Reversine ameliorates hallmarks of cellular senescence in human skeletal myoblasts via reactivation of autophagy

Cellular senescence leads to the depletion of myogenic progenitors and decreased regenerative capacity. We show that the small molecule 2,6-disubstituted purine, reversine, can improve some well-known hallmarks of cellular aging in senescent myoblast cells. Reversine reactivated autophagy and insulin signaling pathway via upregulation of Adenosine Monophosphate-activated protein kinase (AMPK) and Akt2, restoring insulin sensitivity and glucose uptake in senescent cells. Reversine also restored the loss of connectivity of glycolysis to the TCA cycle, thus restoring dysfunctional mitochondria and the impaired myogenic differentiation potential of senescent myoblasts. Altogether, our data suggest that cellular senescence can be reversed by treatment with a single small molecule without employing genetic reprogramming technologies.

Murine muscle stem cell response to perturbations of the neuromuscular junction are attenuated with aging

During aging and neuromuscular diseases, there is a progressive loss of skeletal muscle volume and function impacting mobility and quality of life. Muscle loss is often associated with denervation and a loss of resident muscle stem cells (satellite cells or MuSCs); however, the relationship between MuSCs and innervation has not been established. Herein, we administered severe neuromuscular trauma to a transgenic murine model that permits MuSC lineage tracing. We show that a subset of MuSCs specifically engraft in a position proximal to the neuromuscular junction (NMJ), the synapse between myofibers and motor neurons, in healthy young adult muscles. In aging and in a mouse model of neuromuscular degeneration (Cu/Zn superoxide dismutase knockout - Sod1-/-), this localized engraftment behavior was reduced. Genetic rescue of motor neurons in Sod1-/- mice reestablished integrity of the NMJ in a manner akin to young muscle and partially restored MuSC ability to engraft into positions proximal to the NMJ. Using single cell RNA-sequencing of MuSCs isolated from aged muscle, we demonstrate that a subset of MuSCs are molecularly distinguishable from MuSCs responding to myofiber injury and share similarity to synaptic myonuclei. Collectively, these data reveal unique features of MuSCs that respond to synaptic perturbations caused by aging and other stressors.

Multilayered Human Skeletal Muscle Myoblast Sheets Promote the Healing Process After Colonic Anastomosis in Rats

Colorectal anastomotic leakage is one of the most feared and fatal complications of colorectal surgery. To date, no external coating material that can prevent anastomotic leakage has been developed. As myoblasts possess anti-inflammatory capacity and improve wound healing, we developed a multilayered human skeletal muscle myoblast (HSMM) sheet by periodic exposure to supraphysiological hydrostatic pressure during repeated cell seeding. We assessed whether the application of an HSMM sheet can promote the healing process after colonic anastomosis. Partial colectomy and insufficient suturing were employed to create a high-risk colo-colonic anastomosis model in 60 nude rats. Rats were divided into a control group (n = 30) and an HSMM sheet group (n = 30). Macroscopic findings, anastomotic bursting pressure, and histology at the colonic anastomotic site were evaluated on postoperative day (POD) 3, 5, 7, 14, and 28. The application of an HSMM sheet significantly suppressed abscess formation at the anastomotic site compared to the control group on POD3 and 5. The anastomotic bursting pressure in the HSMM sheet group was higher than that in the control group on POD3 and 5. Inflammatory cell infiltration in the HSMM sheet group was significantly suppressed compared to that in the control group throughout the time course. Collagen deposition in the HSMM sheet group on POD3 was significantly abundant compared to that in the control group. Regeneration of the mucosa at the colonic anastomotic site was promoted in the HSMM sheet group compared to that in the control group on POD14 and 28. Immunohistochemical analysis demonstrated that surviving cells in the HSMM sheet gradually decreased with postoperative time and none were detected on POD14. These results suggest that the application of a multilayered HSMM sheet may prevent postoperative colonic anastomotic leakage.

A Two-Week Insulin Infusion in Intrauterine Growth Restricted Fetal Sheep at 75% Gestation Increases Skeletal Myoblast Replication but Did Not Restore Muscle Mass or Increase Fiber Number

Intrauterine growth restricted (IUGR) fetuses are born with lower skeletal muscle mass, fewer proliferating myoblasts, and fewer myofibers compared to normally growing fetuses. Plasma concentrations of insulin, a myogenic growth factor, are lower in IUGR fetuses. We hypothesized that a two-week insulin infusion at 75% gestation would increase myoblast proliferation and fiber number in IUGR fetal sheep. Catheterized control fetuses received saline (CON-S, n=6), and the IUGR fetuses received either saline (IUGR-S, n=7) or insulin (IUGR-I, 0.014 ± 0.001 units/kg/hr, n=11) for 14 days. Fetal arterial blood gases and plasma amino acid levels were measured. Fetal skeletal muscles (biceps femoris, BF; and flexor digitorum superficialis, FDS) and pancreases were collected at necropsy (126 ± 2 dGA) for immunochemistry analysis, real-time qPCR, or flow cytometry. Insulin concentrations in IUGR-I and IUGR-S were lower vs. CON-S (P ≤ 0.05, group). Fetal arterial P_aO₂, O₂ content, and glucose concentrations were lower in IUGR-I vs. CON-S (P ≤ 0.01) throughout the infusion period. IGF-1 concentrations tended to be higher in IUGR-I vs. IUGR-S (P=0.06), but both were lower vs. CON-S (P ≤ 0.0001, group). More myoblasts were in S/G₂ cell cycle stage in IUGR-I vs. both IUGR-S and CON-S (145% and 113%, respectively, P ≤ 0.01). IUGR-I FDS muscle weighed 40% less and had 40% lower fiber number vs. CON-S (P ≤ 0.05) but were not different from IUGR-S. Myonuclear number per fiber and the mRNA expression levels of muscle regulatory factors were not different between groups. While the pancreatic β-cell mass was lower in both IUGR-I and IUGR-S compared to CON-S, the IUGR groups were not different from each other indicating that feedback inhibition by endogenous insulin did not reduce β-cell mass. A two-week insulin infusion at 75% gestation promoted myoblast proliferation in the IUGR fetus but did not increase fiber or myonuclear number. Myoblasts in the IUGR fetus retain the capacity to proliferate in response to mitogenic stimuli, but intrinsic defects in the fetal myoblast by 75% gestation may limit the capacity to restore fiber number.

The regulatory role of Myomaker and Myomixer-Myomerger-Minion in muscle development and regeneration

Skeletal muscle plays essential roles in motor function, energy, and glucose metabolism. Skeletal muscle formation occurs through a process called myogenesis, in which a crucial step is the fusion of mononucleated myoblasts to form multinucleated myofibers. The myoblast/myocyte fusion is triggered and coordinated in a muscle-specific way that is essential for muscle development and post-natal muscle regeneration. Many molecules and proteins have been found and demonstrated to have the capacity to regulate the fusion of myoblast/myocytes. Interestingly, two newly discovered muscle-specific membrane proteins, Myomaker and Myomixer (also called Myomerger and Minion), have been identified as fusogenic regulators in vertebrates. Both Myomaker and Myomixer-Myomerger-Minion have the capacity to directly control the myogenic fusion process. Here, we review and discuss the latest studies related to these two proteins, including the discovery, structure, expression pattern, functions, and regulation of Myomaker and Myomixer-Myomerger-Minion. We also emphasize and discuss the interaction between Myomaker and Myomixer-Myomerger-Minion, as well as their cooperative regulatory roles in cell-cell fusion. Moreover, we highlight the areas for exploration of Myomaker and Myomixer-Myomerger-Minion in future studies and consider their potential application to control cell fusion for cell-therapy purposes.

