Generation of different fates from multipotent muscle stem cells

SRSF6 and SRSF1 coordinately enhance the inclusion of human MUSK exon 10 to generate a Wnt-sensitive MuSK isoform

Alternative splicing in genes associated with neuromuscular junction (NMJ) often compromises neuromuscular signal transmission and provokes pathological consequences. Muscle-specific receptor tyrosine kinase (MuSK) is an essential molecule in the NMJ. MUSK exon 10 encodes an important part of the frizzled-like cysteine-rich domain, which is necessary for Wnt-mediated acetylcholine receptors clustering at NMJ. MUSK exon 10 is alternatively spliced in humans but not in mice. We reported that humans acquired a unique exonic splicing silencer in exon 10 compared to mice, which promotes exon skipping coordinated by hnRNP C, YB-1, and hnRNP L. Here, we have dissected the underlying mechanisms of exon inclusion. We precisely characterized the exonic splicing enhancer (ESE) elements and determined the functional motifs. We demonstrated that SRSF6 and SRSF1 coordinately enhance exon inclusion through multiple functional motifs in the ESE. Remarkably, SRSF6 exerts a stronger effect than SRSF1, and SRSF6 alone can compensate the function of SRSF1. Interestingly, differentiated muscle reduces the expression of splicing suppressors, rather than enhancers, to generate a functional Wnt-sensitive MuSK isoform to promote neuromuscular signal transmission. Finally, we developed splice-switching antisense oligonucleotides, which could be used to selectively modulate the expression of MUSK isoforms toward a beneficial outcome for therapeutic intervention.

Effects of Long-Term Serum Starvation on Autophagy, Metabolism, and Differentiation of Porcine Skeletal Muscle Satellite Cells

This study investigated the effects of long-term serum starvation on autophagy, metabolism, and differentiation of porcine skeletal muscle satellite cells (SMSCs) and elucidated the role of autophagy in skeletal muscle development. Our findings provide a theoretical basis for improving meat production in domestic pigs. The SMSCs isolated and preserved in our laboratory were revived and divided into six groups based on the culture medium serum concentration to simulate varying levels of serum starvation: 20% serum (control group), 15% serum (mild serum starvation group), 5% serum (severe serum starvation group), and their autophagy inhibition groups supplemented with 3-methyladenine. After 96 h of culture, the apoptosis rate, mitochondrial membrane potential, reactive oxygen species, and ATP were measured to evaluate the effects of serum starvation on the SMSCs' metabolism. Additionally, the levels of autophagy-related proteins, autophagosomes, and autolysosomes were measured to investigate the impact of long-term serum starvation on autophagy. The expression of proteins associated with myogenic and adipogenic differentiation (MHC, MyoD1, peroxisome proliferator-activated receptor γ, and lipoprotein lipase) as well as lipid content were also determined to investigate the effects of long-term serum starvation on SMSC differentiation. The results showed that long-term serum starvation induced autophagy through the AMPK/mTOR signaling pathway, accelerated cell metabolism and apoptosis, exacerbated reactive oxygen species accumulation, and inhibited myogenic and adipogenic differentiation of SMSCs. Moreover, these effects were positively correlated with the level of serum starvation. In addition, serum starvation-induced autophagy moderately promoted the myogenic and adipogenic differentiation of SMSCs; however, these effects were insufficient to counteract the inhibition of cell differentiation by long-term serum starvation. This study provides insight into leveraging serum starvation as a stressor to regulate muscle growth and metabolism in domestic pigs.

Cell transplantation-mediated dystrophin supplementation efficacy in Duchenne muscular dystrophy mouse motor function improvement demonstrated by enhanced skeletal muscle fatigue tolerance

BACKGROUND

Duchenne muscular dystrophy (DMD) is an incurable neuromuscular disease leading to progressive skeletal muscle weakness and fatigue. Cell transplantation in murine models has shown promise in supplementing the lack of the dystrophin protein in DMD muscles. However, the establishment of novel, long-term, relevant methods is needed to assess its efficiency on the DMD motor function. By applying newly developed methods, this study aimed to evaluate the functional and molecular effects of cell therapy-mediated dystrophin supplementation on DMD muscles.

METHODS

Dystrophin was supplemented in the gastrocnemius of a 5-week-old immunodeficient DMD mouse model (Dmd-null/NSG) by intramuscular xenotransplantation of healthy human immortalized myoblasts (Hu5/KD3). A long-term time-course comparative study was conducted between wild-type, untreated DMD, and dystrophin supplemented-DMD mouse muscle functions and histology. A novel GO-ATeam2 transgenic DMD mouse model was also generated to assess in vivo real-time ATP levels in gastrocnemius muscles during repeated contractions.

RESULTS

We found that 10.6% dystrophin supplementation in DMD muscles was sufficient to prevent low values of gastrocnemius maximal isometric contraction torque (MCT) at rest, while muscle fatigue tolerance, assessed by MCT decline after treadmill running, was fully ameliorated in 21-week-old transplanted mice. None of the dystrophin-supplemented fibers were positive for muscle damage markers after treadmill running, with 85.4% demonstrating the utilization of oxidative metabolism. Furthermore, ATP levels in response to repeated muscle contractions tended to improve, and mitochondrial activity was significantly enhanced in dystrophin supplemented-fibers.

CONCLUSIONS

Cell therapy-mediated dystrophin supplementation efficiently improved DMD muscle functions, as evaluated using newly developed evaluation methods. The enhanced muscle fatigue tolerance in 21-week-old mice was associated with the preferential regeneration of damage-resistant and oxidative fibers, highlighting increased mitochondrial activity, after cell transplantation. These findings significantly contribute to a more in-depth understanding of DMD pathogenesis.

Cell Transdifferentiation: A Challenging Strategy with Great Potential

With the discovery and development of somatic cell nuclear transfer, cell fusion, and induced pluripotent stem cells, cell transdifferentiation research has presented unique advantages and stimulated a heated discussion worldwide. Cell transdifferentiation is a phenomenon by which a cell changes its lineage and acquires the phenotype of other cell types when exposed to certain conditions. Indeed, many adult stem cells and differentiated cells were reported to change their phenotype and transform into other lineages. This article reviews the differentiation of stem cells and classification of transdifferentiation, as well as the advantages, challenges, and prospects of cell transdifferentiation. This review discusses new research directions and the main challenges in the use of transdifferentiation in human cells and molecular replacement therapy. Overall, such knowledge is expected to provide a deep understanding of cell fate and regulation, which can change through differentiation, dedifferentiation, and transdifferentiation, with multiple applications.

