Improved muscle healing through enhanced regeneration and reduced fibrosis in myostatin-null mice

The brittle star genome illuminates the genetic basis of animal appendage regeneration

Species within nearly all extant animal lineages are capable of regenerating body parts. However, it remains unclear whether the gene expression programme controlling regeneration is evolutionarily conserved. Brittle stars are a species-rich class of echinoderms with outstanding regenerative abilities, but investigations into the genetic bases of regeneration in this group have been hindered by the limited genomic resources. Here we report a chromosome-scale genome assembly for the brittle star Amphiura filiformis. We show that the brittle star genome is the most rearranged among echinoderms sequenced so far, featuring a reorganized Hox cluster reminiscent of the rearrangements observed in sea urchins. In addition, we performed an extensive profiling of gene expression during brittle star adult arm regeneration and identified sequential waves of gene expression governing wound healing, proliferation and differentiation. We conducted comparative transcriptomic analyses with other invertebrate and vertebrate models for appendage regeneration and uncovered hundreds of genes with conserved expression dynamics, particularly during the proliferative phase of regeneration. Our findings emphasize the crucial importance of echinoderms to detect long-range expression conservation between vertebrates and classical invertebrate regeneration model systems.

The effects of vitamin D on different types of cells

Vitamin D is neccessary for regulation of calcium and phosphorus metabolism in bones, affects imunity, the cardiovascular system, muscles, skin, epithelium, extracellular matrix, the central nervous system, and plays arole in prevention of aging-associated diseases. Vitamin D receptor is expressed in almost all types of cells and its activation leads to modulation of different signaling pathways. In this review, we have analysed the current knowledge of 1,25-dihydroxyvitamin D3 or 25-hydroxyvitamin D3 effects on metabolism of cells important for the function of the cardiovascular system (endothelial cells, vascular smooth muscle cells, cardiac cells and pericytes), tissue healing (fibroblasts), epithelium (various types of epithelial cells) and the central nervous system (neurons, astrocytes and microglia). The goal of this review was to compare the effects of vitamin D on the above mentioned cells in in vitro conditions and to summarize what is known in this field of research.

Functional substitutions of amino acids that differ between GDF11 and GDF8 impact skeletal development and skeletal muscle

Growth differentiation factor 11 (GDF11) and GDF8 (MSTN) are closely related TGF-β family proteins that interact with nearly identical signaling receptors and antagonists. However, GDF11 appears to activate SMAD2/3 more potently than GDF8 in vitro and in vivo. The ligands possess divergent structural properties, whereby substituting unique GDF11 amino acids into GDF8 enhanced the activity of the resulting chimeric GDF8. We investigated potentially distinct endogenous activities of GDF11 and GDF8 in vivo by genetically modifying their mature signaling domains. Full recoding of GDF8 to that of GDF11 yielded mice lacking GDF8, with GDF11 levels ∼50-fold higher than normal, and exhibiting modestly decreased muscle mass, with no apparent negative impacts on health or survival. Substitution of two specific amino acids in the fingertip region of GDF11 with the corresponding GDF8 residues resulted in prenatal axial skeletal transformations, consistent with Gdf11-deficient mice, without apparent perturbation of skeletal or cardiac muscle development or homeostasis. These experiments uncover distinctive features between the GDF11 and GDF8 mature domains in vivo and identify a specific requirement for GDF11 in early-stage skeletal development.

Myostatin is a negative regulator of adult neurogenesis after spinal cord injury in zebrafish

Intrinsic and extrinsic inhibition of neuronal regeneration obstruct spinal cord (SC) repair in mammals. In contrast, adult zebrafish achieve functional recovery after complete SC transection. While studies of innate SC regeneration have focused on axon regrowth as a primary repair mechanism, how local adult neurogenesis affects functional recovery is unknown. Here, we uncover dynamic expression of zebrafish myostatin b (mstnb) in a niche of dorsal SC progenitors after injury. mstnb mutants show impaired functional recovery, normal glial and axonal bridging across the lesion, and an increase in the profiles of newborn neurons. Molecularly, neuron differentiation genes are upregulated, while the neural stem cell maintenance gene fgf1b is downregulated in mstnb mutants. Finally, we show that human fibroblast growth factor 1 (FGF1) treatment rescues the molecular and cellular phenotypes of mstnb mutants. These studies uncover unanticipated neurogenic functions for mstnb and establish the importance of local adult neurogenesis for innate SC repair.

Myostatin inhibits insulin-like growth factor 1-dependent citrate secretion and osteogenesis via nicotinamide adenine dinucleotide phosphate oxidase-4 in a mouse mesenchymal stem cell line

Citrate is an indispensable component of bone. Reduced levels of citrate in bone and serum are reported in the elderly and in osteoporosis patients. Myostatin (Mstn) is implicated in skeletal homeostasis, but its effects on osteogenesis remain incompletely understood. Nox4 has critical roles in bone homeostasis. TGF-β/Mstn-associated Smad2/3 signaling has been linked to Nox4 expression. Insulin-like growth factor (IGF-1) has been shown to counteract many regulatory effects of Mstn. However, the crosstalk among Mstn, IGF-1, and Nox4 is not well understood; the interactive effects of those factors on citrate secretion, osteogenic differentiation, and bone remodeling remain unclear. In this study, we demonstrated that osteogenic differentiation induced an IGF-1-dependent upregulation of citrate secretion that was suppressed by Mstn. Inhibition of Nox4 prevented Mstn-induced reduction of citrate secretion. In addition, Mstn reduced bone nodule formation; these changes were prevented by Nox4 inhibition. Moreover, Mstn increased the ratio of RANKL to OPG mRNAs to favor osteoclast activation. These results indicate that Mstn negatively regulates osteogenesis by increasing levels of Nox4, which reduced IGF-1 expression, citrate secretion, and bone mineralization while also altering the RANKL to OPG ratio. These findings provide new and highly relevant insights into the osseous effects of myostatin.

Myostatin deficiency decreases cardiac extracellular matrix in pigs

Myostatin (MSTN), a member of the TGF-β superfamily, negatively regulates muscle growth. MSTN inhibition has been known to cause a double-muscled phenotype in skeletal muscle and fibrosis reduction in the heart. However, the role of MSTN in the cardiac extracellular matrix (ECM) needs more studies in various species of animal models to draw more objective conclusions. The main objective of the present study was to investigate whether loss of MSTN affects the cardiac extracellular matrix in pigs. Three MSTN knockouts (MSTN-/-) and three wild type (WT) male pigs were generated by crossing MSTN ± heterozygous gilts and boars. Cardiac ECM and underlying mechanisms were determined post-mortem. The role of MSTN on collagen expression was investigated by treating cardiac fibroblasts with active MSTN protein in vitro. MSTN protein was detected in WT hearts, while no expression was detected in MSTN-/- hearts. The heart-to-body weight ratio was significantly decreased in MSTN-/- pigs. The morphometric analyses, including picrosirius red staining, immunofluorescent staining, and ultra-structural thickness examination of the endomysium, revealed a significant reduction of connective tissue content in MSTN-/- hearts compared to WT. Hydroxyproline, type I collagen (Col1A), and p-Smad3/Smad3 levels were significantly lower in MSTN-/- hearts in vivo. On the contrary, cardiac fibroblasts treated with exogenous MSTN protein overexpressed Col1A and activated Smad and AKT signaling pathways in vitro. The present study suggests that inhibition of MSTN decreases cardiac extracellular matrix.