Dystrobrevin alpha gene is a direct target of the vitamin D receptor in muscle

The biologically active metabolite of vitamin D, 1,25-dihydroxyvitamin D3 (VD3), exerts its tissue-specific actions through binding to its intracellular vitamin D receptor (VDR) which functions as a heterodimer with retinoid X receptor (RXR) to recognize vitamin D response elements (VDRE) and activate target genes. Upregulation of VDR in murine skeletal muscle cells occurs concomitantly with transcriptional regulation of key myogenic factors upon VD3 administration, reinforcing the notion that VD3 exerts beneficial effects on muscle. Herein we elucidated the regulatory role of VD3/VDR axis on the expression of dystrobrevin alpha (DTNA), a member of dystrophin-associated protein complex (DAPC). In C2C12 cells, Dtna and VDR gene and protein expression were upregulated by 1-50 nM of VD3 during all stages of myogenic differentiation. In the dystrophic-derived H2K-mdx52 cells, upregulation of DTNA by VD3 occurred upon co-transfection of VDR and RXR expression vectors. Silencing of MyoD1, an E-box binding myogenic transcription factor, did not alter the VD3-mediated Dtna induction, but Vdr silencing abolished this effect. We also demonstrated that VD3 administration enhanced the muscle-specific Dtna promoter activity in presence of VDR/RXR only. Through site-directed mutagenesis and chromatin immunoprecipitation assays, we have validated a VDRE site in Dtna promoter in myogenic cells. We have thus proved that the positive regulation of Dtna by VD3 observed during in vitro murine myogenic differentiation is VDR mediated and specific. The current study reveals a novel mechanism of VDR-mediated regulation for Dtna, which may be positively explored in treatments aiming to stabilize the DAPC in musculoskeletal diseases.

Extracellular matrix remodelling induced by alternating electrical and mechanical stimulations increases the contraction of engineered skeletal muscle tissues

Engineered skeletal muscles are inferior to natural muscles in terms of contractile force, hampering their potential use in practical applications. One major limitation is that the extracellular matrix (ECM) not only impedes the contraction but also ineffectively transmits the forces generated by myotubes to the load. In the present study, ECM remodelling improves contractile force in a short time, and a coordinated, combined electrical and mechanical stimulation induces the desired ECM remodelling. Notably, the application of single and combined stimulations to the engineered muscles remodels the structure of their ECM networks, which determines the mechanical properties of the ECM. Myotubes in the tissues are connected in parallel and in series to the ECM. The stiffness of the parallel ECM must be low not to impede contraction, while the stiffness of the serial ECM must be high to transmit the forces to the load. Both the experimental results and the mechanistic model suggest that the combined stimulation through coordination reorients the ECM fibres in such a way that the parallel ECM stiffness is reduced, while the serial ECM stiffness is increased. In particular, 3 and 20 minutes of alternating electrical and mechanical stimulations increase the force by 18% and 31%, respectively.

Mitochondrial and cytosolic sources of hydrogen peroxide in resting C2C12 myoblasts

The relative contributions of different mitochondrial and cytosolic sources of superoxide and hydrogen peroxide in cells are not well established because of a lack of suitable quantitative assays. To address this problem using resting C2C12 myoblasts we measured the effects of specific inhibitors that do not affect other pathways on the rate of appearance of hydrogen peroxide in the extracellular medium. We used inhibitors of NADPH oxidases (NOXs), suppressors of site I_Q electron leak (S1QELs) at mitochondrial Complex I, and suppressors of site III_Qo electron leak (S3QELs) at mitochondrial Complex III. Around 40% of net cellular hydrogen peroxide release was from NOXs and approximately 45% was from the two mitochondrial sites; 30% from site III_Qo and 15% from site I_Q. As expected, decreasing cytosolic antioxidant capacity by lowering glutathione levels increased the absolute rates from all sites without changing their proportions, whereas decreasing antioxidant defenses in the mitochondrial matrix increased only the absolute and relative contributions of the two mitochondrial sites. These results show directly that mitochondria are a major contributor to cytosolic hydrogen peroxide in resting C2C12 myoblasts, and provide the first direct evidence of superoxide/hydrogen peroxide production from site I_Q in unstressed cells.

Preventing muscle wasting by osteoporosis drug alendronate in vitro and in myopathy models via sirtuin-3 down-regulation

BACKGROUND

A global consensus on the loss of skeletal muscle mass and function in humans refers as sarcopenia and cachexia including diabetes, obesity, renal failure, and osteoporosis. Despite a current improvement of sarcopenia or cachexia with exercise training and supportive therapies, alternative and specific managements are needed to discover for whom are unable or unwilling to embark on these treatments. Alendronate is a widely used drug for osteoporosis in the elderly and postmenopausal women. Osteopenic menopausal women with 6 months of alendronate therapy have been observed to improve not only lumbar bone mineral density but also handgrip strength. However, the effect and mechanism of alendronate on muscle strength still remain unclear. Here, we investigated the therapeutic potential and the molecular mechanism of alendronate on the loss of muscle mass and strength in vitro and in vivo.

METHODS

Mouse myoblasts and primary human skeletal muscle-derived progenitor cells were used to assess the effects of low-dose alendronate (0.1-1 μM) combined with or without dexamethasone on myotube hypertrophy and myogenic differentiation. Moreover, we also evaluated the effects of low-dose alendronate (0.5 and 1 mg/kg) by oral administration on the limb muscle function and morphology of mice with denervation-induced muscle atrophy and glycerol-induced muscle injury.

RESULTS

Alendronate inhibited dexamethasone-induced myotube atrophy and myogenic differentiation inhibition in mouse myoblasts and primary human skeletal muscle-derived progenitor cells. Alendronate significantly abrogated dexamethasone-up-regulated sirtuin-3 (SIRT3), but not SIRT1, protein expression in myotubes. Both SIRT3 inhibitor AKG7 and SIRT3-siRNA transfection could also reverse dexamethasone-up-regulated atrogin-1 and SIRT3 protein expressions. Animal studies showed that low-dose alendronate by oral administration ameliorated the muscular malfunction in mouse models of denervation-induced muscle atrophy and glycerol-induced muscle injury with a negative regulation of SIRT3 expression.

CONCLUSIONS

The putative mechanism by which muscle atrophy was improved with alendronate might be through the SIRT3 down-regulation. These findings suggest that alendronate can be a promising therapeutic strategy for management of muscle wasting-related diseases and sarcopenia.

Myocyte enhancer factor 2A promotes proliferation and its inhibition attenuates myogenic differentiation via myozenin 2 in bovine skeletal muscle myoblast

Myocyte enhancer factor 2A (MEF2A) is widely distributed in various tissues or organs and plays crucial roles in multiple biological processes. To examine the potential effects of MEF2A on skeletal muscle myoblast, the functional role of MFE2A in myoblast proliferation and differentiation was investigated. In this study, we found that the mRNA expression level of Mef2a was dramatically increased during the myogenesis of bovine skeletal muscle primary myoblast. Overexpression of MEF2A significantly promoted myoblast proliferation, while knockdown of MEF2A inhibited the proliferation and differentiation of myoblast. RT-PCR and western blot analysis revealed that this positive effect of MEF2A on the proliferation of myoblast was carried out by triggering cell cycle progression by activating CDK2 protein expression. Besides, MEF2A was found to be an important transcription factor that bound to the myozenin 2 (MyoZ2) proximal promoter and performed upstream of MyoZ2 during myoblast differentiation. This study provides the first experimental evidence that MEF2A is a positive regulator in skeletal muscle myoblast proliferation and suggests that MEF2A regulates myoblast differentiation via regulating MyoZ2.

Optimal Ultrasound Exposure Conditions for Maximizing C2C12 Muscle Cell Proliferation and Differentiation

Described here is an in vitro systematic investigation of the effects on C2C12 myoblasts of exposure to finely controlled and repeatable low-intensity pulsed ultrasound of different frequencies (500 kHz, 1 MHz, 3 MHz and 5 MHz) and different intensities (250, 500 and 1000 mW/cm²). An in-house stimulation system and an ultrasound-transparent cell culture well minimized reflections and attenuations, allowing precise control of ultrasound delivery. Results indicated that a 3 MHz stimulation at 1 W/cm² intensity maximized cell proliferation in comparison with the other exposure conditions and untreated controls. In contrast, cell differentiation and the consequent formation of multinucleated myotubes were maximized by 1 MHz stimulation at 500 mW/cm² intensity. The highly controlled exposure conditions employed allowed precise correlation of the ultrasound delivery to the bio-effects produced, thus overcoming the inconsistency of some results available in the literature and contributing to the potential of ultrasound treatment for muscle therapy and regeneration.