The Role of Supporting Cell Populations in Satellite Cell Mediated Muscle Repair

Skeletal muscle has a high capacity to repair and remodel in response to damage, largely through the action of resident muscle stem cells, termed satellite cells. Satellite cells are required for the proper repair of skeletal muscle through a process known as myogenesis. Recent investigations have observed relationships between satellite cells and other cell types and structures within the muscle microenvironment. These findings suggest that the crosstalk between inflammatory cells, fibrogenic cells, bone-marrow-derived cells, satellite cells, and the vasculature is essential for the restoration of muscle homeostasis. This review will discuss the influence of the cells and structures within the muscle microenvironment on satellite cell function and muscle repair.

Possible Mechanisms Linking Obesity, Steroidogenesis, and Skeletal Muscle Dysfunction

Increasing evidence suggests that skeletal muscles may play a role in the pathogenesis of obesity and associated conditions due to their impact on insulin resistance and systemic inflammation. Skeletal muscles, as well as adipose tissue, are largely recognized as endocrine organs, producing biologically active substances, such as myokines and adipokines. They may have either beneficial or harmful effects on the organism and its functions, acting through the endocrine, paracrine, and autocrine pathways. Moreover, the collocation of adipose tissue and skeletal muscles, i.e., the amount of intramuscular, intermuscular, and visceral adipose depots, may be of major importance for metabolic health. Traditionally, the generalized and progressive loss of skeletal muscle mass and strength or physical function, named sarcopenia, has been thought to be associated with age. That is why most recently published papers are focused on the investigation of the effect of obesity on skeletal muscle function in older adults. However, accumulated data indicate that sarcopenia may arise in individuals with obesity at any age, so it seems important to clarify the possible mechanisms linking obesity and skeletal muscle dysfunction regardless of age. Since steroids, namely, glucocorticoids (GCs) and sex steroids, have a major impact on the amount and function of both adipose tissue and skeletal muscles, and are involved in the pathogenesis of obesity, in this review, we will also discuss the role of steroids in the interaction of these two metabolically active tissues in the course of obesity.

Simple and efficient differentiation of human iPSCs into contractible skeletal muscles for muscular disease modeling

Pathophysiological analysis and drug discovery targeting human diseases require disease models that suitably recapitulate patient pathology. Disease-specific human induced pluripotent stem cells (hiPSCs) differentiated into affected cell types can potentially recapitulate disease pathology more accurately than existing disease models. Such successful modeling of muscular diseases requires efficient differentiation of hiPSCs into skeletal muscles. hiPSCs transduced with doxycycline-inducible MYOD1 (MYOD1-hiPSCs) have been widely used; however, they require time- and labor-consuming clonal selection, and clonal variations must be overcome. Moreover, their functionality should be carefully examined. Here, we demonstrated that bulk MYOD1-hiPSCs established with puromycin selection rather than G418 selection showed rapid and highly efficient differentiation. Interestingly, bulk MYOD1-hiPSCs exhibited average differentiation properties of clonally established MYOD1-hiPSCs, suggesting that it is possible to minimize clonal variations. Moreover, disease-specific hiPSCs of spinal bulbar muscular atrophy (SBMA) could be efficiently differentiated via this method into skeletal muscle that showed disease phenotypes, suggesting the applicability of this method for disease analysis. Finally, three-dimensional muscle tissues were fabricated from bulk MYOD1-hiPSCs, which exhibited contractile force upon electrical stimulation, indicating their functionality. Thus, our bulk differentiation requires less time and labor than existing methods, efficiently generates contractible skeletal muscles, and may facilitate the generation of muscular disease models.

Continuous fish muscle cell line with capacity for myogenic and adipogenic-like phenotypes

Cell-cultivated fish offers the potential for a more ethical, sustainable, and safe seafood system. However, fish cell culture is relatively understudied in comparison to mammalian cells. Here, we established and characterized a continuous Atlantic mackerel (Scomber scombrus) skeletal muscle cell line ("Mack" cells). The cells were isolated from muscle biopsies of fresh-caught fish, with separate isolations performed from two distinct fish. Mack1 cells (cells from the first isolation) were cultured for over a year and subcultured over 130 times. The cells proliferated at initial doubling times of 63.9 h (± 19.1 SD). After a spontaneous immortalization crisis from passages 37-43, the cells proliferated at doubling times of 24.3 h (± 4.91 SD). A muscle phenotype was confirmed through characterization of muscle stemness and differentiation via paired-box protein 7 and myosin heavy chain immunostaining, respectively. An adipocyte-like phenotype was also demonstrated for the cells through lipid accumulation, confirmed via Oil Red O staining and quantification of neutral lipids. New qPCR primers (HPRT, PAX3B, MYOD1, MYOG, TNNT3A, and PPARG) were tailored to the mackerel genome and used to characterize mackerel cell genotypes. This work provides the first spontaneously immortalized fish muscle cell line for research, ideally serving as a reference for subsequent investigation.

Intermuscular adipose tissue in metabolic disease

Intermuscular adipose tissue (IMAT) is a distinct adipose depot described in early reports as a 'fatty replacement' or 'muscle fat infiltration' that was linked to ageing and neuromuscular disease. Later studies quantifying IMAT with modern in vivo imaging methods (computed tomography and magnetic resonance imaging) revealed that IMAT is proportionately higher in men and women with type 2 diabetes mellitus and the metabolic syndrome than in people without these conditions and is associated with insulin resistance and poor physical function with ageing. In parallel, agricultural research has provided extensive insight into the role of IMAT and other muscle lipids in muscle (that is, meat) quality. In addition, studies using rodent models have shown that IMAT is a bona fide white adipose tissue depot capable of robust triglyceride storage and turnover. Insight into the importance of IMAT in human biology has been limited by the dearth of studies on its biological properties, that is, the quality of IMAT. However, in the past few years, investigations have begun to determine that IMAT has molecular and metabolic features that distinguish it from other adipose tissue depots. These studies will be critical to further decipher the role of IMAT in health and disease and to better understand its potential as a therapeutic target.

Satellite cell-specific deletion of Cipc alleviates myopathy in mdx mice

Skeletal muscle regeneration relies on satellite cells that can proliferate, differentiate, and form new myofibers upon injury. Emerging evidence suggests that misregulation of satellite cell fate and function influences the severity of Duchenne muscular dystrophy (DMD). The transcription factor Pax7 determines the myogenic identity and maintenance of the pool of satellite cells. The circadian clock regulates satellite cell proliferation and self-renewal. Here, we show that the CLOCK-interacting protein Circadian (CIPC) a negative-feedback regulator of the circadian clock, is up-regulated during myoblast differentiation. Specific deletion of Cipc in satellite cells alleviates myopathy, improves muscle function, and reduces fibrosis in mdx mice. Cipc deficiency leads to activation of the ERK1/2 and JNK1/2 signaling pathways, which activates the transcription factor SP1 to trigger the transcription of Pax7 and MyoD. Therefore, CIPC is a negative regulator of satellite cell function, and loss of Cipc in satellite cells promotes muscle regeneration.