Myostatin: a multifunctional role in human female reproduction and fertility - a short review

Myostatin (MSTN) is member of the transforming growth factor β (TGF-β) superfamily and was originally identified in the musculoskeletal system as a negative regulator of skeletal muscle growth. The functional roles of MSTN outside of the musculoskeletal system have aroused researchers' interest in recent years, with an increasing number of studies being conducted in this area. Notably, the expression of MSTN and its potential activities in various reproductive organs, including the ovary, placenta, and uterus, have recently been examined. Numerous studies published in the last few years demonstrate that MSTN plays a critical role in human reproduction and fertility, including the regulation of follicular development, ovarian steroidogenesis, granule-cell proliferation, and oocyte maturation regulation. Furthermore, findings from clinical samples suggest that MSTN may play a key role in the pathogenesis of several reproductive disorders such as uterine myoma, preeclampsia (PE), ovary hyperstimulation syndrome (OHSS), and polycystic ovarian syndrome (PCOS). There is no comprehensive review regarding to MSTN related to the female reproductive system in the literature. This review serves as a summary of the genes in reproductive medicine and their potential influence. We summarized MSTN expression in different compartments of the female reproductive system. Subsequently, we discuss the role of MSTN in both physiological and several pathological conditions related to the female fertility and reproduction-related diseases.

Lack of Tgfbr1 and Acvr1b synergistically stimulates myofibre hypertrophy and accelerates muscle regeneration

In skeletal muscle, transforming growth factor-β (TGF-β) family growth factors, TGF-β1 and myostatin, are involved in atrophy and muscle wasting disorders. Simultaneous interference with their signalling pathways may improve muscle function; however, little is known about their individual and combined receptor signalling. Here, we show that inhibition of TGF-β signalling by simultaneous muscle-specific knockout of TGF-β type I receptors Tgfbr1 and Acvr1b in mice, induces substantial hypertrophy, while such effect does not occur by single receptor knockout. Hypertrophy is induced by increased phosphorylation of Akt and p70S6K and reduced E3 ligases expression, while myonuclear number remains unaltered. Combined knockout of both TGF-β type I receptors increases the number of satellite cells, macrophages and improves regeneration post cardiotoxin-induced injury by stimulating myogenic differentiation. Extra cellular matrix gene expression is exclusively elevated in muscle with combined receptor knockout. Tgfbr1 and Acvr1b are synergistically involved in regulation of myofibre size, regeneration, and collagen deposition.

Antibody-Based Therapeutic Interventions for Amyotrophic Lateral Sclerosis: A Systematic Literature Review

Amyotrophic Lateral Sclerosis (ALS) is a mid-life onset neurodegenerative disease that manifests its symptomatology with motor impairments and cognitive deficits overlapping with Frontotemporal Lobar Degeneration (FTLD). The etiology of ALS remains elusive, with various mechanisms and cellular targets implicated, and no treatment can reverse or stop the progression of the pathology. Therapeutic interventions based on passive immunization are gaining attention for neurodegenerative diseases, and FDA recently approved the first antibody-based approach for Alzheimer's disease. The present systematic review of the literature aims to highlight the efforts made over the past years at developing antibody-based strategies to cure ALS. Thirty-one original research papers have been selected where the therapeutic efficacy of antibodies were investigated and described in patients and animal models of ALS. Antibody-based interventions analyzed, target both extracellular molecules implicated in the pathology and intracellular pathogenic proteins known to drive the disease, such as SOD1, TDP-43 or C9ORF72 repeats expansions. The potentials and limitations of these therapeutic interventions have been described and discussed in the present review.

Lessons Learned from Discontinued Clinical Developments in Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked condition caused by a deficiency of functional dystrophin protein. Patients experience progressive muscle weakness, cardiomyopathy and have a decreased life expectancy. Standards of care, including treatment with steroids, and multidisciplinary approaches have extended the life expectancy and improved the quality of life of patients. In the last 30 years, several compounds have been assessed in preclinical and clinical studies for their ability to restore functional dystrophin levels or to modify pathways involved in DMD pathophysiology. However, there is still an unmet need with regards to a disease-modifying treatment for DMD and the attrition rate between early-phase and late-phase clinical development remains high. Currently, there are 40 compounds in clinical development for DMD, including gene therapy and antisense oligonucleotides for exon skipping. Only five of them have received conditional approval in one jurisdiction subject to further proof of efficacy. In this review, we present data of another 16 compounds that failed to complete clinical development, despite positive results in early phases of development in some cases. We examine the reasons for the high attrition rate and we suggest solutions to avoid similar mistakes in the future.

Factors Involved in Morphogenesis in the Muscle-Tendon-Bone Complex

A decline in the body's motor functions has been linked to decreased muscle mass and function in the oral cavity and throat; however, aging of the junctions of the muscles and bones has also been identified as an associated factor. Basic and clinical studies on the muscles, tendons and bones, each considered independently, have been published. In recent years, however, research has focused on muscle attachment as the muscle-tendon-bone complex from various perspectives, and there is a growing body of knowledge on SRY-box9 (Sox9) and Mohawk(Mkx), which has been identified as a common controlling factor and a key element. Myostatin, a factor that inhibits muscle growth, has been identified as a potential key element in the mechanisms of lifetime structural maintenance of the muscle-tendon-bone complex. Findings in recent studies have also uncovered aspects of the mechanisms of motor organ complex morphostasis in the superaged society of today and will lay the groundwork for treatments to prevent motor function decline in older adults.

Glucocorticoid Receptor Alpha Targets SLC2A4 to Regulate Protein Synthesis and Breakdown in Porcine Skeletal Muscle Cells

In the presence of stress, the hypothalamic-pituitary-adrenal (HPA) axis activity can be enhanced to promote the secretion of a large amount of glucocorticoids (GCs), which play an important role in the anabolism and catabolism of skeletal muscle. When the endogenous and exogenous glucocorticoids are deficient or excessive, the body will produce stress-related resistance and change the protein metabolism. In this study, we investigated the effect of GC receptor GRα on protein breakdown and synthesis in porcine skeletal muscle cells (PSCs). Overexpression of GRα was shown to increase the expression of protein degradation-related genes, while knockdown of GRα decreased the expression of these genes. Additionally, we found a relationship between GRα and solute carrier family 2 member 4 (SLC2A4), SLC2A4 expression level increases when stress occurs, suggesting that increasing SLC2A4 expression can partially alleviate stress-induced damage, and we found that there is a combination between them via luciferase reporter assays, which still needs to be confirmed in further studies.