Rev-Erb co-regulates muscle regeneration via tethered interaction with the NF-Y cistrome

Objective

The loss of skeletal muscle mass and strength are a central feature of traumatic injury and degenerative myopathies. Unfortunately, pharmacological interventions typically fail to stem the long-term decline in quality of life. Reduced Rev-Erb-mediated gene suppression in cultured C2C12 myoblasts has been shown to stimulate myoblast differentiation. Yet the mechanisms that allow Rev-Erb to pleiotropically inhibit muscle differentiation are not well understood. In this study, we sought to elucidate the role of Rev-Erb in the regulation of muscle differentiation and regeneration in vivo.

Methods

Using Rev-Erbα/β shRNAs, pharmacological ligands, and Rev-Erbα null and heterozygous mice, we probed the mechanism of Rev-Erbα/β regulation of muscle differentiation and muscle regeneration.

Results

ChIP seq analysis of Rev-Erb in differentiating myoblasts showed that Rev-Erbα did not transcriptionally regulate muscle differentiation through cognate Rev-Erb/ROR-response elements but through possible interaction with the cell fate regulator NF-Y at CCAAT-motifs. Muscle differentiation is stimulated by Rev-Erb release from CCAAT-motifs at promoter and enhancer elements of a number of myogenesis proteins. Partial loss of Rev-Erb expression in mice heterozygous for Rev-Erbα accelerated muscle repair in vivo whereas Rev-Erb knockout mice showed deficiencies in regenerative repair compared to wild type mice. These phenotypic differences between heterozygous and knockout mice were not apparently dependent on MRF induction in response to injury. Similarly, pharmacological disruption of Rev-Erb suppressive activity in injured muscle accelerated regenerative repair in response to acute injury.

Conclusions

Disrupting Rev-Erb activity in injured muscle accelerates regenerative muscle repair/differentiation through transcriptional de-repression of myogenic programs. Rev-Erb, therefore, may be a potent therapeutic target for a myriad of muscular disorders.

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The nuclear receptor Rev-Erb is released from myogenic gene promoter and enhancer regions during myoblast differentiation.

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Rev-Erb regulates myoblast differentiation through interaction with the cell-fate regulator NF-Y at CCAAT-motifs.

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Rev-Erb antagonists accelerate muscle regeneration following acute muscle injury by stimulating myoblast differentiation.

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Human muscle differentiation can be regulated using pharmacological ligands that target Rev-Erb.

Exercise training alters DNA methylation patterns in genes related to muscle growth and differentiation in mice

The adaptive response of skeletal muscle to exercise training is tightly controlled and therefore requires transcriptional regulation. DNA methylation is an epigenetic mechanism known to modulate gene expression, but its contribution to exercise-induced adaptations in skeletal muscle is not well studied. Here, we describe a genome-wide analysis of DNA methylation in muscle of trained mice (n = 3). Compared with sedentary controls, 2,762 genes exhibited differentially methylated CpGs (P < 0.05, meth diff >5%, coverage >10) in their putative promoter regions. Alignment with gene expression data (n = 6) revealed 200 genes with a negative correlation between methylation and expression changes in response to exercise training. The majority of these genes were related to muscle growth and differentiation, and a minor fraction involved in metabolic regulation. Among the candidates were genes that regulate the expression of myogenic regulatory factors (Plexin A2) as well as genes that participate in muscle hypertrophy (Igfbp4) and motor neuron innervation (Dok7). Interestingly, a transcription factor binding site enrichment study discovered significantly enriched occurrence of CpG methylation in the binding sites of the myogenic regulatory factors MyoD and myogenin. These findings suggest that DNA methylation is involved in the regulation of muscle adaptation to regular exercise training.

Peroxiredoxin 3 has a crucial role in the contractile function of skeletal muscle by regulating mitochondrial homeostasis

Antioxidant systems against reactive oxygen species (ROS) are important factors in regulating homeostasis in various cells, tissues, and organs. Although ROS are known to cause to muscular disorders, the effects of mitochondrial ROS in muscle physiology have not been fully understood. Here, we investigated the effects of ROS on muscle mass and function using mice deficient in peroxiredoxin 3 (Prx3), which is a mitochondrial antioxidant protein. Ablation of Prx3 deregulated the mitochondrial network and membrane potential of myotubes, in which ROS levels were increased. We showed that the DNA content of mitochondria and ATP production were also reduced in Prx3-KO muscle. Of note, the mitofusin 1 and 2 protein levels decreased in Prx3-KO muscle, a biochemical evidence of impaired mitochondrial fusion. Contractile dysfunction was examined by measuring isometric forces of isolated extensor digitorum longus (EDL) and soleus muscles. Maximum absolute forces in both the EDL and the soleus muscles were not significantly affected in Prx3-KO mice. However, fatigue trials revealed that the decrease in relative force was greater and more rapid in soleus from Prx3-KO compared to wild-type mice. Taken together, these results suggest that Prx3 plays a crucial role in mitochondrial homeostasis and thereby controls the contractile functions of skeletal muscle.

Inositol 1,4,5 trisphosphate receptor 1 is a key player of human myoblast differentiation

Cytosolic Ca(2+) signals are fundamental for the early and late steps of myoblast differentiation and are, as in many cells, generated by Ca(2+) release from internal stores as well as by plasma membrane Ca(2+) entry. Our recent studies identified the store-operated Ca(2+) channels, Orai1 and TRPC1&C4, as crucial for the early steps of human myogenesis and for the late fusion events. In the present work, we assessed the role of the inositol-1,4,5 tris-phosphate receptor (IP3R) type 1 during human myoblast differentiation. We demonstrated, using siRNA strategy that IP3R1 is required for the expression of muscle-specific transcription factors such as myogenin and MEF2 (myocyte enhancer factor 2), and for the formation of myotubes. The knockdown of IP3R1 strongly reduced endogenous spontaneous Ca(2+) transients, and attenuated store-operated Ca(2+) entry. As well, two Ca(2+)-dependent key enzymes of muscle differentiation, NFAT and CamKII are down-regulated upon siIP3R1 treatment. On the contrary, the overexpression of IP3R1 accelerated myoblasts differentiation. These findings identify Ca(2+) release mediated by IP3R1 as an essential mechanism during the early steps of myoblast differentiation.

Rapid depletion of muscle progenitor cells in dystrophic mdx/utrophin-/- mice

Duchenne muscular dystrophy (DMD) patients lack dystrophin from birth; however, muscle weakness becomes apparent only at 3-5 years of age, which happens to coincide with the depletion of the muscle progenitor cell (MPC) pools. Indeed, MPCs isolated from older DMD patients demonstrate impairments in myogenic potential. To determine whether the progression of muscular dystrophy is a consequence of the decline in functional MPCs, we investigated two animal models of DMD: (i) dystrophin-deficient mdx mice, the most commonly utilized model of DMD, which has a relatively mild dystrophic phenotype and (ii) dystrophin/utrophin double knock-out (dKO) mice, which display a similar histopathologic phenotype to DMD patients. In contrast to age-matched mdx mice, we observed that both the number and regeneration potential of dKO MPCs rapidly declines during disease progression. This occurred in MPCs at both early and late stages of myogenic commitment. In fact, early MPCs isolated from 6-week-old dKO mice have reductions in proliferation, resistance to oxidative stress and multilineage differentiation capacities compared with age-matched mdx MPCs. This effect may potentially be mediated by fibroblast growth factor overexpression and/or a reduction in telomerase activity. Our results demonstrate that the rapid disease progression in the dKO model is associated, at least in part, with MPC depletion. Therefore, alleviating MPC depletion could represent an approach to delay the onset of the histopathologies associated with DMD patients.

