Myostatin is a potent inhibitor of myogenesis; thus differential expression might be expected across individuals varying in responsiveness to myogenic stimuli. We hypothesized that myostatin would be differentially regulated across humans with markedly different hypertrophic responses to resistance training (RT; 16 wk). Targets were assessed in muscle biopsies at baseline (T1) and 24 h after the first (T2) and last (T3) loading bouts in previously untrained subjects statistically clustered based on mean myofiber hypertrophy as extreme (Xtr; n = 17, 2,475 microm(2)), modest (n = 32, 1,111 microm(2)), and nonresponders (n = 17, -16 microm(2)). We assessed protein levels of latent full-length myostatin protein complex and its propeptide; mRNA levels of myostatin, cyclin D1, p21(cip1), p27(kip1), and activin receptor IIB; and serum myostatin protein concentration. Total RNA concentration increased by T3 in nonresponders (37%) and modest responders (40%), while it increased acutely (T2) only in Xtr (26%), remaining elevated at T3 (40%). Myostatin mRNA decreased at T2 (-44%) and remained suppressed at T3 (-52%), but not differentially across clusters. Cyclin D1 mRNA increased robustly by T2 (38%) and T3 (74%). The increase at T2 was driven by Xtr (62%, P < 0.005), and Xtr had the largest elevation at T3 (82%, P < 0.001). No effects were found for other target transcripts. Myostatin protein complex increased 44% by T3 (P < 0.001), but not differentially by cluster. Myostatin protein complex propeptide and circulating myostatin were not influenced by RT or cluster. Overall, we found no compelling evidence that myostatin is differentially regulated in humans demonstrating robust RT-mediated myofiber hypertrophy vs. those more resistant to growth.

Relationships between transforming growth factor-beta1, myostatin, and decorin: implications for skeletal muscle fibrosis

Recent studies have shown that myostatin, first identified as a negative regulator of skeletal muscle growth, may also be involved in the formation of fibrosis within skeletal muscle. In this study, we further explored the potential role of myostatin in skeletal muscle fibrosis, as well as its interaction with both transforming growth factor-beta1 and decorin. We discovered that myostatin stimulated fibroblast proliferation in vitro and induced its differentiation into myofibroblasts. We further found that transforming growth factor-beta1 stimulated myostatin expression, and conversely, myostatin stimulated transforming growth factor-beta1 secretion in C2C12 myoblasts. Decorin, a small leucine-rich proteoglycan, was found to neutralize the effects of myostatin in both fibroblasts and myoblasts. Moreover, decorin up-regulated the expression of follistatin, an antagonist of myostatin. The results of in vivo experiments showed that myostatin knock-out mice developed significantly less fibrosis and displayed better skeletal muscle regeneration when compared with wild-type mice at 2 and 4 weeks following gastrocnemius muscle laceration injury. In wild-type mice, we found that transforming growth factor-beta1 and myostatin co-localize in myofibers in the early stages of injury. Recombinant myostatin protein stimulated myofibers to express transforming growth factor-beta1 in skeletal muscles at early time points following injection. In summary, these findings define a fibrogenic property of myostatin and suggest the existence of co-regulatory relationships between transforming growth factor-beta1, myostatin, and decorin.

Molecular cloning of the Atlantic salmon activin receptor IIB cDNA - Localization of the receptor and myostatin in vivo and in vitro in muscle cells

In mammals, the activin receptor type IIB (ActRIIB) binds with high affinity several members of the transforming growth factor-beta (TGF-beta) superfamily, including the negative muscle regulator myostatin (MSTN). In this study, an actRIIB cDNA of 1443 bp was isolated by reverse transcription (RT)-PCR from the liver of Atlantic salmon (Salmo salar) encoding almost the complete receptor. The deduced salmon ActRIIB of 481 amino acids (aa) contained the conserved catalytic domain of serine/threonine protein kinases, and showed the highest sequence identity (83-87%) to the zebrafish, chicken and goldfish ActRIIB. Salmon actRIIB mRNA was identified by RT-PCR in all the examined tissues of juvenile fish that was confirmed by in situ hybridization. In comparison, the salmon MSTN signal was less widespread, and co-expression of the receptor and this putative ligand was only demonstrated in skeletal muscle. Consistently, both ActRIIB and MSTN were immunocytologically identified in salmon myoblasts and differentiated myotubes in culture.