The elusive promise of myostatin inhibition for muscular dystrophy

PURPOSE OF REVIEW

Recent terminations of clinical trials of myostatin inhibitors in muscular dystrophy have raised questions about the predictiveness of mouse models for this therapeutic strategy.

RECENT FINDINGS

A variety of myostatin inhibitors have been developed for preclinical and clinical studies. These inhibitors have ameliorated the phenotype of many but not all mouse models of muscular dystrophy. However, randomized double-blinded placebo controlled trials in both pediatric and adult muscular dystrophies have, as of yet, not demonstrated functional improvement.

SUMMARY

The present article will review the preclinical promise of myostatin inhibitors, the clinical trial experience to date of these inhibitors in muscular dystrophy, and the potential reasons for the lack of observed translation.

Exercise Training as Therapeutic Approach in Cancer Cachexia: A Review of Potential Anti-inflammatory Effect on Muscle Wasting

Cachexia is a multifactorial inflammatory syndrome with high prevalence in cancer patients. It is characterized by a metabolic chaos culminating in drastic reduction in body weight, mainly due to skeletal muscle and fat depletion. Currently, there is not a standard intervention for cachexia, but it is believed that a dynamic approach should be applied early in the course of the disease to maintain or slow the loss of physical function. The present review sought to explain the different clinical and experimental applications of different models of exercise and their contribution to a better prognosis of the disease. Here the advances in knowledge about the application of physical training in experimental models are elucidated, tests that contribute substantially to elucidate the cellular and biochemical mechanisms of exercise in different ways, as well as clinical trials that present not only the impacts of exercise in front cachexia but also the challenges of its application in clinical practice.

Candidate rejuvenating factor GDF11 and tissue fibrosis: friend or foe?

Growth differentiation factor 11 (GDF11 or bone morphogenetic protein 11, BMP11) belongs to the transforming growth factor-β superfamily and is closely related to other family member-myostatin (also known as GDF8). GDF11 was firstly identified in 2004 due to its ability to rejuvenate the function of multiple organs in old mice. However, in the past few years, the heralded rejuvenating effects of GDF11 have been seriously questioned by many studies that do not support the idea that restoring levels of GDF11 in aging improves overall organ structure and function. Moreover, with increasing controversies, several other studies described the involvement of GDF11 in fibrotic processes in various organ setups. This review paper focuses on the GDF11 and its pro- or anti-fibrotic actions in major organs and tissues, with the goal to summarize our knowledge on its emerging role in regulating the progression of fibrosis in different pathological conditions, and to guide upcoming research efforts.

Regenerating Urethral Striated Muscle by CRISPRi/dCas9-KRAB-Mediated Myostatin Silencing for Obesity-Associated Stress Urinary Incontinence

Overweight females are prone to obesity-associated stress urinary incontinence (OA-SUI), and there are no definitive medical therapies for this common urologic condition. This study was designed to test the hypothesis that regenerative therapy to restore urethral striated muscle (stM) and pelvic floor muscles might represent a valuable therapeutic approach. For the in vitro experiment, single-guide RNAs targeting myostatin (MSTN) were used for CRISPRi/dCas9-Kruppel associated box (KRAB)-mediated gene silencing. For the in vivo experiment, a total of 14 female lean ZUC-Leprfa 186 and 14 fatty ZUC-Leprfa 185 rats were used as control and CRISPRi-MSTN treated groups, respectively. The results indicated that lentivirus-mediated expression of MSTN CRISPRi/dCas9-KRAB caused sustained downregulation of MSTN in rat L6 myoblast cells and significantly enhanced myogenesis in vitro. In vivo, the urethral sphincter injection of lentiviral-MSTN sgRNA and lentiviral-dCas9-KRAB significantly increased the leak point pressure, the thickness of the stM layer, the ratio of stM to smooth muscle, and the number of neuromuscular junctions. Downregulation of MSTN with CRISPRi/dCas9-KRAB-mediated gene silencing significantly enhanced myogenesis in vitro and in vivo. It also improved urethral continence in the OA-SUI rat model.

Similar sequences but dissimilar biological functions of GDF11 and myostatin

Growth differentiation factor 11 (GDF11) and myostatin (MSTN) are closely related TGFβ family members that are often believed to serve similar functions due to their high homology. However, genetic studies in animals provide clear evidence that they perform distinct roles. While the loss of Mstn leads to hypermuscularity, the deletion of Gdf11 results in abnormal skeletal patterning and organ development. The perinatal lethality of Gdf11-null mice, which contrasts with the long-term viability of Mstn-null mice, has led most research to focus on utilizing recombinant GDF11 proteins to investigate the postnatal functions of GDF11. However, the reported outcomes of the exogenous application of recombinant GDF11 proteins are controversial partly because of the different sources and qualities of recombinant GDF11 used and because recombinant GDF11 and MSTN proteins are nearly indistinguishable due to their similar structural and biochemical properties. Here, we analyze the similarities and differences between GDF11 and MSTN from an evolutionary point of view and summarize the current understanding of the biological processing, signaling, and physiological functions of GDF11 and MSTN. Finally, we discuss the potential use of recombinant GDF11 as a therapeutic option for a wide range of medical conditions and the possible adverse effects of GDF11 inhibition mediated by MSTN inhibitors.

Activin type II receptor ligand signaling inhibition after experimental ischemic heart failure attenuates cardiac remodeling and prevents fibrosis

Myostatin (MSTN) is a transforming growth factor (TGF)-β superfamily member that acts as a negative regulator of muscle growth and may play a role in cardiac remodeling. We hypothesized that inhibition of activin type II receptors (ACTRII) to reduce MSTN signaling would reduce pathological cardiac remodeling in experimental heart failure (HF). C57BL/6J mice underwent left anterior descending coronary artery ligation under anesthesia to induce myocardial infarction (MI) or no ligation (sham). MI and sham animals were each randomly divided into groups (n ≥ 10 mice/group) receiving an ACTRII or ACTRII/TGFβ receptor-signaling inhibiting strategy: 1) myo-Fc group (weekly 10 mg/kg Myo-Fc) or 2) Fol + TGFi group (daily 12 µg/kg follistatin plus 2 mg/kg TGFβ receptor inhibitor), versus controls. ACTRII/TGFBR signaling inhibition preserved cardiac function by echocardiography and prevented an increase in brain natriuretic peptide (BNP). ACTRII/TGFBR inhibition resulted in increased phosphorylation (P) of Akt and decreased P-p38 mitogen-activated protein kinase (MAPK) in MI mice. In vitro, Akt contributed to P-SMAD2,3, P-p38, and BNP regulation in cardiomyocytes. ACTRII/TGFBR inhibition increased sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA2a) levels and decreased unfolded protein response (UPR) markers in MI mice. ACTRII/TGFBR inhibition was associated with a decrease in cardiac fibrosis and fibrosis markers, connective tissue growth factor (CTGF), type I collagen, fibronectin, α-smooth muscle actin, and matrix metalloproteinase (MMP)-12 in MI mice. MSTN exerted a direct regulation on the UPR marker eukaryotic translation initiation factor-2α (eIf2α) in cardiomyocytes. Our study suggests that ACTRII ligand inhibition has beneficial effects on cardiac signaling and fibrosis after ischemic HF.NEW & NOTEWORTHY Activin type II receptor ligand inhibition resulted in preserved cardiac function, a decrease in cardiac fibrosis, improved SERCA2a levels, and a prevention of the unfolded protein response in mice with myocardial infarction.