Induction of skeletal muscle differentiation in vitro by therapeutic ultrasound

Therapeutic ultrasound (TU) has been used for the last 50 y in rehabilitation, including treatment of soft tissues. Ultrasound waves can be employed in two different modes of operation, continuous and pulsed, which produce both thermal and non-thermal effects. Despite the large-scale use of TU, there are few scientific studies on its biologic effects during skeletal muscle differentiation. To better analyze the cellular effects of TU, we decided to follow cells in vitro. The main purpose of this study was to evaluate the effects of TU in primary chick myogenic cell cultures using phase contrast optical microscopy and immunofluorescence microscopy, followed by image analysis and quantification. Our results indicate that TU can stimulate the differentiation of skeletal muscle cells in vitro, as measured by the thickness of multinucleated myotubes, the ratio of mononucleated cells to multinucleated cells and expression of the muscle-specific protein desmin. This study is a first step toward a metrologic and science-based protocol for cell treatment under different ultrasound field exposures.

[Effects of mechanical stimulation on expression of autoantigens in myoblasts]

OBJECTIVE

To explore the effects of mechanical stimulation on the expression of autoantigens in myoblasts.

METHODS

According to different processing methods, C2C12 cells were divided into the experimental group and control group; the experimental group was divided into 4 subgroups: 2-, 4-, and 6-day and 1-day stretch groups. In 2-, 4-, and 6-day stretch groups, mechanical loading was added on the C2C12 cells at a stretching frequency of 0.25 Hz and cellular deformation amplitude of 10%, 2 hours a day for 2, 4, and 6 days respectively by Flexercell 5000 strain unit, and at a stretching frequency of 1 Hz and cellular deformation amplitude of 15% for 1 hour in 1-day stretch group. In the control group, the cells were routinely cultured for 1, 2, 4, and 6 days (1-, 2-, 4-, and 6-day control). The cells were observed by inverted phase contrast microscope. The cell proliferation was detected by flow cytometry; the expressions of autoantigens were detected by Western blot method, including the Ku/the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs), U1-70 (A part of ATP-dependent DNA helicase II), histidyl tRNA synthetase (HRS), and Mi-2 (reconfigurable components deacetylase complexes of NuRD).

RESULTS

The exfoliated cells were found in 1-day stretch group, but no exfoliated cell was seen in the control group for 1-day culture. The cells proliferated more obviously in 2-day stretch group than in the control group for 2-day culture; cell differentiation was found in 4-day stretch group, and cell fusion in 6-day stretch group, which were similar to those in the control group for 4- and 6-day culture. After single stretching, cell apoptosis was found in 1-day stretch group, showing no significant difference in the relative DNA proliferation index (DPI) when compared with DPI of control group for 1-day culture (t = 0.346, P = 0.747). After cyclic stretching, DPIs of 2- and 4- day stretch groups were significantly increased when compared with those of the control group for 2- and 4-day culture (P < 0.05), but no significant difference was found between control group for 6-day culture and 6-day stretch group (t = 1.191, P = 0.303). Compared with the control group for 2-day culture, the relative protein expression of autoantigens (DNA-Pkcs, Mi-2, HRS, and U1-70) in 2-day stretch group decreased significantly (P < 0.05), but no significant difference was found between control group for 4-day culture and 4-day stretch group (P > 0.05). The relative protein expressions of autoantigens in 4-day stretch group significantly increased when compared with those of 2-day stretch group (P < 0.05), but the relative protein expressions of autoantigens in the control group for 4-day culture significantly decreased when compared with those of the control group for 2-day culture (P < 0.05).

CONCLUSION

Short-term mechanical stimulation can inhibit the expressions of autoantigens in myoblasts, but with the time prolonging, cell differentiation and fusion and adaptation to mechanical stimulation would result in diminished inhibitory effect.

The combination of glycosaminoglycans and fibrous proteins improves cell proliferation and early differentiation of bovine primary skeletal muscle cells

Primary muscle cell model systems from farm animals are widely used to acquire knowledge about muscle development, muscle pathologies, overweight issues and tissue regeneration. The morphological properties of a bovine primary muscle cell model system, in addition to cell proliferation and differentiation features, were characterized using immunocytochemistry, western blotting and real-time PCR. We observed a reorganization of the Golgi complex in differentiated cells. The Golgi complex transformed to a highly fragmented network of small stacks of cisternae positioned throughout the myotubes as well as around the nucleus. Different extracellular matrix (ECM) components were used as surface coatings in order to improve cell culture conditions. Our experiments demonstrated improved proliferation and early differentiation for cells grown on surface coatings containing a mixture of both glycosaminoglycans (GAGs) and fibrous proteins. We suggest that GAGs and fibrous proteins mixed together into a composite biomaterial can mimic a natural ECM, and this could improve myogenesis for in vitro cell cultures.

Epigenetic regulation of skeletal muscle development and differentiation

Skeletal muscle cells have served as a paradigm for understanding mechanisms leading to cellular differentiation. Formation of skeletal muscle involves a series of steps in which cells are committed towards the myogenic lineage, undergo expansion to give rise to myoblasts that differentiate into multinucleated myotubes, and mature to form adult muscle fibers. The commitment, proliferation, and differentiation of progenitor cells involve both genetic and epigenetic changes that culminate in alterations in gene expression. Members of the Myogenic regulatory factor (MRF), as well as the Myocyte Enhancer Factor (MEF2) families control distinct steps of skeletal muscle proliferation and differentiation. In addition, -growing evidence indicates that chromatin modifying enzymes and remodeling complexes epigenetically reprogram muscle promoters at various stages that preclude or promote MRF and MEF2 activites. Among these, histone deacetylases (HDACs), histone acetyltransferases (HATs), histone methyltransferases (HMTs) and SWI/SNF complexes alter chromatin structure through post-translational modifications to impact MRF and MEF2 activities. With such new and emerging knowledge, we are beginning to develop a true molecular understanding of the mechanisms by which skeletal muscle development and differentiation is regulated. Elucidation of the mechanisms by which epigenetic regulators control myogenesis will likely provide a new foundation for the development of novel therapeutic drugs for muscle dystrophies, ageing-related regeneration defects that occur due to altered proliferation and differentiation, and other malignancies.

In vitro susceptibility of thioredoxins and glutathione to redox modification and aging-related changes in skeletal muscle

Thioredoxins (Trx's) regulate redox signaling and are localized to various cellular compartments. Specific redox-regulated pathways for adaptation of skeletal muscle to contractions are attenuated during aging, but little is known about the roles of Trx's in regulating these pathways. This study investigated the susceptibility of Trx1 and Trx2 in skeletal muscle to oxidation and reduction in vitro and the effects of aging and contractions on Trx1, Trx2, and thioredoxin reductase (TrxR) 1 and 2 contents and nuclear and cytosolic Trx1 and mitochondrial Trx2 redox potentials in vivo. The proportions of cytosolic and nuclear Trx1 and mitochondrial Trx2 in the oxidized or reduced forms were analyzed using redox Western blotting. In myotubes, the mean redox potentials were nuclear Trx1, -251 mV; cytosolic Trx1, -242mV; mitochondrial Trx2, -346mV, data supporting the occurrence of differing redox potentials between cell compartments. Exogenous treatment of myoblasts and myotubes with hydrogen peroxide or dithiothreitol modified glutathione redox status and nuclear and cytosolic Trx1, but mitochondrial Trx2 was unchanged. Tibialis anterior muscles from young and old mice were exposed to isometric muscle contractions in vivo. Aging increased muscle contents of Trx1, Trx2, and TrxR2, but neither aging nor endogenous ROS generated during contractions modified Trx redox potentials, although oxidation of glutathione and other thiols occurred. We conclude that glutathione redox couples in skeletal muscle are more susceptible to oxidation than Trx and that Trx proteins are upregulated during aging, but do not appear to modulate redox-regulated adaptations to contractions that fail during aging.