Regulation of A-to-I RNA editing and stop codon recoding to control selenoprotein expression during skeletal myogenesis

Selenoprotein N (SELENON), a selenocysteine (Sec)-containing protein with high reductive activity, maintains redox homeostasis, thereby contributing to skeletal muscle differentiation and function. Loss-of-function mutations in SELENON cause severe neuromuscular disorders. In the early-to-middle stage of myoblast differentiation, SELENON maintains redox homeostasis and modulates endoplasmic reticulum (ER) Ca2+ concentration, resulting in a gradual reduction from the middle-to-late stages due to unknown mechanisms. The present study describes post-transcriptional mechanisms that regulate SELENON expression during myoblast differentiation. Part of an Alu element in the second intron of SELENON pre-mRNA is frequently exonized during splicing, resulting in an aberrant mRNA that is degraded by nonsense-mediated mRNA decay (NMD). In the middle stage of myoblast differentiation, ADAR1-mediated A-to-I RNA editing occurs in the U1 snRNA binding site at 5' splice site, preventing Alu exonization and producing mature mRNA. In the middle-to-late stage of myoblast differentiation, the level of Sec-charged tRNASec decreases due to downregulation of essential recoding factors for Sec insertion, thereby generating a premature termination codon in SELENON mRNA, which is targeted by NMD.

Role of skeletal muscle satellite cells in the repair of osteoporotic fractures mediated by β-catenin

BACKGROUND

Osteoporosis is a metabolic disease, and osteoporotic fracture (OPF) is one of its most serious complications. It is often ignored that the influence of the muscles surrounding the fracture on the healing of OPF. We aimed to clarify the role of skeletal muscle satellite cells (SMSCs) in promoting OPF healing by β-catenin, to improve our understanding of SMSCs, and let us explore its potential as a therapeutic target.

METHODS

Skeletal muscles were obtained from control non-OPF or OPF patients for primary SMSCs culture (n = 3, 33% females, mean age 60 ± 15.52). Expression of SMSCs was measured. In vivo, 3-month-old female C57BL/6 mice underwent OVX surgery. Three months later, the left tibia fracture model was again performed. The control and the treatment group (n = 24, per group, female). The treatment group was treated with an agonist (osthole). Detection of SMSCs in muscles and fracture healing at 7, 14, and 28 three time points (n = 8, 8, 8, female). To further clarify the scientific hypothesis, we innovatively used Pax7-Cre^ERT2/+ ;β-catenin^fx/fx transgenic mice (n = 12, per group, male). Knock out β-catenin in SMSC to observe the proliferation and osteogenic differentiation of SMSCs, and OPF healing. In vitro primary cells of SMSCs from 3-month-old litter-negative β-catenin^fx/fx transgenic mice. After adenovirus-CRE transfection, the myogenic and osteogenic differentiation of SMSC was observed.

RESULTS

We find that human SMSCs reduced proliferation and osteogenic differentiation in patients with OPF (-38.63%, P < 0.05). And through animal experiments, it was found that activation of β-catenin promoted the proliferation and osteogenic differentiation of SMSC at the fracture site, thereby accelerating the healing of the fracture site (189.47%, P < 0.05). To prove this point of view, in the in vivo Pax7-Cre^ERT2/+ ;β-catenin^fx/fx transgenic mouse experiment, we innovatively found that knocking out β-catenin in SMSC will cause a decrease in bone mass and bone microstructure, and accompanied by delayed fracture healing (-35.04%, P < 0.001). At the same time, through in vitro SMSC culture experiments, it was found that their myogenic (-66.89%, P < 0.01) and osteogenic differentiation (-16.5%, P < 0.05) ability decreased.

CONCLUSIONS

These results provide the first practical evidence for a direct contribution of SMSCs to promote the healing of OPF with important clinical implications as it may help in the treatment of delayed healing and non-union of OPFs, and mobilization of autologous stem cell therapy in orthopaedic applications.

Meclozine ameliorates skeletal muscle pathology and increases muscle forces in mdx mice

We screened pre-approved drugs for the survival of the Hu5/KD3 human myogenic progenitors. We found that meclozine, an anti-histamine drug that has long been used for motion sickness, promoted the proliferation and survival of Hu5/KD3 cells. Meclozine increased expression of MyoD, but reduced expression of myosin heavy chain and suppressed myotube formation. Withdrawal of meclozine, however, resumed the ability of Hu5/KD3 cells to differentiate into myotubes. We examined the effects of meclozine on mdx mouse carrying a nonsense mutation in the dystrophin gene and modeling for Duchenne muscular dystrophy. Intragastric administration of meclozine in mdx mouse increased the body weight, the muscle mass in the lower limbs, the cross-sectional area of the paravertebral muscle, and improved exercise performances. Previous reports show that inhibition of phosphorylation of ERK1/2 improves muscle functions in mouse models for Emery-Dreifuss muscular dystrophy and cancer cachexia, as well as in mdx mice. We and others previously showed that meclozine blocks the phosphorylation of ERK1/2 in cultured cells. We currently showed that meclozine decreased phosphorylation of ERK1/2 in muscles in mdx mice but not in wild-type mice. This was likely to be one of the underlying mechanisms of the effects of meclozine on mdx mice.