Tissue repair brakes: A common paradigm in the biology of regeneration

To date, most attention on tissue regeneration has focused on the exploration of positive cues promoting or allowing the engagement of natural cellular restoration upon injury. In contrast, the signals fostering cell identity maintenance in the vertebrate body have been poorly investigated; yet they are crucial, for their counteraction could become a powerful method to induce and modulate regeneration. Here we review the mechanisms inhibiting pro-regenerative spontaneous adaptive cell responses in different model organisms and organs. The pharmacological or genetic/epigenetic modulation of such regenerative brakes could release a dormant but innate adaptive competence of certain cell types and therefore boost tissue regeneration in different situations.

Featured characteristics and pivotal roles of satellite cells in skeletal muscle regeneration

Skeletal muscle, the essential organ for locomotion, as well as energy reservoir and expenditure, has robust regenerative capacity in response to mechanical stress and injury. As muscle-specific stem cells, satellite cells are responsible for providing new myoblasts during the process of muscle growth and regeneration. Self-renewal capacity and the fate of satellite cells are highly regulated and influenced by their surrounding factors, such as extracellular matrix and soluble proteins. The strong myogenic potential of satellite cells makes them a potential resource for stem cell therapy to cure genetic muscle disease and repair injured muscle. Here, we both review key features of satellite cells during skeletal muscle development and regeneration and summarize recent outcomes of satellite cell transplantation studies.

Muscle fibrosis in the soft palate: Delivery of cells, growth factors and anti-fibrotics

The healing of skeletal muscle injuries after major trauma or surgical reconstruction is often complicated by the development of fibrosis leading to impaired function. Research in the field of muscle regeneration is mainly focused on the restoration of muscle mass while far less attention is paid to the prevention of fibrosis. In this review, we take as an example the reconstruction of the muscles in the soft palate of cleft palate patients. After surgical closure of the soft palate, muscle function during speech is often impaired by a shortage of muscle tissue as well as the development of fibrosis. We will give a short overview of the most common approaches to generate muscle mass and then focus on strategies to prevent fibrosis. These include anti-fibrotic strategies that have been developed for muscle and other organs by the delivery of small molecules, decorin and miRNAs. Anti-fibrotic compounds should be delivered in aligned constructs in order to obtain the organized architecture of muscle tissue. The available techniques for the preparation of aligned muscle constructs will be discussed. The combination of approaches to generate muscle mass with anti-fibrotic components in an aligned muscle construct may greatly improve the functional outcome of regenerative therapies for muscle injuries.

Signals from the Niche: Insights into the Role of IGF-1 and IL-6 in Modulating Skeletal Muscle Fibrosis

Muscle regeneration, characterized by the activation and proliferation of satellite cells and other precursors, is accompanied by an inflammatory response and the remodeling of the extracellular matrix (ECM), necessary to remove cellular debris and to mechanically support newly generated myofibers and activated satellite cells. Muscle repair can be considered concluded when the tissue architecture, vascularization, and innervation have been restored. Alterations in these connected mechanisms can impair muscle regeneration, leading to the replacement of functional muscle tissue with a fibrotic scar. In the present review, we will discuss the cellular mediators of fibrosis and how the altered expression and secretion of soluble mediators, such as IL-6 and IGF-1, can modulate regulatory networks involved in the altered regeneration and fibrosis during aging and diseases.

Effects of myostatin on the mechanical properties of muscles during repeated active lengthening in the mouse

The aim of the present study was to investigate how myostatin dysfunction affects fast and slow muscle stiffness and viscosity during severe repeated loading. Isolated extensor digitorum longus (EDL) and soleus muscles of young adult female mice of the BEH (dysfunctional myostatin) and BEH+/+ (functional myostatin) strains were subjected to 100 contraction-stretching loading cycles during which contractile and mechanical properties were assessed. BEH mice exhibited greater exercise-induced muscle damage, although the effect was muscle- and age-dependent and limited to the early phases of simulated exercise. The relative reduction of the EDL muscle isometric force recorded during the initial 10-30 loading cycles was greater in BEH mice than in BEH+/+ mice and exceeded that of the soleus muscle of either strain. The induced damage was associated with lower muscle stiffness. The effects of myostatin on the mechanical properties of muscles depend on muscle type and maturity.

Inhibition of the Activin Receptor Type-2B Pathway Restores Regenerative Capacity in Satellite Cell-Depleted Skeletal Muscle

Degenerative myopathies typically display a decline in satellite cells coupled with a replacement of muscle fibers by fat and fibrosis. During this pathological remodeling, satellite cells are present at lower numbers and do not display a proper regenerative function. Whether a decline in satellite cells directly contributes to disease progression or is a secondary result is unknown. In order to dissect these processes, we used a genetic model to reduce the satellite cell population by ~70–80% which leads to a nearly complete loss of regenerative potential. We observe that while no overt tissue damage is observed following satellite cell depletion, muscle fibers atrophy accompanied by changes in the stem cell niche cellular composition. Treatment of these mice with an Activin receptor type-2B (AcvR2B) pathway blocker reverses muscle fiber atrophy as expected, but also restores regenerative potential of the remaining satellite cells. These findings demonstrate that in addition to controlling fiber size, the AcvR2B pathway acts to regulate the muscle stem cell niche providing a more favorable environment for muscle regeneration.