Comparative expression profiling identifies differential roles for Myogenin and p38α MAPK signaling in myogenesis

Skeletal muscle differentiation is mediated by a complex gene expression program requiring both the muscle-specific transcription factor Myogenin (Myog) and p38α MAPK (p38α) signaling. However, the relative contribution of Myog and p38α to the formation of mature myotubes remains unknown. Here, we have uncoupled the activity of Myog from that of p38α to gain insight into the individual roles of these proteins in myogenesis. Comparative expression profiling confirmed that Myog activates the expression of genes involved in muscle function. Furthermore, we found that in the absence of p38α signaling, Myog expression leads to the down-regulation of genes involved in cell cycle progression. Consistent with this, the expression of Myog is sufficient to induce cell cycle exit. Interestingly, p38α-defective, Myog-expressing myoblasts fail to form multinucleated myotubes, suggesting an important role for p38α in cell fusion. Through the analysis of p38α up-regulated genes, the tetraspanin CD53 was identified as a candidate fusion protein, a role confirmed both ex vivo in primary myoblasts, and in vivo during myofiber regeneration in mice. Thus, our study has revealed an unexpected role for Myog in mediating cell cycle exit and has identified an essential role for p38α in cell fusion through the up-regulation of CD53.

The roles of RGD and grooved topography in the adhesion, morphology, and differentiation of C2C12 skeletal myoblasts

Both chemical and topographic cues are crucial for the development of skeletal muscle. In this study, the relative roles of both signals in regard to cell adhesion, morphology, and differentiation of C2C12 skeletal myoblasts were investigated. Grooved polystyrene substrates containing grooves with approximately 900 nm in width with 600 nm ridge spans and 665 nm in depth were conjugated with the cell adhesion peptide arginine-glycine-aspartic acid (RGD). RGD conjugation significantly enhanced the adhesion, growth and differentiation of C2C12 cells. On the other hand, anisotropic topography primarily directed the direction and alignment of myoblasts and myotubes. The results in this study provide information regarding the relative roles of chemical and topographic cues in musculoskeletal myogenesis, and are of interest to applications in muscle tissue engineering.

Regulation of Pax7 protein levels by caspase-3 and proteasome activity in differentiating myoblasts

The transcription factor Pax7 negatively regulates the activity of the muscle regulatory transcription factor MyoD, preventing muscle precursor cells from undergoing terminal differentiation. In this context, the ratio between Pax7 and MyoD protein levels is thought to be critical in allowing myogenesis to proceed or to maintain the undifferentiated muscle precursor state. We have previously shown that Pax7 is subject to rapid down regulation in differentiating myoblasts, via a proteasome-dependent pathway. Here we present evidence indicating that Pax7 is also subject to caspase-3-dependent regulation. Furthermore, simultaneous inhibition of caspase-3 and proteasome activity induced further accumulation of Pax7 protein in differentiating myoblasts. These results suggest that at early stages of muscle differentiation, Pax7 levels are regulated by at least two independent mechanisms involving caspase-3 and proteasome activity.

Novel hyperbranched polyamidoamine nanoparticles for transfecting skeletal myoblasts with vascular endothelial growth factor gene for cardiac repair

We investigated the feasibility and efficacy of hyperbranched polyamidoamine (hPAMAM) mediated human vascular endothelial growth factor-165 (hVEGF(165)) gene transfer into skeletal myoblasts for cardiac repair. The hPAMAM was synthesized using a modified one-pot method. Encapsulated DNA was protected by hPAMAM from degradation for over 120 min. The transfection efficiency of hPAMAM in myoblasts was 82.6 ± 7.0% with cell viability of 94.6 ± 1.4% under optimal conditions. The hPAMAM showed much higher transfection efficiency (P < 0.05) than polyetherimide and Lipofectamine 2000 with low cytotoxicity. The transfected skeletal myoblasts gave stable hVEGF(165) expression for 18 days. After transplantation of hPAMAM-hVEGF(165) transfected cells, apoptotic myocardial cells decreased at day 1 and heart function improved at day 28, with increased neovascularization (P < 0.05). These results indicate that hPAMAM-based gene delivery into myoblasts is feasible and effective and may serve as a novel and promising non-viral DNA vehicle for gene therapy in myocardial infarction.

Ornithine decarboxylase is upregulated by the androgen receptor in skeletal muscle and regulates myoblast proliferation

The aim of this study is to determine if the Odc1 gene, which encodes ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine biosynthesis, is directly regulated by the androgen receptor (AR) in skeletal muscle myoblasts and if Odc1 regulates myoblast proliferation and differentiation. We previously showed that expression of Odc1 is decreased in muscle from AR knockout male mice. In this study, we show in vivo that Odc1 expression is also decreased >60% in muscle from male muscle-specific AR knockout mice. In normal muscle homeostasis, Odc1 expression is regulated by age and sex, reflecting testosterone levels, as muscle of adult male mice expresses high levels of Odc1 compared with age-matched females and younger males. In vitro, expression of Odc1 is 10- and 1.5-fold higher in proliferating mouse C(2)C(12) and human skeletal muscle myoblasts, respectively, than in differentiated myotubes. Dihydrotestosterone increases Odc1 levels 2.7- and 1.6-fold in skeletal muscle cell myoblasts after 12 and 24 h of treatment, respectively. Inhibition of ODC activity in C(2)C(12) myoblasts by α-difluoromethylornithine decreases myoblast number by 40% and 66% following 48 and 72 h of treatment, respectively. In contrast, overexpression of Odc1 in C(2)C(12) myoblasts results in a 27% increase in cell number vs. control when cells are grown under differentiation conditions for 96 h. This prolonged proliferation is associated with delayed differentiation, with reduced expression of the differentiation markers myogenin and Myf6 in Odc1-overexpressing cells. In conclusion, androgens act via the AR to upregulate Odc1 in skeletal muscle myoblasts, and Odc1 promotes myoblast proliferation and delays differentiation.

No effect of low-level lasers on in vitro myoblast culture

Effects of phototherapy using low-level lasers depend on irradiation parameters and the type of laser used. The aim of the present study was to evaluate the effect of phototherapy on the proliferation of cultured C2C12 myoblasts under different nutritional conditions using low-level GaAlAs and InGaAlP lasers with different parameters and incubation periods. C2C12 cells cultured in regular and nutrient-deficient medium were irradiated with low-level GaAlAs (780 nm) and InGaA1P (660 nm) lasers with energy densities of 3.8, 6.3 and 10 J/cm2, and 3.8, 10 and 17.5 J/cm2, respectively. Cell proliferation was assessed 48 and 72 h after irradiation by MTT assay. There were no significant differences in cell proliferation between laser-treated myoblasts and control cultures for any of the parameters and incubation periods. Further studies are necessary to determine the correct laser parameters for optimizing the biostirhulation of myoblasts.

Myoblast models of skeletal muscle hypertrophy and atrophy

PURPOSE OF REVIEW

To highlight recent breakthroughs and controversies in the use of myoblast models to uncover cellular and molecular mechanisms regulating skeletal muscle hypertrophy and atrophy.

RECENT FINDINGS

Myoblast cultures provide key mechanistic models of the signalling and molecular pathways potentially employed by skeletal muscle in-vivo to regulate hypertrophy and atrophy. Recently the controversy as to whether insulin-like growth factor (IGF)-I is important in hypertrophy following mechanical stimuli vs. alternative pathways has been hotly debated and is discussed. The role of myostatin in myoblast models of atrophy and interactions between protein synthetic pathways including Akt/mTOR and the 'atrogenes' are explored.

SUMMARY

Targeted in-vivo experimentation directed by skeletal muscle cell culture and bioengineering (three-dimensional skeletal muscle cell culture models) will provide key biomimetic and mechanistic data regarding hypertrophy and atrophy and thus enable the development of important strategies for tackling muscle wasting associated with ageing and disease processes.