Heterotopic ossification in mice overexpressing Bmp2 in Tie2+ lineages

Bone morphogenetic protein (Bmp) signaling is critical for organismal development and homeostasis. To elucidate Bmp2 function in the vascular/hematopoietic lineages we generated a new transgenic mouse line in which ectopic Bmp2 expression is controlled by the Tie2 promoter. Tie2CRE/+;Bmp2tg/tg mice develop aortic valve dysfunction postnatally, accompanied by pre-calcific lesion formation in valve leaflets. Remarkably, Tie2CRE/+;Bmp2tg/tg mice develop extensive soft tissue bone formation typical of acquired forms of heterotopic ossification (HO) and genetic bone disorders, such as Fibrodysplasia Ossificans Progressiva (FOP). Ectopic ossification in Tie2CRE/+;Bmp2tg/tg transgenic animals is accompanied by increased bone marrow hematopoietic, fibroblast and osteoblast precursors and circulating pro-inflammatory cells. Transplanting wild-type bone marrow hematopoietic stem cells into lethally irradiated Tie2CRE/+;Bmp2tg/tg mice significantly delays HO onset but does not prevent it. Moreover, transplanting Bmp2-transgenic bone marrow into wild-type recipients does not result in HO, but hematopoietic progenitors contribute to inflammation and ectopic bone marrow colonization rather than to endochondral ossification. Conversely, aberrant Bmp2 signaling activity is associated with fibroblast accumulation, skeletal muscle fiber damage, and expansion of a Tie2+ fibro-adipogenic precursor cell population, suggesting that ectopic bone derives from a skeletal muscle resident osteoprogenitor cell origin. Thus, Tie2CRE/+;Bmp2tg/tg mice recapitulate HO pathophysiology, and might represent a useful model to investigate therapies seeking to mitigate disorders associated with aberrant extra-skeletal bone formation.

The small molecule DIPQUO promotes osteogenic differentiation via inhibition of glycogen synthase kinase 3-beta signaling

Bone fractures are common impact injuries typically resolved through natural processes of osteogenic regeneration and bone remodeling, restoring the biological and mechanical function. However, dysfunctionality in bone healing and repair often arises in the context of aging-related chronic disorders, such as Alzheimer's disease (AD). There is unmet need for effective pharmacological modulators of osteogenic differentiation and an opportunity to probe the complex links between bone biology and cognitive disorders. We previously discovered the small molecule DIPQUO, which promotes osteoblast differentiation and bone mineralization in mouse and human cell culture models, and in zebrafish developmental and regenerative models. Here, we examined the detailed function of this molecule. First, we used kinase profiling, cellular thermal shift assays, and functional studies to identify glycogen synthase kinase 3-beta (GSK3-β) inhibition as a mechanism of DIPQUO action. Treatment of mouse C2C12 myoblasts with DIPQUO promoted alkaline phosphatase expression and activity, which could be enhanced synergistically by treatment with other GSK3-β inhibitors. Suppression of the expression or function of GSK3-β attenuated DIPQUO-dependent osteogenic differentiation. In addition, DIPQUO synergized with GSK3-β inhibitors to stimulate expression of osteoblast genes in human multipotent progenitors. Accordingly, DIPQUO promoted accumulation and activation of β-catenin. Moreover, DIPQUO suppressed activation of tau microtubule-associated protein, an AD-related effector of GSK3-β signaling. Therefore, DIPQUO has potential as both a lead candidate for bone therapeutic development and a pharmacological modulator of GSK3-β signaling in cell culture and animal models of disorders including AD.

Oleic acid in the absence of a PPARγ agonist increases adipogenic gene expression in bovine muscle satellite cells1

We hypothesized that oleic acid (OA) in the absence of a thiazolidinedione (i.e., a synthetic peroxisome proliferator-activated receptorγ [PPARγ] agonist) would increase adipogenic gene expression in bovine muscle satellite cells (BSC). The BSC were cultured in differentiation medium containing 10 µM ciglitazone (CI), 100 µM OA, or 100 µM OA plus 10 µM CI (CI-OA). Control (CON) BSC were cultured only in differentiation media (containing 2% horse serum). The presence of myogenin, desmin, and paired box 7 proteins was confirmed in the BSC by immunofluorescence staining, demonstrating that we had isolated myogenic cells. The OA BSC had lesser paired box 3 (Pax3) and myogenic differentiation 1 expression but greater Pax7 and mygogenin (MYOG) expression (P < 0.05), than the CON BSC. The CI BSC had greater Pax3, Pax7, and MYOG expression than CON BSC (P < 0.05), suggesting that CI would promote BSC myogenesis under pro-myogenic conditions (i.e., when cultured with horse serum). However, both the OA and CI treatments upregulated the expression of PPARγ, CCAAT/enhancer-binding protein alpha (C/EBPα) and C/EBPß, sterol regulatory element-binding protein 1, lipoprotein lipase, and glycerol-3-phosphate acyltransferase 3 gene expression, as well as media adiponectin concentration (P < 0.05). The CI, OA, and CI-OA treatments also increased triacylglycerol and lipid droplet accumulation, in spite of upregulation (relative to CON BSC) of adenosine monophosphate-activated protein kinase alpha-1, perilipin 2 (PLIN2), and PLIN3 in BSC and downregulation of G protein-coupled protein receptor 43, acyl-CoA synthetase long chain family member 3, and stearoyl-CoA desaturase (P < 0.05). These results indicate that OA in the absence of a synthetic PPARγ agonist can effectively increase adipogenic gene expression in BSC.

Identification and Characterization of Skeletal Muscle Stem Cells from Human Orbicularis Oculi Muscle

Skeletal muscle stem cell (SMSC) transplantation has shown great therapeutical potential in repairing muscle loss and dysfunction, but the muscle acquisition is usually a traumatic procedure causing pain and morbidity to the donor. In this study, we investigated the feasibility of isolating SMSCs from human orbicularis oculi muscle (OOM), which is routinely removed and discarded during ophthalmic cosmetic surgeries. OOM fragments were harvested from 18 female healthy donors undergoing upper eyelid plasties. Plastic-adherent cells were isolated from the muscles using a two-step plating method combined with collagenase digestion. A total of 15 cell cultures were successfully established from the muscle samples. These adherent cells were positive for the specific markers of SMSCs and could be directed toward the osteogenic, adipogenic, chondrogenic, and myogenic phenotypes in the presence of lineage-specific inductive media. Moreover, after cultured in the myogenic inductive medium for 3 weeks, the muscle cells were injected into the tibialis anterior muscles of nude mice and the cell fate was detected using a DiI-labeling technique. In vivo myogenesis was evidenced by the expression of DiI fluorescence after cell transplantation. The donor cells could be found in the satellite cell position and incorporated into the host myofibers. Our results demonstrated that human OOM represents a novel source of myogenic precursors with stem cell-like properties, which may provide a foundation for the SMSC-based therapeutics of skeletal muscle diseases.

The effect of type 2 diabetes mellitus and obesity on muscle progenitor cell function

In addition to its primary function to provide movement and maintain posture, the skeletal muscle plays important roles in energy and glucose metabolism. In healthy humans, skeletal muscle is the major site for postprandial glucose uptake and impairment of this process contributes to the pathogenesis of type 2 diabetes mellitus (T2DM). A key component to the maintenance of skeletal muscle integrity and plasticity is the presence of muscle progenitor cells, including satellite cells, fibroadipogenic progenitors, and some interstitial progenitor cells associated with vessels (myo-endothelial cells, pericytes, and mesoangioblasts). In this review, we aim to discuss the emerging concepts related to these progenitor cells, focusing on the identification and characterization of distinct progenitor cell populations, and the impact of obesity and T2DM on these cells. The recent advances in stem cell therapies by targeting diabetic and obese muscle are also discussed.