Exercise mitigates the effects of hyperhomocysteinemia on adverse muscle remodeling

Hyperhomocysteinemia (HHcy) is known for causing inflammation and vascular remodeling, particularly through production of reactive oxygen species (ROS) and matrix metalloproteinase-9 (MMP-9) activation. Although its effect on the skeletal muscle is unclear, HHcy can cause skeletal muscle weakness and functional impairment by induction of inflammatory mediators and macrophage mediated injury. Exercise has been shown to reduce homocysteine levels and therefore, could serve as a promising intervention for HHcy. The purpose of this study was to investigate whether HHcy causes skeletal muscle fibrosis through induction of inflammation and determine whether exercise can mitigate these effects. C57BL/6J (WT) and CBS+/- (HHcy) mice were administered a 6 weeks treadmill exercise protocol. Hindlimb perfusion was measured via laser Doppler. Measurement of skeletal muscle protein expression was done by western blot. Levels of skeletal muscle MMP-9 mRNA were determined by qPCR. Collagen deposition in the skeletal muscle was measured using Masson's trichrome staining. In CBS+/- mice, HHcy manifested with decreased body weight and femoral artery lumen diameter, as well as a trend of lower hindlimb perfusion. These mice displayed increased wall to lumen ratio, mean arterial blood pressure, collagen deposition, and elevated myostatin protein expression. Exercise mitigated the effects above in CBS+/- mice. Skeletal muscle from CBS+/- mice had elevated markers of remodeling and hypoxia: iNOS, EMMPRIN, and MMP-9. We conclude that HHcy causes skeletal muscle fibrosis possibly through induction of EMMPRIN/MMP-9 and exercise is capable of mitigating the pathologies associated with HHcy.

Regulation of fibrosis in muscular dystrophy

The production of force and power are inherent properties of skeletal muscle, and regulated by contractile proteins within muscle fibers. However, skeletal muscle integrity and function also require strong connections between muscle fibers and their extracellular matrix (ECM). A well-organized and pliant ECM is integral to muscle function and the ability for many different cell populations to efficiently migrate through ECM is critical during growth and regeneration. For many neuromuscular diseases, genetic mutations cause disruption of these cytoskeletal-ECM connections, resulting in muscle fragility and chronic injury. Ultimately, these changes shift the balance from myogenic pathways toward fibrogenic pathways, culminating in the loss of muscle fibers and their replacement with fatty-fibrotic matrix. Hence a common pathological hallmark of muscular dystrophy is prominent fibrosis. This review will cover the salient features of muscular dystrophy pathogenesis, highlight the signals and cells that are important for myogenic and fibrogenic actions, and discuss how fibrosis alters the ECM of skeletal muscle, and the consequences of fibrosis in developing therapies.

Identification of satellite cells from anole lizard skeletal muscle and demonstration of expanded musculoskeletal potential

The lizards are evolutionarily the closest vertebrates to humans that demonstrate the ability to regenerate entire appendages containing cartilage, muscle, skin, and nervous tissue. We previously isolated PAX7-positive cells from muscle of the green anole lizard, Anolis carolinensis, that can differentiate into multinucleated myotubes and express the muscle structural protein, myosin heavy chain. Studying gene expression in these satellite/progenitor cell populations from A. carolinensis can provide insight into the mechanisms regulating tissue regeneration. We generated a transcriptome from proliferating lizard myoprogenitor cells and compared them to transcriptomes from the mouse and human tissues from the ENCODE project using XGSA, a statistical method for cross-species gene set analysis. These analyses determined that the lizard progenitor cell transcriptome was most similar to mammalian satellite cells. Further examination of specific GO categories of genes demonstrated that among genes with the highest level of expression in lizard satellite cells were an increased number of genetic regulators of chondrogenesis, as compared to mouse satellite cells. In micromass culture, lizard PAX7-positive cells formed Alcian blue and collagen 2a1 positive nodules, without the addition of exogenous morphogens, unlike their mouse counterparts. Subsequent quantitative RT-PCR confirmed up-regulation of expression of chondrogenic regulatory genes in lizard cells, including bmp2, sox9, runx2, and cartilage specific structural genes, aggrecan and collagen 2a1. Taken together, these data suggest that tail regeneration in lizards involves significant alterations in gene regulation with expanded musculoskeletal potency.

Effects of a systemic enzyme therapy in healthy active adults after exhaustive eccentric exercise: a randomised, two-stage, double-blinded, placebo-controlled trial

Background

Systemic enzyme therapy may improve symptoms of exhaustive eccentric exercise due to anti-inflammatory properties.

Methods

In a randomised, placebo-controlled, two-stage clinical trial, systemic enzyme therapy (Wobenzym) was administered for 72 hours before and 72 hours following a day on which subjects performed an exhaustive eccentric exercise (isokinetic loading of the quadriceps). Efficacy criteria (maximal strength and pain) and time points were selected to account for the multidimensional nature of exercise-induced muscle damage symptoms. Subjects were randomised in a crossover (stage I, n=28) and parallel group design (stage II, n=44).

Results

Analysis of stage I data demonstrated a significant superiority (Mann-Whitney=0.6153; p=0.0332; one sided) for systemic enzyme therapy compared with placebo. Stage II was designed as a randomised controlled parallel group comparison. Heterogeneity (I²>0.5) between stages led to separate analyses of stage I (endurance-trained subjects) and stage II (strength-trained subjects). Combined analysis resulted in no evidence for corresponding treatment effects. Analysis of pooled biomarker data, however, demonstrated significant favourable effects for systemic enzyme therapy in both stages.

Conclusion

Systemic enzyme therapy before and after exhaustive eccentric exercise resulted in higher maximal concentric strength in the less strength-trained subjects (stage I) and in significant favourable effects on biomarkers (inflammatory, metabolic and immune) in all subjects. The application of these findings needs further evaluation.

Myostatin inhibitor ACE-031 treatment of ambulatory boys with Duchenne muscular dystrophy: Results of a randomized, placebo-controlled clinical trial

INTRODUCTION

ACE-031 is a fusion protein of activin receptor type IIB and IgG1-Fc, which binds myostatin and related ligands. It aims to disrupt the inhibitory effect on muscle development and provide potential therapy for myopathies like Duchenne muscular dystrophy (DMD).

METHODS

ACE-031 was administered subcutaneously every 2-4 weeks to DMD boys in a randomized, double-blind, placebo-controlled, ascending-dose trial. The primary objective was safety evaluation. Secondary objectives included characterization of pharmacokinetics and pharmacodynamics.

RESULTS

ACE-031 was not associated with serious or severe adverse events. The study was stopped after the second dosing regimen due to potential safety concerns of epistaxis and telangiectasias. A trend for maintenance of the 6-minute walk test (6MWT) distance in the ACE-031 groups compared with a decline in the placebo group (not statistically significant) was noted, as was a trend for increased lean body mass and bone mineral density (BMD) and reduced fat mass.

CONCLUSION

ACE-031 use demonstrated trends for pharmacodynamic effects on lean mass, fat mass, BMD, and 6MWT. Non-muscle-related adverse events contributed to the decision to discontinue the study. Myostatin inhibition is a promising therapeutic approach for DMD. Muscle Nerve 55: 458-464, 2017.