Cell cycle regulation during proliferation and differentiation of mammalian muscle precursor cells

Proliferation and differentiation of muscle precursor cells are intensively studied not only in the developing mouse embryo but also using models of skeletal muscle regeneration or analyzing in vitro cultured cells. These analyses allowed to show the universality of the cell cycle regulation and also uncovered tissue-specific interplay between major cell cycle regulators and factors crucial for the myogenic differentiation. Examination of the events accompanying proliferation and differentiation leading to the formation of functional skeletal muscle fibers allows understanding the molecular basis not only of myogenesis but also of skeletal muscle regeneration. This chapter presents the basis of the cell cycle regulation in proliferating and differentiating muscle precursor cells during development and after muscle injury. It focuses at major cell cycle regulators, myogenic factors, and extracellular environment impacting on the skeletal muscle.

Biological and morphological characterization of in vitro expanded human muscle-derived stem cells

Stem cells are generally characterised as clonogenic and undifferentiated cells with the capacity of self-renewal and plasticity. Over the past few years, the adult stem cells have been derived from various types of tissues including the skeletal muscle. The main goal of the present study was the isolation, in vitro expansion and characterisation of muscle-derived stem cells (MDSCs). Thereby obtained results showed that MDSCs have a fibroblast-like shape with a large nucleus having one to four nucleoli. The cytoplasm was transparent without any signs of vacuolisation. TEM analysis showed an ultrastructure of cells with high proteosynthetic activity. MDSCs had a large and irregular nucleus with variable number of nucleoli. The cytoplasm contained a richly developed and rough endoplasmic reticulum, prominent Golgi apparatus cisterns as well as transport vesicles containing glycogen granules and variable microvilli and filopodia. They expressed alpha-actin and desmin. Results of the phenotypic characterization showed that the analyzed cells were positive for CD29, CD34, CD44, CD90, CD105 and HLA Class I. They did not express CD14, CD45, CD235a, HLA Class II and human fibroblast surface protein. According to these results it should be emphasised that MDSCs after performing the detailed studies focused on their immunological properties and differentiation potential may be used in the cell therapy of many degenerative diseases.

TRAF6 regulates proliferation and differentiation of skeletal myoblasts

We could recently demonstrate an important role of receptor interacting protein-2 (RIP2), an activator of nuclear factor kappa B (NF-κB) and a target of activated receptors of the tumor necrosis factor receptor (TNFR) type, in myogenic differentiation and regeneration. Here, we analyze a potential role of TNFR associated factor 6 (TRAF6), which also associates with the cytoplasmic domain of TNFR type, but also IL-1-R and TLR type receptors, and activates NF-κB, in these processes. Specifically, we show that during myogenic differentiation in vitro, traf6 gene expression is downregulated in normal myoblasts, but not in rhabdomyosarcoma cells, suggesting a role of the TRAF6 protein in this process. Inhibition of traf6 expression using specific siRNAs led to an inhibition of both myoblast proliferation and differentiation, whereas inhibition of the TRAF6 effector NF-κB alone in our system only blocked proliferation. Finally, we demonstrate that the traf6 gene is downregulated in skeletal muscle tissue of the dystrophic mdx mouse. Taken together, these data argue for a role of TRAF6 in the regulation of skeletal muscle differentiation and regeneration.

Ferritin overexpression for noninvasive magnetic resonance imaging-based tracking of stem cells transplanted into the heart

An unmet need in cardiac cell therapy is a noninvasive imaging technique capable of tracking changes in graft size over time and monitoring cell dynamics such as replication and death, factors to which commonly used superparamagnetic nanoparticles are insensitive. Our goal was to explore if overexpression of ferritin, a nontoxic iron-binding protein, can be used for noninvasive magnetic resonance imaging (MRI) of cells transplanted into the infarcted heart. Mouse skeletal myoblasts (C2C12 cells) were engineered to overexpress ferritin. Ferritin overexpression did not interfere with cell viability, proliferation, or differentiation into multinucleated myotubes. Ferritin overexpression caused a 25% decrease in T2 relaxation time in vitro compared to wild-type cells. Transgenic grafts were detected in vivo 3 weeks after transplantation into infarcted hearts of syngeneic mice as areas of hypointensity caused by iron accumulation in overexpressed ferritin complexes. Graft size evaluation by MRI correlated tighly with histologic measurements (R2 = .8). Our studies demonstrated the feasibility of ferritin overexpression in mouse skeletal myoblasts and the successful detection of transgenic cells by MRI in vitro and in vivo after transplantation into the infarcted mouse heart. These experiments lay the groundwork for using the MRI gene reporter ferritin to track stem cells transplanted to the heart.

AlphaB-crystallin is involved in oxidative stress protection determined by VEGF in skeletal myoblasts

Recent studies suggest that the effects of VEGF-A, the prototype VEGF ligand, may extend to a variety of cell types other than endothelial cells. The expression of VEGF-A and its main receptors, Flt-1/VEGFR-1 and KDR/Flk-1/VEGFR-2, was indeed detected in several cell types, including cardiac myocytes and regenerating myotubes. In addition to its proangiogenic activity, evidence indicates that VEGF-A can sustain skeletal muscle regeneration by enhancing the survival and migration of myogenic cells and by promoting the growth of myogenic fibers. In this study, our aim was to investigate whether VEGF could protect skeletal muscle satellite cells from apoptotic cell death triggered by reactive oxygen species and to identify the main molecular mechanisms. C2C12 mouse myoblasts, cultured in vitro in the presence of exogenous VEGF or stably transfected with a plasmid vector expressing VEGF-A, were subjected to oxidative stress and analyzed for cell growth and survival, induction of apoptosis, and molecular signaling. The results of our study demonstrated that VEGF protects C2C12 myoblasts from apoptosis induced by oxidative or hypoxic-like stress. This protection did not correlate with the modulation of the expression of VEGF receptors, but is clearly linked to the phosphorylation of the KDR/Flk-1 receptor, the activation of NF-kappaB, and/or the overexpression of the antiapoptotic protein alphaB-crystallin.

AMPK inhibits myoblast differentiation through a PGC-1alpha-dependent mechanism

Elevated phosphorylation of AMP-activated protein kinase (AMPK) has been shown to inhibit skeletal muscle growth in both culture and animal models, but its role in differentiation of muscle cells is less clear. p21 is known to have an important role in differentiation, but AMPK's role regulating p21 in differentiation in muscle cultures is unknown. Therefore, the purpose of this study was to determine the role of p21 in differentiation of skeletal muscle cells under conditions of elevated AMPK phosphorylation. Treating C(2)C(12) myoblast cultures with 1 mM 5-aminoimidazole-4-carboxamide 1-beta-D-ribonucleoside (AICAR) for up to 24 h induced AMPK phosphorylation. Activation of AMPK reduced p21 protein and mRNA expression, which was associated with reduced G(1)/S cell cycle transition and p21 promoter activity. AICAR-treated myoblasts undergoing differentiation also had reduced p21 protein expression, reduced myotube formation, and myosin accumulation. When myotube cultures were treated with AICAR for 24 h, p21, myosin protein expression, and MyoD were significantly reduced. Myotube atrophy was also apparent compared with control conditions. Addition of compound C, an AMPK inhibitor, attenuated AICAR's negative effects on the myotube cultures. The nuclear expression of p21 protein appeared to be more affected by AICAR-treated myotubes than the cytosolic portion of p21 protein, which was attenuated with compound C treatment. Further analysis revealed that AICAR treatment increased PGC-1alpha and decreased FOXO3A protein expression, which was reversed with compound C cotreatment. Knockdown of PGC-1alpha with shRNA corroborated the compound C data, preserving nuclear FOXO3A and p21 protein expression. These data demonstrate that AICAR-induced AMPK phosphorylation inhibits cell cycle transition, reducing differentiation of myoblasts into myotubes, through PGC-1alpha-FOXO3A-p21.