Stem cell based therapies to treat muscular dystrophy

Muscular dystrophies comprise a heterogeneous group of neuromuscular disorders, characterized by progressive muscle wasting, for which no satisfactory treatment exists. Multiple stem cell populations, both of adult or embryonic origin, display myogenic potential and have been assayed for their ability to correct the dystrophic phenotype. To date, many of these described methods have failed, underlying the need to identify the mechanisms controlling myogenic potential, homing of donor populations to the musculature, and avoidance of the immune response. Recent results focus on the fresh isolation of satellite cells and the use of multiple growth factors to promote mesangioblast migration, both of which promote muscle regeneration. Throughout this chapter, various stem cell based therapies will be introduced and evaluated based on their potential to treat muscular dystrophy in an effective and efficient manner.

Myostatin regulation of muscle development: Molecular basis, natural mutations, physiopathological aspects

Since its identification in 1997, myostatin has been considered as a novel and unique negative regulator of muscle growth, as mstn-/- mice display a dramatic and widespread increase in skeletal muscle mass. Myostatin also appears to be involved in muscle homeostasis in adults as its expression is regulated during muscle atrophy. Moreover, deletion of the myostatin gene seems to affect adipose tissue mass in addition to skeletal muscle mass. Natural myostatin gene mutations occur in cattle breeds such as Belgian Blue, exhibiting an obviously increased muscle mass, but also in humans, as has recently been demonstrated. Here we review these natural mutations and their associated phenotypes as well as the physiological influence of the alterations in myostatin expression and the physiopathological consequences of changes in myostatin expression, especially with regard to satellite cells. Interestingly, studies have demonstrated some rescue effects of myostatin in muscular pathologies such as myopathies, providing a novel pharmacological strategy for treatment. Furthermore, the myostatin pathway is now better understood thanks to in vitro studies and it consists of inhibition of myoblast progression in the cell cycle, inhibition of myoblast terminal differentiation, in both cases associated to protection from apoptosis. The molecular pathway driving the myogenic myostatin influence is currently under extensive study and many molecular partners of myostatin have been identified, suggesting novel potent muscle growth enhancers for both human and agricultural applications.

Gene expression profiling highlights defective myogenesis in DMD patients and a possible role for bone morphogenetic protein 4

Duchenne Muscular Dystrophy (DMD) is characterized by progressive muscle weakness and wasting. Despite the sustained presence of satellite cells in their skeletal muscles, muscle regeneration in DMD patients seems inefficient and unable to compensate for the continuous muscle fiber loss. To find a molecular explanation, we compared the gene expression profiles of myoblasts from healthy individuals and DMD patients during activation and differentiation in culture. DMD cultures showed significant gene expression changes, even before dystrophin is expressed. We found a higher expression level of bone morphogenetic protein 4 (BMP4) in DMD cultures, which we demonstrate to inhibit differentiation into myotubes. In the later stages of differentiation, we observed a significant decline in expression of sarcomeric genes in the absence of dystrophin, probably contributing to sarcomeric instability. These results support the hypothesis that inefficient muscle regeneration is caused by impaired myoblast differentiation and impaired maintenance of the myotubes.

Therapeutics in duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a fatal disorder affecting approximately 1 in 3,500 live born males, characterized by progressive muscle weakness. Several different strategies are being investigated in developing a cure for this disorder. Until a cure is found, therapeutic and supportive care is essential in preventing complications and improving the afflicted child's quality of life. Currently, corticosteroids are the only class of drug that has been extensively studied in this condition, with controversy existing over the use of these drugs, especially in light of the multiple side effects that may occur. The use of nutritional supplements has expanded in recent years as researchers improve our abilities to use gene and stem cell therapies, which will hopefully lead to a cure soon. This article discusses the importance of therapeutic interventions in children with DMD, the current debate over the use of corticosteroids to treat this disease, the growing use of natural supplements as a new means of treating these boys and provides an update on the current state of gene and stem cell therapies.

Muscle regeneration through myostatin inhibition

PURPOSE OF REVIEW

Myostatin is an endogenous, negative regulator of muscle growth. Selective inhibition of myostatin may have broad clinical utility by improving regeneration in diverse and burdensome muscle disorders. An understanding of this potential is relevant because inhibitors of myostatin have recently entered clinical trials.

RECENT FINDINGS

This article reviews the structure and function of myostatin, the effect of inhibiting myostatin in models of disease, and potential therapeutic approaches to blocking myostatin pharmacologically. The possibility that a myostatin inhibitor will promote muscle regeneration in human disease, as seen in animal models, is suggested by the observation that loss of myostatin results in muscle hypertrophy in a human subject.

SUMMARY

Multiple approaches to inhibiting myostatin are suggested by the recent elucidation of its signaling pathway. An inhibitor of myostatin may be the first drug specifically designed to enhance muscle growth and regeneration.