PGC-1α modulates necrosis, inflammatory response, and fibrotic tissue formation in injured skeletal muscle

Background

Skeletal muscle tissue has an enormous regenerative capacity that is instrumental for a successful defense against muscle injury and wasting. The peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) exerts therapeutic effects in several muscle pathologies, but its role in damage-induced muscle regeneration is unclear.

Methods

Using muscle-specific gain- and loss-of-function models for PGC-1α in combination with the myotoxic agent cardiotoxin (CTX), we explored the role of this transcriptional coactivator in muscle damage and inflammation.

Results

Interestingly, we observed PGC-1α-dependent effects at the early stages of regeneration, in particular regarding macrophage accumulation and polarization from the pro-inflammatory M1 to the anti-inflammatory M2 type, a faster resolution of necrosis and protection against the development of fibrosis after multiple CTX-induced injuries.

Conclusions

PGC-1α exerts beneficial effects on muscle inflammation that might contribute to the therapeutic effects of elevated muscle PGC-1α in different models of muscle wasting.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-016-0110-x) contains supplementary material, which is available to authorized users.

Developmental Biology and Regenerative Medicine: Addressing the Vexing Problem of Persistent Muscle Atrophy in the Chronically Torn Human Rotator Cuff

Persistent muscle atrophy in the chronically torn rotator cuff is a significant obstacle for treatment and recovery. Large atrophic changes are predictive of poor surgical and nonsurgical outcomes and frequently fail to resolve even following functional restoration of loading and rehabilitation. New insights into the processes of muscle atrophy and recovery gained through studies in developmental biology combined with the novel tools and strategies emerging in regenerative medicine provide new avenues to combat the vexing problem of muscle atrophy in the rotator cuff. Moving these treatment strategies forward likely will involve the combination of surgery, biologic/cellular agents, and physical interventions, as increasing experimental evidence points to the beneficial interaction between biologic therapies and physiologic stresses. Thus, the physical therapy profession is poised to play a significant role in defining the success of these combinatorial therapies. This perspective article will provide an overview of the developmental biology and regenerative medicine strategies currently under investigation to combat muscle atrophy and how they may integrate into the current and future practice of physical therapy.

KLF7 Regulates Satellite Cell Quiescence in Response to Extracellular Signaling

Retaining muscle stem satellite cell (SC) quiescence is important for the maintenance of stem cell population and tissue regeneration. Accumulating evidence supports the model where key extracellular signals play crucial roles in maintaining SC quiescence or activation, however, the intracellular mechanisms that mediate niche signals to control SC behavior are not fully understood. Here, we reported that KLF7 functioned as a key mediator involved in low-level TGF-β signaling and canonical Notch signaling-induced SC quiescence and myoblast arrest. The data obtained showed that KLF7 was upregulated in quiescent SCs and nonproliferating myoblasts. Silence of KLF7 promoted SCs activation and myoblasts proliferation, but overexpression of KLF7 induced myogenic cell arrest. Notably, the expression of KLF7 was regulated by TGF-β and Notch3 signaling. Knockdown of KLF7 diminished low-level TGF-β and canonical Notch signaling-induced SC quiescence. Investigation into the mechanism revealed that KLF7 regulation of SC function was dependent on p21 and acetylation of Lys227 and/or 231 in the DNA binding domain of KLF7. Our study provides new insights into the regulatory network of muscle stem cell quiescence. Stem Cells 2016;34:1310-1320.

Nfix Regulates Temporal Progression of Muscle Regeneration through Modulation of Myostatin Expression

Nfix belongs to a family of four highly conserved proteins that act as transcriptional activators and/or repressors of cellular and viral genes. We previously showed a pivotal role for Nfix in regulating the transcriptional switch from embryonic to fetal myogenesis. Here, we show that Nfix directly represses the Myostatin promoter, thus controlling the proper timing of satellite cell differentiation and muscle regeneration. Nfix-null mice display delayed regeneration after injury, and this deficit is reversed upon in vivo Myostatin silencing. Conditional deletion of Nfix in satellite cells results in a similar delay in regeneration, confirming the functional requirement for Nfix in satellite cells. Moreover, mice lacking Nfix show reduced myofiber cross sectional area and a predominant slow twitching phenotype. These data define a role for Nfix in postnatal skeletal muscle and unveil a mechanism for Myostatin regulation, thus providing insights into the modulation of its complex signaling pathway.

Suppression of Myostatin Stimulates Regenerative Potential of Injured Antigravitational Soleus Muscle in Mice under Unloading Condition

Effects of myostatin (MSTN)-suppression on the regeneration of injured skeletal muscle under unloading condition were investigated by using transgenic mice expressing a dominant-negative form of MSTN (MSTN-DN). Both MSTN-DN and wild-type (WT) mice were subjected to continuous hindlimb suspension (HS) for 6 weeks. Cardiotoxin (CTX) was injected into left soleus muscle under anesthesia 2 weeks after the initiation of HS. Then, the soleus muscles were excised following 6-week HS (4 weeks after CTX-injection). CTX-injection caused to reduce the soleus fiber cross-sectional area (CSA) in WT mice under both unloading and weight-bearing conditions, but not in MSTN-DN mice. Under unloading condition, CTX-injected muscle weight and fiber CSA in MSTN-DN mice were significantly higher than those in WT mice. CTX-injected muscle had many damaged and regenerating fibers having central nuclei in both WT and MSTN-DN mice. Significant increase in the population of Pax7-positive nuclei in CTX-injected muscle was observed in MSTN-DN mice, but not in WT mice. Evidences indicate that the suppression of MSTN cause to increase the regenerative potential of injured soleus muscle via the increase in the population of muscle satellite cells regardless of unloading conditions.

Mammalian Mss51 is a skeletal muscle-specific gene modulating cellular metabolism

BACKGROUND

The transforming growth factor β (TGF-β) signaling pathways modulate skeletal muscle growth, regeneration, and cellular metabolism. Several recent gene expression studies have shown that inhibition of myostatin and TGF-β1 signaling consistently leads to a significant reduction in expression of Mss51, also named Zmynd17. The function of mammalian Mss51 is unknown although a putative homolog in yeast is a mitochondrial translational activator.

OBJECTIVE

The objective of this work was to characterize mammalian Mss51.

METHODS

Quantitative RT-PCR and immunoblot of subcellular fractionation were used to determine expression patterns and localization of Mss51. The CRISPR/Cas9 system was used to reduce expression of Mss51 in C2C12 myoblasts and the function of Mss51 was evaluated in assays of proliferation, differentiation and cellular metabolism.

RESULTS

Mss51 was predominantly expressed in skeletal muscle and in those muscles dominated by fast-twitch fibers. In vitro, its expression was upregulated upon differentiation of C2C12 myoblasts into myotubes. Expression of Mss51 was modulated in response to altered TGF-β family signaling. In human muscle, Mss51 localized to the mitochondria. Its genetic disruption resulted in increased levels of cellular ATP, β-oxidation, glycolysis, and oxidative phosphorylation.