Human myoblasts modulate the function of antigen-presenting cells

Muscle biopsy specimens of myositis patients were analyzed for the presence of dendritic cells (DC) and macrophages (MPh) by immunohistochemistry. The interaction of DC and myoblasts (MB) was studied by coculture and effects on DC phenotype and function were assessed by flow cytometry and T-cell proliferation assays. Effects of MB-lysates on the phagocytic capacity of MPh were analyzed in bead-incorporation assays. Myositis specimens revealed a tendency towards more immature DC. MB modulated the maturation state of DC and DC recovered from MB-coculture had an inhibitory effect on T-cell proliferation. MB-lysates strongly stimulated MPh phagocytosis. Hypothetically, MB might modulate APC, counterbalancing immune-mediated damage.

Isolation and Characterization of Mesoangioblasts from Facioscapulohumeral Muscular Dystrophy Muscle Biopsies

Facioscapulohumeral muscular dystrophy (FSHD) is the third most frequent inherited muscle disease. Because in FSHD patients the coexistence of affected and unaffected muscles is common, myoblasts expanded from unaffected FSHD muscles have been proposed as suitable tools for autologous cell transplantation. Mesoangioblasts are a new class of adult stem cells of mesodermal origin, potentially useful for the treatment of primitive myopathies of different etiology. Here, we report the isolation and characterization of mesoangioblasts from FSHD muscle biopsies and describe morphology, proliferation, and differentiation abilities of both mesoangioblasts and myoblasts derived from various affected and unaffected muscles of nine representative FSHD patients. We demonstrate that mesoangioblasts can be efficiently isolated from FSHD muscle biopsies and expanded to an amount of cells necessary to transplant into an adult patient. Proliferating mesoangioblasts from all muscles examined did not differ from controls in terms of morphology, phenotype, proliferation rate, or clonogenicity. However, their differentiation ability into skeletal muscle was variably impaired, and this defect correlated with the overall disease severity and the degree of histopathologic abnormalities of the muscle of origin. A remarkable differentiation defect was observed in mesoangioblasts from all mildly to severely affected FSHD muscles, whereas mesoangioblasts from morphologically normal muscles showed no myogenic differentiation block. Our study could open the way to cell therapy for FSHD patients to limit muscle damage in vivo through the use of autologous mesoangioblasts capable of reaching damaged muscles and engrafting into them, without requiring immune suppression or genetic correction in vitro. Disclosure of potential conflicts of interest is found at the end of this article.

Transplantation of nanoparticle transfected skeletal myoblasts overexpressing vascular endothelial growth factor-165 for cardiac repair

BACKGROUND

We investigated the feasibility and efficacy of polyethylenimine (PEI) based human vascular endothelial growth factor-165 (hVEGF165) gene transfer into human skeletal myoblasts (HSM) for cell based delivery to the infarcted myocardium.

METHODS AND RESULTS

Based on optimized transfection procedure using enhanced green fluorescent protein (pEGFP), HSM were transfected with plasmid-hVEGF165 (phVEGF165) carried by PEI (PEI-phVEGF165) nanoparticles. The transfected HSM were characterized for transfection and expression of hVEGF165 in vitro and transplanted into rat heart model of acute myocardial infarction (AMI): group-1=DMEM injection, group-2= HSM transplantation, group-3= PEI-phVEGF165-transfected HSM (PEI-phVEGF165 myoblast) transplantation. A total of 48 rats received cyclosporine injection from 3 days before and until 4 weeks after cell transplantation. Echocardiography was performed to assess the heart function. Animals were sacrificed for molecular and histological studies on the heart tissue at 4 weeks after treatment. Based on optimized transfection conditions, transfected HSM expressed hVEGF165 for 18 days with >90% cell viability in vitro. Apoptotic index was reduced in group-2 and group-3 as compared with group-1. Blood vessel density (x400) by immunostaining for PECAM-1 in group-3 was significantly higher (P=0.043 for both) as compared with group-1 and group-2 at 4 weeks. Regional blood flow (ml/min/g) in the left ventricular anterior wall was higher in group-3 (P=0.043 for both) as compared with group-1 and group-2. Improved ejection fraction was achieved in group-3 (58.44+/-4.92%) as compared with group-1 (P=0.004).

CONCLUSION

PEI nanoparticle mediated hVEGF165 gene transfer into HSM is feasible and safe. It may serve as a novel and efficient alternative for angiomyogenesis in cardiac repair.

Molecular imaging of cardiac stem cell transplantation

In recent years, stem cell therapy for the treatment of heart disease has translated from the imagination of investigators to the bedside of patients. The initial results from trials evaluating cell therapy for the heart are encouraging. As this new field of cellular transplantation matures, it is imperative that novel methodologies for evaluating cell therapy are developed and applied to guide therapy. Molecular imaging is a discipline that is evolving to address these needs and is expected to play an increasing role in the characterization and assessment of cell therapy. This article provides a focused overview of clinical stem cell therapy for the heart, followed by a discussion of how novel molecular imaging techniques are presently being applied to monitor cell therapy.

Multiple effects of high mobility group box protein 1 in skeletal muscle regeneration

OBJECTIVE

High mobility group box 1 protein (HMGB1) is a cytokine released by necrotic and inflammatory cells in response to injury. We examined the role of HMGB1 in skeletal muscle regeneration after hindlimb ischemia.

METHODS AND RESULTS

Unilateral hindlimb ischemia was induced in mice by femoral artery dissection. HMGB1 levels increased in regenerating skeletal muscle and the blockade of endogenous HMGB1 by the administration of its truncated form, the BoxA, resulted in the reduction of vessel density. In contrast, intramuscular administration of HMGB1 enhanced perfusion and increased the number of regenerating fibers. To separately study the myogenic and the angiogenic effects of HMGB1, in vitro experiments were performed with isolated myoblasts and endothelial cells. Myoblasts were found to express the HMGB1 receptor RAGE and TLR4 which were downregulated during in vitro myogenic differentiation. HMGB1 was extracellularly released by differentiated myoblasts and exerted a chemotactic activity on myogenic cells. This effect was partially dependent on RAGE and was inhibited by BoxA treatment. Finally, HMGB1 stimulated tubular-like structure formation by endothelial cells through the activation of extracellular signal-regulated kinase (ERK) and JNK signal transduction pathways.

CONCLUSIONS

HMGB1 plays a role in skeletal muscle regeneration modulating, in an autocrine-paracrine manner, myoblast and endothelial cell functions.

A role for the Myoblast city homologues Dock1 and Dock5 and the adaptor proteins Crk and Crk-like in zebrafish myoblast fusion

Myoblast fusion follows a defined sequence of events that is strikingly similar in vertebrates and invertebrates. Genetic analysis in Drosophila has identified many of the molecules that mediate the different steps in the fusion process; by contrast, the molecular basis of myoblast fusion during vertebrate embryogenesis remains poorly characterised. A key component of the intracellular fusion pathway in Drosophila is the protein encoded by the myoblast city (mbc) gene, a close homologue of the vertebrate protein dedicator of cytokinesis 1 (DOCK1,formerly DOCK180). Using morpholino antisense-oligonucleotide-mediated knockdown of gene activity in the zebrafish embryo, we show that the fusion of embryonic fast-twitch myoblasts requires the activities of Dock1 and the closely related Dock5 protein. In addition, we show that the adaptor proteins Crk and Crk-like (Crkl), with which Dock proteins are known to interact physically, are also required for myoblast fusion.

Recipient age determines the cardiac functional improvement achieved by skeletal myoblast transplantation

OBJECTIVES

The aim of the current study was to evaluate the effect of recipient age on the regenerative response to implantation with young skeletal myoblasts (SKMCs) after a coronary artery ligation.

BACKGROUND

In contrast with previous findings in animals, the initial clinical trials of cell transplantation after a myocardial infarction have reported only limited improvements in ventricular function. The restricted regenerative capacity of cells isolated from older patients is certainly a factor; however, the present study investigated the impact of another potentially significant factor: recipient age.