Reduced Myogenic and Increased Adipogenic Differentiation Capacity of Rotator Cuff Muscle Stem Cells

BACKGROUND

Fat accumulation commonly occurs in chronically torn rotator cuff muscles, and increased fat within the rotator cuff is correlated with poor clinical outcomes. The extent of lipid deposition is particularly pronounced in injured rotator cuff muscles compared with other commonly injured muscles such as the gastrocnemius. Satellite cells, which are a tissue-resident muscle stem-cell population, can differentiate into fat cells. We hypothesized that satellite cells from the rotator cuff have greater intrinsic adipogenic differentiation potential than do gastrocnemius satellite cells, and this difference is due to variations in epigenetic imprinting between the cells.

METHODS

Satellite cells from gastrocnemius and rotator cuff muscles of mice were cultured in adipogenic media, and the capacity to differentiate into mature muscle cells and adipogenic cells was assessed (n ≥ 9 plates per muscle group). We also performed DNA methylation analysis of gastrocnemius and rotator cuff satellite cells to determine whether epigenetic differences were present between the 2 groups (n = 5 mice per group).

RESULTS

Compared with the gastrocnemius, satellite cells from the rotator cuff had a 23% reduction in myogenic differentiation and an 87% decrease in the expression of the differentiated muscle cell marker MRF4 (myogenic regulatory factor 4). With respect to adipogenesis, rotator cuff satellite cells had a 4.3-fold increase in adipogenesis, a 12-fold increase in the adipogenic transcription factor PPARγ (peroxisome proliferator-activated receptor gamma), and a 65-fold increase in the adipogenic marker FABP4 (fatty-acid binding protein 4). Epigenetic analysis identified 355 differentially methylated regions of DNA between rotator cuff and gastrocnemius satellite cells, and pathway enrichment analysis suggested that these regions were involved with lipid metabolism and adipogenesis.

CONCLUSIONS

Satellite cells from rotator cuff muscles have reduced myogenic and increased adipogenic differentiation potential compared with gastrocnemius muscles. There appears to be a cellular and genetic basis behind the generally poor rates of rotator cuff muscle healing.

CLINICAL RELEVANCE

The reduced myogenic and increased adipogenic capacity of rotator cuff satellite cells is consistent with the increased fat content and poor muscle healing rates often observed for chronically torn rotator cuff muscles. For patients undergoing rotator cuff repair, transplantation of autologous satellite cells from other muscles less prone to fatty infiltration may improve clinical outcomes.

Progress in research into spinal cord injury repair: Tissue engineering scaffolds and cell transdifferentiation

As with all tissues of the central nervous system, the low regeneration ability of spinal cord tissue after injury decreases the potential for repair and recovery. Initially, in spinal cord injuries (SCI), often the surgeon can only limit further damage by early surgical decompression. However, with the development of basic science, especially the development of genetic engineering, molecular biology, tissue engineering, and materials science, some promising progress has been made in promoting the repair of central nervous system injuries. For example, transplantation of neural stem cells (NSCs), olfactory ensheathing cells (OECs), and gene- mediated transdifferentiation to repair central nervous system injury. This paper summarizes the progress and prospects of SCI repair with tissue engineering scaffold and cell transdifferentiation from an extensive literatures.

Stem cells and heterotopic ossification: Lessons from animal models

Put most simply, heterotopic ossification (HO) is the abnormal formation of bone at extraskeletal sites. HO can be classified into two main subtypes, genetic and acquired. Acquired HO is a common complication of major connective tissue injury, traumatic central nervous system injury, and surgical interventions, where it can cause significant pain and postoperative disability. A particularly devastating form of HO is manifested in the rare genetic disorder, fibrodysplasia ossificans progressiva (FOP), in which progressive heterotopic bone formation occurs throughout life, resulting in painful and disabling cumulative immobility. While the central role of stem/progenitor cell populations in HO is firmly established, the identity of the offending cell type(s) remains to be conclusively determined, and little is known of the mechanisms that direct these progenitor cells to initiate cartilage and bone formation. In this review, we summarize current knowledge of the cells responsible for acquired HO and FOP, highlighting the strengths and weaknesses of animal models used to interrogate the cellular origins of HO.

Local myogenic pulp-derived cell injection enhances craniofacial muscle regeneration in vivo

OBJECTIVES

To enhance myogenic differentiation in pulp cells isolated from extracted premolars by epigenetic modification using a DNA demethylation agent, 5-aza-2'-deoxycytidine (5-Aza), and to evaluate the potent stimulatory effect of 5-Aza-treated pulp cell injection for craniofacial muscle regeneration in vivo.

SETTING AND SAMPLE POPULATION

Pulp cells were isolated from premolars extracted for orthodontic purposes from four adults (age range, 18-22.1 years).

MATERIAL AND METHODS

Levels of myogenic differentiation and functional contraction response in vitro were compared between pulp cells with or without pre-treatment of 5-Aza. Changes in muscle regeneration in response to green fluorescent protein (GFP)-labelled myogenic pulp cell injection in vivo were evaluated using a cardiotoxin (CTX)-induced muscle injury model of the gastrocnemius as well as the masseter muscle in mice.

RESULTS

Pre-treatment of 5-Aza in pulp cells stimulated myotube formation, myogenic differentiation in terms of desmin and myogenin expression, and the level of collagen gel contraction. The local injection of 5-Aza pre-treated myogenic pulp cells was engrafted into the host tissue and indicated signs of enhanced muscle regeneration in both the gastrocnemius and the masseter muscles.

CONCLUSION

The epigenetic modification of pulp cells from extracted premolars and the local injection of myogenic pulp cells may stimulate craniofacial muscles regeneration in vivo.