CONCLUSIONS

Mss51 is a novel, skeletal muscle-specific gene and a key target of myostatin and TGF-β1 signaling. Unlike myostatin, TGF-β1 and IGF-1, Mss51 does not regulate myoblast proliferation or differentiation. Rather, Mss51 appears to be one of the effectors of these growth factors on metabolic processes including fatty acid oxidation, glycolysis and oxidative phosphorylation.

Myostatin: expanding horizons

Myostatin is a secreted growth and differentiation factor that belongs to the TGF-β superfamily. Myostatin is predominantly synthesized and expressed in skeletal muscle and thus exerts a huge impact on muscle growth and function. In keeping with its negative role in myogenesis, myostatin expression is tightly regulated at several levels including epigenetic, transcriptional, post-transcriptional, and post-translational. New revelations regarding myostatin regulation also offer mechanisms that could be exploited for developing myostatin antagonists. Increasingly, it is becoming clearer that besides its conventional role in muscle, myostatin plays a critical role in metabolism. Hence, molecular mechanisms by which myostatin regulates several key metabolic processes need to be further explored.

Runx1 Transcription Factor Is Required for Myoblasts Proliferation during Muscle Regeneration

Following myonecrosis, muscle satellite cells proliferate, differentiate and fuse, creating new myofibers. The Runx1 transcription factor is not expressed in naïve developing muscle or in adult muscle tissue. However, it is highly expressed in muscles exposed to myopathic damage yet, the role of Runx1 in muscle regeneration is completely unknown. Our study of Runx1 function in the muscle’s response to myonecrosis reveals that this transcription factor is activated and cooperates with the MyoD and AP-1/c-Jun transcription factors to drive the transcription program of muscle regeneration. Mice lacking dystrophin and muscle Runx1 (mdx^-/Runx1^f/f), exhibit impaired muscle regeneration leading to age-dependent muscle waste, gradual decrease in motor capabilities and a shortened lifespan. Runx1-deficient primary myoblasts are arrested at cell cycle G₁ and consequently differentiate. Such premature differentiation disrupts the myoblasts’ normal proliferation/differentiation balance, reduces the number and size of regenerating myofibers and impairs muscle regeneration. Our combined Runx1-dependent gene expression, ChIP-seq, ATAC-seq and histone H3K4me1/H3K27ac modification analyses revealed a subset of Runx1-regulated genes that are co-occupied by MyoD and c-Jun in mdx^-/Runx1^f/f muscle. The data provide unique insights into the transcriptional program driving muscle regeneration and implicate Runx1 as an important participant in the pathology of muscle wasting diseases.

In response to muscle injury, the muscle initiates a repair process that calls for the proliferation of muscle stem cells, which differentiate and fuse to create the myofibers that regenerate the tissue. Maintaining the balance between myoblast proliferation and differentiation is crucial for proper regeneration, with disruption leading to impaired regeneration characteristic of muscle-wasting diseases. Our study highlights the important role the Runx1 transcription factor plays in muscle regeneration and in regulating the balance between muscle stem cell proliferation and differentiation. While not expressed in healthy muscle tissue, Runx1 level significantly increases in response to various types of muscle damage. This aligns with our finding that mice lacking Runx1 in their muscles suffer from impaired muscle regeneration. Their muscles contained a significantly low number of regenerating myofibers, which were also relatively smaller in size, resulting in loss of muscle mass and motor capabilities. Our results indicate that Runx1 regulates muscle regeneration by preventing premature differentiation of proliferating myoblasts, thereby facilitating the buildup of the myoblast pool required for proper regeneration. Through genome-wide gene-expression analysis we identify a set of Runx1-regulated genes responsible for muscle regeneration thereby implicating Runx1 in the pathology of muscle wasting diseases such as Duchenne muscular dystrophy.

Myostatin induces interstitial fibrosis in the heart via TAK1 and p38

Myostatin, a member of the TGF-β superfamily of secreted growth factors, is a negative regulator of skeletal muscle growth. In the heart, it is expressed at lower levels compared to skeletal muscle but up-regulated under disease conditions. Cre recombinase-mediated inactivation of myostatin in adult cardiomyocytes leads to heart failure and increased mortality but cardiac function of surviving mice is restored after several weeks probably due to compensatory expression in non-cardiomyocytes. To study long-term effects of increased myostatin expression in the heart and to analyze the putative crosstalk between cardiomyocytes and fibroblasts, we overexpressed myostatin in cardiomyocytes. Increased expression of myostatin in heart muscle cells caused interstitial fibrosis via activation of the TAK-1-MKK3/6-p38 signaling pathway, compromising cardiac function in older mice. Our results uncover a novel role of myostatin in the heart and highlight the necessity for tight regulation of myostatin to maintain normal heart function.

Changes in skeletal muscle and tendon structure and function following genetic inactivation of myostatin in rats

Myostatin is a negative regulator of skeletal muscle and tendon mass. Myostatin deficiency has been well studied in mice, but limited data are available on how myostatin regulates the structure and function of muscles and tendons of larger animals. We hypothesized that, in comparison to wild-type (MSTN(+/+) ) rats, rats in which zinc finger nucleases were used to genetically inactivate myostatin (MSTN(Δ/Δ) ) would exhibit an increase in muscle mass and total force production, a reduction in specific force, an accumulation of type II fibres and a decrease and stiffening of connective tissue. Overall, the muscle and tendon phenotype of myostatin-deficient rats was markedly different from that of myostatin-deficient mice, which have impaired contractility and pathological changes to fibres and their extracellular matrix. Extensor digitorum longus and soleus muscles of MSTN(Δ/Δ) rats demonstrated 20-33% increases in mass, 35-45% increases in fibre number, 20-57% increases in isometric force and no differences in specific force. The insulin-like growth factor-1 pathway was activated to a greater extent in MSTN(Δ/Δ) muscles, but no substantial differences in atrophy-related genes were observed. Tendons of MSTN(Δ/Δ) rats had a 20% reduction in peak strain, with no differences in mass, peak stress or stiffness. The general morphology and gene expression patterns were similar between tendons of both genotypes. This large rodent model of myostatin deficiency did not have the negative consequences to muscle fibres and extracellular matrix observed in mouse models, and suggests that the greatest impact of myostatin in the regulation of muscle mass may not be to induce atrophy directly, but rather to block hypertrophy signalling.