METHODS

We compared the myogeneic capacities of SKMCs isolated from young rats (3 months old) and older rats (24 months old). Highly myogenic SKMCs derived from young rats (or culture media, in control rats) were then transplanted into the infarcted myocardium of young and older recipients at 1 week after coronary ligation.

RESULTS

In vitro, proliferation and myotube formation were significantly greater in SKMCs derived from young rats than from older rats. In vivo, young and older recipients of SKMCs exhibited increases in cell density, vascular density, and collagen preservation relative to age-matched control animals. However, cell therapy produced significantly greater functional improvements in young recipients than in older, along with relative increases in stem cell factor, cell density, cell survival, and angiogenesis.

CONCLUSIONS

Functional improvement after the post-myocardial infarction implantation of young SKMCs was limited in older recipients, likely due to reductions in their cardiac and systemic responses to cell transplantation.

Lack of galectin-1 results in defects in myoblast fusion and muscle regeneration

Galectin-1 has been implicated in the development of skeletal muscle, being maximally expressed at the time of myofiber formation. Furthermore, in the presence of exogenous galectin-1, mononuclear myoblasts show increased fusion in vitro. In the current study, we have used the galectin-1 null mouse to elucidate the role of galectin-1 in skeletal muscle development and regeneration. Myoblasts derived from the galectin-1 mutant showed a reduced ability to fuse in vitro. In galectin-1 null mutants, there was evidence of a delay in muscle fiber development at the neonatal stage and muscle fiber diameter was reduced when compared with wild-type at the adult stage. Muscle regeneration was also compromised in the galectin-1 mutant with the process being delayed and a reduced fiber size being maintained. These results, therefore, show a definitive role for galectin-1 in fusion of myoblasts both in vitro, in vivo, and in regeneration after recovery from induced injury.

Stem cells and their potential in cell-based cardiac therapies

Stem cells are potential agents for the treatment of myocardial infarcts among other heart diseases. Over the past decade, the scientific community has extensively used a wide variety of cells and examined their capacity to both regenerate the infarcted myocardium and improve functionally the diseased hearts. Some of the cells used include skeletal myoblasts, bone marrow-derived cells, adult cardiac resident stem cells, mesenchymal stem cells, and both mouse and human embryonic stem cells (Nat Biotechnol 2005;23:845-856). The reported cardiogenic capacity of the utilitized stem cells is assayed both in vitro through the use of differentiation paradigms and in vivo through transplantation into a variety of animal models of cardiac disease. The purpose of this review article is to summarize recent stem cell applications in cell-based cardiac therapies and their outcomes.

Advances in cell-based therapy for structural heart disease

Congestive heart failure and coronary artery disease are the leading causes of morbidity and mortality in the United States despite substantial therapeutic advances in the last half century. Only very recently have studies arisen that support possibility of regenerating tissue of damaged human organs including the heart. In this regard, there is growing pre-clinical and clinical evidence demonstrating the safety and efficacy of cell-based myocardial regeneration using a variety of cell lines. Although the data on the exact mechanism of action and the fate of the administered cells is controversial, there is consistent evidence for improved cardiac function and myocardial regeneration using different cell types. This extraordinarily exciting scientific advance has forced cardiovascular scientists to re-evaluate the long-held paradigm of cardiac myocyte terminal differentiation and life-long longevity of the cardiac myocytes that comprise the heart. Whereas, these new ideas originated with attempts to perform cellular transplantation using exogenous stem or precursor cells, mechanistic insights have rapidly evolved to the realization that adult organs harbor stem cells with significant plasticity, capable of repopulating their respective organ. Indeed these cells may be harnessed as a therapeutic agent or may represent the target of regenerative therapeutic strategies.

Skeletal Myoblasts as a Therapeutic Agent

Cell transplantation is emerging as a new treatment designed to improve the poor outcome of patients with cardiac failure. Its rationale is that implantation of contractile cells into postinfarction scars could functionally rejuvenate these areas. Primarily for practical reasons, autologous skeletal myoblasts have been the first to be considered for a clinical use. A large number of experimental studies have consistently documented a robust engraftment of myoblasts, their in-scar differentiation into myotubes, and an associated improvement in left ventricular function. The early results of phase I clinical trials have then established both the feasibility and safety of this procedure with the caveat of arrhythmic events. Efficacy data are equally encouraging but definitely need to be validated by large prospective placebo-controlled, double-blind randomized trials such as the Myoblast Autologous Grafting in Ischemic Cardiomyopathy (MAGIC) study, the results of which are now pending. In addition to assessing the effect of myoblast transplantation on regional and global heart function, these results will also provide comprehensive safety data and thus allow a more objective assessment of the risk-benefit ratio. However, it is already apparent that the outcome of myoblast transfer could most likely be improved by optimizing the purity of the cell yield (by selecting muscle-derived progenitors less lineage-committed than the myoblasts), the mode of delivery (by increasing the accuracy of cell injections while decreasing their invasiveness), and the survival of the engrafted cells (by concomitant graft vascularization and incorporation of cells in three-dimensional matrices). Most, if not all, of these changes will have to be incorporated before skeletal myoblasts can acquire the status of therapeutic agents. Furthermore, there is increasing evidence that myoblasts may act by attenuating left ventricular remodeling or paracrinally affecting the surrounding myocardium but not by generating new cardiomyocytes because of their strict commitment to a myogenic lineage. Thus, improvement of function is not tantamount of myocardial regeneration, and if such a regeneration remains the primary objective, it is worth considering alternate cell types able to generate new cardiac cells that will be electromechanically coupled with the host cardiomyocytes. In the setting of this second generation of cells, human cardiac-specified embryonic stem cells may hold the greatest promise.

Expression of mdr1 is required for efficient long term regeneration of dystrophic muscle

The mouse mdr1a and mdr1b genes are expressed in skeletal muscle, though their precise role in muscle is unknown. Dystrophic muscle is characterized by repeated cycles of degeneration and regeneration. To explore the role of the mdr1 genes during muscle regeneration, we have created a triple knockout mouse lacking the mdr1a, mdr1b, and the dystrophin genes. The resulting ReX mice developed normally and were fertile. However, as adults, ReX had a higher proportion of degenerating muscle fibers and greater long-term loss of muscle mass than mdx. ReX muscles were also characterized by a reduced proportion of muscle side population (mSP) cells, of myogenic cells, and a reduced capacity for muscle regeneration. We found too that mSP cells derived from dystrophic muscle are more myogenic than those from normal muscle. Thus, in dystrophic muscle, the mdr1 gene plays an important role in the preservation of the mSP and of the myogenic regenerative potential. Moreover, our results suggest a hitherto unappreciated role of mdr1 in precursor cells of regenerating tissue; they therefore provide an important clue to the physiological significance of mdr1 expression in stem cells.

Sema4C participates in myogenic differentiation in vivo and in vitro through the p38 MAPK pathway

Sema4C is a member of transmembrane semaphorin proteins which regulate axonal guidance in the developing nervous system. The expression of Sema4C was dramatically induced not only during differentiation of C2C12 mouse myoblasts, but also during injury-induced skeletal muscle regeneration. C2C12 cells stably or transiently expressing Sema4C both showed increased myogenic differentiation reflected by accelerated myotube formation and expression of muscle-specific proteins. Overexpression of Sema4C elicited p38 phosphorylation directly, and the effects of Sema4C during myogenic differentiation could be abolished by the p38alpha-specific inhibitor SB203580. Knockdown of Sema4C by siRNA transfection during C2C12 myoblasts differentiation could suppress the phosphorylation of p38 followed by dramatically diminished myotube formation. Sema4C could activate the myogenin promoter during myogenic differentiation. This activation could be abolished by p38 inhibitor SB203580. Taken together, these observations reveal novel functional potentialities of Sema4C which suggest that Sema4C promotes terminal myogenic differentiation in a p38 MAPK-dependent manner.