Activin-dependent signaling in fibro/adipogenic progenitors causes fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare autosomal-dominant disorder characterized by progressive and profoundly disabling heterotopic ossification (HO). Here we show that fibro/adipogenic progenitors (FAPs) are a major cell-of-origin of HO in an accurate genetic mouse model of FOP (Acvr1^tnR206H). Targeted expression of the disease-causing type I bone morphogenetic protein (BMP) receptor, ACVR1(R206H), to FAPs recapitulates the full spectrum of HO observed in FOP patients. ACVR1(R206H)-expressing FAPs, but not wild-type FAPs, activate osteogenic signaling in response to activin ligands. Conditional loss of the wild-type Acvr1 allele dramatically exacerbates FAP-directed HO, suggesting that mutant and wild-type ACVR1 receptor complexes compete for activin ligands or type II BMP receptor binding partners. Finally, systemic inhibition of activin A completely blocks HO and restores wild-type-like behavior to transplanted Acvr1^R206H/+ FAPs. Understanding the cells that drive HO may facilitate the development of cell-specific therapeutic approaches to inhibit catastrophic bone formation in FOP.

Fibrodysplasia ossificans progressiva is a severe disorder characterized by heterotopic ossification, and is caused by mutations in ACVR1. Here, the authors show that expression of mutant ACVR1 in fibro/adipogenic progenitors recapitulates disease progression, and that this can be halted by systemic inhibition of activin A in mice.

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.

Reversible differentiation of immortalized human bladder smooth muscle cells accompanied by actin bundle reorganization

Previous studies have shown that phenotypic modulation of smooth muscle cells (SMCs) plays a pivotal role in human diseases. However, the molecular mechanisms underlying the reversible differentiation of SMCs remain elusive particularly because cultured SMCs that reproducibly exhibit bidirectional phenotypic modulation have not been established. Here we established an immortalized human bladder SMC line designated as hBS11. Under differentiation-inducing conditions, hBS11 cells underwent smooth muscle differentiation accompanied by the robust expression of smooth muscle differentiation markers and isoform-dependent reorganization of actin bundles. The cholinergic receptor agonist carbachol increased intracellular calcium in differentiated hBS11 cells in an acetylcholine muscarinic receptor-dependent manner. Differentiated hBS11 cells displayed contractile properties depending on the elevation in the levels of intracellular calcium. Depolarization of membrane potential triggered inward sodium current in differentiated hBS11 cells. However, differentiated hBS11 cells lost the differentiated phenotype and resumed mitosis when re-fed with growth medium. Our study provides direct evidence pertaining to the human bladder SMCs being able to retain the capacity of reversible differentiation and that the reorganization of actin bundles is involved in the reinstatement of contractility. Moreover, we have established a human SMC line retaining high proliferating potential without compromising differentiation potential.

Transdifferentiation of myoblasts into osteoblasts – possible use for bone therapy

Objectives

Transdifferentiation is defined as the conversion of one cell type to another and is an ever-expanding field with a growing number of cells found to be capable of such a process. To date, the fact remains that there are limited treatment options for fracture healing, osteoporosis and bone repair post-destruction by bone tumours. Hence, this review focuses on the transdifferentiation of myoblast to osteoblast as a means to further understand the transdifferentiation process and to investigate a potential therapeutic option if successful.

Key findings

The potent osteoinductive effects of the bone morphogenetic protein-2 are largely implicated in the transdifferentiation of myoblast to osteoblast. Bone morphogenetic protein-2-induced activation of the Smad1 protein ultimately results in JunB synthesis, the first transcriptional step in myoblast dedifferentiation. The upregulation of the activating protein-1 binding activity triggers the transcription of the runt-related transcription factor 2 gene, a transcription factor that plays a major role in osteoblast differentiation.

Summary

This potential transdifferentiation treatment may be utilised for dental implants, fracture healing, osteoporosis and bone repair post-destruction by bone tumours.

Bisulfite Sequencing for DNA Methylation Analysis of Primary Muscle Stem Cells

In skeletal muscle, DNA methylation contributes to the suppression of gene expression in several biological processes and diseases. A protocol for the detection of methylated cytosine was thus established based on methylation-sensitive enzymes, immunoprecipitation, and bisulfite conversion. DNA methylation analysis, with bisulfite conversion and sequencing, enables the quantification of methylation at each single base position. Here, we describe a basic method of bisulfite sequencing that can be used to analyze local DNA methylation status to confirm genome-wide DNA methylation analysis or correlation of gene expression regulatory mechanisms.

Role of osteoclasts in heterotopic ossification enhanced by fibrodysplasia ossificans progressiva-related activin-like kinase 2 mutation in mice

Fibrodysplasia ossificans progressiva (FOP) is a disorder of skeletal malformations and progressive heterotopic ossification. The constitutively activating mutation (R206H) of the bone morphogenetic protein type 1 receptor, activin-like kinase 2 (ALK2), is responsible for the pathogenesis of FOP. Although transfection of the causal mutation of FOP into myoblasts enhances osteoclast formation by transforming growth factor-β (TGF-β), the role of osteoclasts in heterotopic ossification is unknown. We therefore examined the effects of alendronate, SB431542 and SB203580 on heterotopic ossification induced by the causal mutation of FOP. Total bone mineral content as well as numbers of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated and alkaline phosphatase (ALP)-positive cells in heterotopic bone were significantly higher in muscle tissues implanted with ALK2 (R206H)-transfected mouse myoblastic C2C12 cells than in the tissues implanted with empty vector-transfected cells in nude mice. Alendronate, an aminobisphosphonate, did not affect total mineral content or numbers of TRAP-positive multinucleated and ALP-positive cells in heterotopic bone, which were enhanced by the implantation of ALK2 (R206H)-transfected C2C12 cells, although it significantly decreased serum levels of cross-linked C-telopeptide of type I collagen, a bone resorption index. Moreover, neither SB431542, an inhibitor of TGF-β receptor type I kinase, nor SB203580, an inhibitor of p38 mitogen-activated protein kinase, affected the increase in heterotopic ossification due to the implantation of ALK2 (R206H)-transfected C2C12 cells. In conclusion, the present study indicates that osteoclast inhibition does not affect heterotopic ossification enhanced by FOP-related mutation.

Dnmt3a Regulates Proliferation of Muscle Satellite Cells via p57Kip2

Cell differentiation status is defined by the gene expression profile, which is coordinately controlled by epigenetic mechanisms. Cell type-specific DNA methylation patterns are established by chromatin modifiers including de novo DNA methyltransferases, such as Dnmt3a and Dnmt3b. Since the discovery of the myogenic master gene MyoD, myogenic differentiation has been utilized as a model system to study tissue differentiation. Although knowledge about myogenic gene networks is accumulating, there is only a limited understanding of how DNA methylation controls the myogenic gene program. With an aim to elucidate the role of DNA methylation in muscle development and regeneration, we investigate the consequences of mutating Dnmt3a in muscle precursor cells in mice. Pax3 promoter-driven Dnmt3a-conditional knockout (cKO) mice exhibit decreased organ mass in the skeletal muscles, and attenuated regeneration after cardiotoxin-induced muscle injury. In addition, Dnmt3a-null satellite cells (SCs) exhibit a striking loss of proliferation in culture. Transcriptome analysis reveals dysregulated expression of p57Kip2, a member of the Cip/Kip family of cyclin-dependent kinase inhibitors (CDKIs), in the Dnmt3a-KO SCs. Moreover, RNAi-mediated depletion of p57Kip2 replenishes the proliferation activity of the SCs, thus establishing a role for the Dnmt3a-p57Kip2 axis in the regulation of SC proliferation. Consistent with these findings, Dnmt3a-cKO muscles exhibit fewer Pax7⁺ SCs, which show increased expression of p57Kip2 protein. Thus, Dnmt3a is found to maintain muscle homeostasis by epigenetically regulating the proliferation of SCs through p57Kip2.