Alternative binding modes identified for growth and differentiation factor-associated serum protein (GASP) family antagonism of myostatin

Myostatin, a member of the TGF-β family of ligands, is a strong negative regulator of muscle growth. As such, it is a prime therapeutic target for muscle wasting disorders. Similar to other TGF-β family ligands, myostatin is neutralized by binding one of a number of structurally diverse antagonists. Included are the antagonists GASP-1 and GASP-2, which are unique in that they specifically antagonize myostatin. However, little is known from a structural standpoint describing the interactions of GASP antagonists with myostatin. Here, we present the First low resolution solution structure of myostatin-free and myostatin-bound states of GASP-1 and GASP-2. Our studies have revealed GASP-1, which is 100 times more potent than GASP-2, preferentially binds myostatin in an asymmetrical 1:1 complex, whereas GASP-2 binds in a symmetrical 2:1 complex. Additionally, C-terminal truncations of GASP-1 result in less potent myostatin inhibitors that form a 2:1 complex, suggesting that the C-terminal domains of GASP-1 are the primary mediators for asymmetric complex formation. Overall, this study provides a new perspective on TGF-β antagonism, where closely related antagonists can utilize different ligand-binding strategies.

A phase 1/2a follistatin gene therapy trial for becker muscular dystrophy

Becker muscular dystrophy (BMD) is a variant of dystrophin deficiency resulting from DMD gene mutations. Phenotype is variable with loss of ambulation in late teenage or late mid-life years. There is currently no treatment for this condition. In this BMD proof-of-principle clinical trial, a potent myostatin antagonist, follistatin (FS), was used to inhibit the myostatin pathway. Extensive preclinical studies, using adeno-associated virus (AAV) to deliver follistatin, demonstrated an increase in strength. For this trial, we used the alternatively spliced FS344 to avoid potential binding to off target sites. AAV1.CMV.FS344 was delivered to six BMD patients by direct bilateral intramuscular quadriceps injections. Cohort 1 included three subjects receiving 3 × 10(11) vg/kg/leg. The distance walked on the 6MWT was the primary outcome measure. Patients 01 and 02 improved 58 meters (m) and 125 m, respectively. Patient 03 showed no change. In Cohort 2, Patients 05 and 06 received 6 × 10(11) vg/kg/leg with improved 6MWT by 108 m and 29 m, whereas, Patient 04 showed no improvement. No adverse effects were encountered. Histological changes corroborated benefit showing reduced endomysial fibrosis, reduced central nucleation, more normal fiber size distribution with muscle hypertrophy, especially at high dose. The results are encouraging for treatment of dystrophin-deficient muscle diseases.

Interrelationship between bone substitution materials and skeletal muscle tissue

Bone density and quantity are primary conditions for the insertion and stability of dental implants. In cases of a lack of adequate maxillary or mandibulary bone, bone augmentation will be necessary. The use of synthetic bioactive bone substitution materials is of increasing importance as alternatives to autogenously bone grafts. It is well known that bone can influence muscle function and muscle function can influence bone structures. Muscles have a considerable potential of adaptation and muscle tissue surrounding an inserted implant or bone surrogate can integrate changes in mechanical load of the muscle and hereupon induce signaling cascades with protein synthesis and arrangement of the cytoskeleton. The Musculus latissimus dorsi is very often used for the analyses of the in vivo biocompatibility of newly designed biomaterials. Beside macroscopically and histologically examination, biocompatibility can be assessed by analyses of the biomaterial influence of gene expression. This review discusses changes in the fiber type distribution, myosin heavy chain isoform composition, histological appearance and vascularization of the skeletal muscle after implantation of bone substitution materials. Especially, the effects of bone surrogates should be described at the molecular-biological and cellular level.

Myostatin inhibits proliferation and insulin-stimulated glucose uptake in mouse liver cells

Although myostatin functions primarily as a negative regulator of skeletal muscle growth and development, accumulating biological and epidemiological evidence indicates an important contributing role in liver disease. In this study, we demonstrate that myostatin suppresses the proliferation of mouse Hepa-1c1c7 murine-derived liver cells (50%; p < 0.001) in part by reducing the expression of the cyclins and cyclin-dependent kinases that elicit G1-S phase transition of the cell cycle (p < 0.001). Furthermore, real-time PCR-based quantification of the long noncoding RNA metastasis associated lung adenocarcinoma transcript 1 (Malat1), recently identified as a myostatin-responsive transcript in skeletal muscle, revealed a significant downregulation (25% and 50%, respectively; p < 0.05) in the livers of myostatin-treated mice and liver cells. The importance of Malat1 in liver cell proliferation was confirmed via arrested liver cell proliferation (p < 0.05) in response to partial Malat1 siRNA-mediated knockdown. Myostatin also significantly blunted insulin-stimulated glucose uptake and Akt phosphorylation in liver cells while increasing the phosphorylation of myristoylated alanine-rich C-kinase substrate (MARCKS), a protein that is essential for cancer cell proliferation and insulin-stimulated glucose transport. Together, these findings reveal a plausible mechanism by which circulating myostatin contributes to the diminished regenerative capacity of the liver and diseases characterized by liver insulin resistance.

Negative auto-regulation of myostatin expression is mediated by Smad3 and microRNA-27

Growth factors, such as myostatin (Mstn), play an important role in regulating post-natal myogenesis. In fact, loss of Mstn has been shown to result in increased post-natal muscle growth through enhanced satellite cell functionality; while elevated levels of Mstn result in dramatic skeletal muscle wasting through a mechanism involving reduced protein synthesis and increased ubiquitin-mediated protein degradation. Here we show that miR-27a/b plays an important role in feed back auto-regulation of Mstn and thus regulation of post-natal myogenesis. Sequence analysis of Mstn 3' UTR showed a single highly conserved miR-27a/b binding site and increased expression of miR-27a/b was correlated with decreased expression of Mstn and vice versa both in vitro and in mice in vivo. Moreover, we also show that Mstn gene expression was regulated by miR-27a/b. Treatment with miR-27a/b-specific AntagomiRs resulted in increased Mstn expression, reduced myoblast proliferation, impaired satellite cell activation and induction of skeletal muscle atrophy that was rescued upon either blockade of, or complete absence of, Mstn. Consistent with this, miR-27a over expression resulted in reduced Mstn expression, skeletal muscle hypertrophy and an increase in the number of activated satellite cells, all features consistent with impaired Mstn function. Loss of Smad3 was associated with increased levels of Mstn, concomitant with decreased miR-27a/b expression, which is consistent with impaired satellite cell function and muscular atrophy previously reported in Smad3-null mice. Interestingly, treatment with Mstn resulted in increased miR-27a/b expression, which was shown to be dependent on the activity of Smad3. These data highlight a novel auto-regulatory mechanism in which Mstn, via Smad3 signaling, regulates miR-27a/b and in turn its own expression. In support, Mstn-mediated inhibition of Mstn 3' UTR reporter activity was reversed upon miR-27a/b-specific AntagomiR transfection. Therefore, miR-27a/b, through negatively regulating Mstn, plays a role in promoting satellite cell activation, myoblast proliferation and preventing muscle wasting.