How muscle homeostasis is maintained is not completely elucidated yet. Epigenetic disorders such as Beckwith-Wiedemann syndrome, which causes hypergrowth of skeletal muscles and rhabdomyosarcoma, indicate that epigenetic regulations such as DNA methylation, contribute to this homeostasis control. DNA methylation is mediated by DNA methyltransferases, such as Dnmt3a and Dnmt3b, which are de novo DNA methyltransferases. The role of DNA methylation in somatic stem cells is not completely understood, although it has been shown to be indispensable in differentiation of primordial germ cells and embryonic stem cells. In this report, we investigated the role of Dnmt3a in muscle satellite cells by analyzing Dnmt3a-conditional knockout (cKO) mice in which Dnmt3a loci are deleted utilizing Cre-recombinase driven by Pax7 or Pax3 promoters that are specifically activated in the muscle precursor lineage. The loss of Dnmt3a in cKO mice causes decreased muscle mass and significantly impaired muscle regeneration. Moreover, Dnmt3a loss also results in a striking loss of proliferation of SCs, which is caused by mis-expression of a cyclin-dependent kinase inhibitor, p57Kip2. Therefore, our findings suggest that DNA methylation plays an essential role in muscle homeostasis.

Muscle Satellite Cells: Exploring the Basic Biology to Rule Them

Adult skeletal muscle is a postmitotic tissue with an enormous capacity to regenerate upon injury. This is accomplished by resident stem cells, named satellite cells, which were identified more than 50 years ago. Since their discovery, many researchers have been concentrating efforts to answer questions about their origin and role in muscle development, the way they contribute to muscle regeneration, and their potential to cell-based therapies. Satellite cells are maintained in a quiescent state and upon requirement are activated, proliferating, and fusing with other cells to form or repair myofibers. In addition, they are able to self-renew and replenish the stem pool. Every phase of satellite cell activity is highly regulated and orchestrated by many molecules and signaling pathways; the elucidation of players and mechanisms involved in satellite cell biology is of extreme importance, being the first step to expose the crucial points that could be modulated to extract the optimal response from these cells in therapeutic strategies. Here, we review the basic aspects about satellite cells biology and briefly discuss recent findings about therapeutic attempts, trying to raise questions about how basic biology could provide a solid scaffold to more successful use of these cells in clinics.

Articular cartilage: from formation to tissue engineering

Hyaline cartilage is the nonlinear, inhomogeneous, anisotropic, poro-viscoelastic connective tissue that serves as friction-reducing and load-bearing cushion in synovial joints and is vital for mammalian skeletal movements. Due to its avascular nature, low cell density, low proliferative activity and the tendency of chondrocytes to de-differentiate, cartilage cannot regenerate after injury, wear and tear, or degeneration through common diseases such as osteoarthritis. Therefore severe damage usually requires surgical intervention. Current clinical strategies to generate new tissue include debridement, microfracture, autologous chondrocyte transplantation, and mosaicplasty. While articular cartilage was predicted to be one of the first tissues to be successfully engineered, it proved to be challenging to reproduce the complex architecture and biomechanical properties of the native tissue. Despite significant research efforts, only a limited number of studies have evolved up to the clinical trial stage. This review article summarizes the current state of cartilage tissue engineering in the context of relevant biological aspects, such as the formation and growth of hyaline cartilage, its composition, structure and biomechanical properties. Special attention is given to materials development, scaffold designs, fabrication methods, and template-cell interactions, which are of great importance to the structure and functionality of the engineered tissue.

Changing Paradigms in Cranio-Facial Regeneration: Current and New Strategies for the Activation of Endogenous Stem Cells

Craniofacial area represent a unique district of human body characterized by a very high complexity of tissues, innervation and vascularization, and being deputed to many fundamental function such as eating, speech, expression of emotions, delivery of sensations such as taste, sight, and earing. For this reasons, tissue loss in this area following trauma or for example oncologic resection, have a tremendous impact on patients' quality of life. In the last 20 years regenerative medicine has emerged as one of the most promising approach to solve problem related to trauma, tissue loss, organ failure etc. One of the most powerful tools to be used for tissue regeneration is represented by stem cells, which have been successfully implanted in different tissue/organs with exciting results. Nevertheless, both autologous and allogeneic stem cell transplantation raise many practical and ethical concerns that make this approach very difficult to apply in clinical practice. For this reason different cell free approaches have been developed aiming to the mobilization, recruitment, and activation of endogenous stem cells into the injury site avoiding exogenous cells implant but instead stimulating patients' own stem cells to repair the lesion. To this aim many strategies have been used including functionalized bioscaffold, controlled release of stem cell chemoattractants, growth factors, BMPs, Platelet-Rich-Plasma, and other new strategies such as ultrasound wave and laser are just being proposed. Here we review all the current and new strategies used for activation and mobilization of endogenous stem cells in the regeneration of craniofacial tissue.

Identifying the Cellular Mechanisms Leading to Heterotopic Ossification

Heterotopic ossification (HO) is a debilitating condition defined by the de novo development of bone within non-osseous soft tissues, and can be either hereditary or acquired. The hereditary condition, fibrodysplasia ossificans progressiva is rare but life threatening. Acquired HO is more common and results from a severe trauma that produces an environment conducive for the formation of ectopic endochondral bone. Despite continued efforts to identify the cellular and molecular events that lead to HO, the mechanisms of pathogenesis remain elusive. It has been proposed that the formation of ectopic bone requires an osteochondrogenic cell type, the presence of inductive agent(s) and a permissive local environment. To date several lineage-tracing studies have identified potential contributory populations. However, difficulties identifying cells in vivo based on the limitations of phenotypic markers, along with the absence of established in vitro HO models have made the results difficult to interpret. The purpose of this review is to critically evaluate current literature within the field in an attempt identify the cellular mechanisms required for ectopic bone formation. The major aim is to collate all current data on cell populations that have been shown to possess an osteochondrogenic potential and identify environmental conditions that may contribute to a permissive local environment. This review outlines the pathology of endochondral ossification, which is important for the development of potential HO therapies and to further our understanding of the mechanisms governing bone formation.

Molecular mechanisms of skeletal muscle development, regeneration, and osteogenic conversion

Both skeletal muscle and bone are of mesodermal origin and derived from somites during embryonic development. Somites differentiate into the dorsal dermomyotome and the ventral sclerotome, which give rise to skeletal muscle and bone, respectively. Extracellular signaling molecules, such as Wnt and Shh, secreted from the surrounding environment, determine the developmental fate of skeletal muscle. Dermomyotome cells are specified as trunk muscle progenitor cells by transcription factor networks involving Pax3. These progenitor cells delaminate and migrate to form the myotome, where they are determined as myoblasts that differentiate into myotubes or myofibers. The MyoD family of transcription factors plays pivotal roles in myogenic determination and differentiation. Adult skeletal muscle regenerates upon exercise, muscle injury, or degeneration. Satellite cells are muscle-resident stem cells and play essential roles in muscle growth and regeneration. Muscle regeneration recapitulates the process of muscle development in many aspects. In certain muscle diseases, ectopic calcification or heterotopic ossification, as well as fibrosis and adipogenesis, occurs in skeletal muscle. Muscle-resident mesenchymal progenitor cells, which may be derived from vascular endothelial cells, are responsible for the ectopic osteogenesis, fibrogenesis, and adipogenesis. The small GTPase M-Ras is likely to participate in the ectopic calcification and ossification, as well as in osteogenesis during development. This article is part of a Special Issue entitled "Muscle Bone Interactions".

JNK implication in adipocyte-like cell death induced by chemotherapeutic drug cisplatin

Recent evidence shows that tumor microenvironment containing heterogeneous cells may be involved in cancer initiation, growth and tumor cell response to anticancer therapy. Chemotherapy was designed to make toxic impact on malicious cells in organisms, however, the means to protect healthy cells against chemical toxicity are still unsuccessful. As known, the majority of tumor surrounding cells are cancer-associated adipocytes which influence cancer development, progression and treatment. Targeting the components of tumor microenvironment in combination with conventional cancer treatment may become an effective cancer therapy strategy. However, little is known about adipocyte death mechanisms during combined chemo- and targeted therapy. The importance of c-Jun-NH<inf>2</inf>-terminal kinase (JNK) signaling in tumor development and treatment has been demonstrated using various in vitro and in vivo cancer models. The aim of this study was to ascertain adipocyte viability during simultaneous stress kinase JNK inhibition and exposure to one of the most commonly used anticancer drugs cis-diamminedichloroplatinum II (cisplatin). Our model involved adipocyte-like cells (ADC) which were obtained during in vitro differentiation of adult rabbit muscle-derived stem cells. Cisplatin induced apoptotic cell death. During 24-hr cisplatin treatment gradual, strong and prolonged increase of both JNK and its target protein c-Jun phosphorylation was found in ADC. Pre-treatment of cells with SP600125 decreased cisplatin-induced activation of c-Jun and promoted apoptosis. Upregulation of proapoptotic Bax and downregulation of antiapoptotic Bcl-2 proteins were found to be regulated in JNK-dependent manner. Thus, the results prove the antiapoptotic role of activated JNK in adipocyte-like cells treated with cisplatin.

SRSF1 and hnRNP H antagonistically regulate splicing of COLQ exon 16 in a congenital myasthenic syndrome

The catalytic subunits of acetylcholinesterase (AChE) are anchored in the basal lamina of the neuromuscular junction using a collagen-like tail subunit (ColQ) encoded by COLQ. Mutations in COLQ cause endplate AChE deficiency. An A-to-G mutation predicting p.E415G in COLQ exon 16 identified in a patient with endplate AChE deficiency causes exclusive skipping of exon 16. RNA affinity purification, mass spectrometry, and siRNA-mediated gene knocking down disclosed that the mutation disrupts binding of a splicing-enhancing RNA-binding protein, SRSF1, and de novo gains binding of a splicing-suppressing RNA-binding protein, hnRNP H. MS2-mediated artificial tethering of each factor demonstrated that SRSF1 and hnRNP H antagonistically modulate splicing by binding exclusively to the target in exon 16. Further analyses with artificial mutants revealed that SRSF1 is able to bind to degenerative binding motifs, whereas hnRNP H strictly requires an uninterrupted stretch of poly(G). The mutation compromised splicing of the downstream intron. Isolation of early spliceosome complex revealed that the mutation impairs binding of U1-70K (snRNP70) to the downstream 5′ splice site. Global splicing analysis with RNA-seq revealed that exons carrying the hnRNP H-binding GGGGG motif are predisposed to be skipped compared to those carrying the SRSF1-binding GGAGG motif in both human and mouse brains.

Isolation and characterization of human mesenchymal stem cells from gingival connective tissue

BACKGROUND AND OBJECTIVE

The main purpose of this study was to isolate and characterize gingival connective tissue-derived mesenchymal stem cells (GMSCs). The secondary purpose was to present a modified isolation method for the GMSCs.

MATERIAL AND METHODS

Collected healthy gingival tissue samples were de-epithelialized and minced into small fragments. The tissues were digested by dispase and collagenase IV for 30 min. The first digested cell suspension was discarded, and then additional digestion was performed to the remaining cells in the same solution for 90 min. The isolated cells from gingiva was incubated in 37°C humidified condition and observed by inverted microscope. Cytoskeletal morphology was evaluated by phalloidin immunofluorescence. Potency of the cells was tested by colony-forming unit fibroblast assay. GMSCs were characterized by osteogenic, adipogenic and chondrogenic differentiation, and flow cytometric, immunofluorescence analysis.

RESULTS

GMSCs showed spindle-shaped, fibroblast-like morphology, colony-forming abilities, adherence to plastic and multilineage differentiation (osteogenic, adipogenic, chondrogenic) potency. GMSCs expressed CD44, CD73, CD90 and CD105, but did not express CD14, CD45, CD34 and CD19 in flow cytometry. Expression of stem cell markers (SSEA-4, STRO-1, CD146, CD166 and CD271) and a mesenchymal marker (vimentin) were observed by immunofluorescence.

CONCLUSIONS

In conclusion, we isolated and characterized stem cells from human gingival connective tissue with modified protocol. GMSCs showed multipotency with high proliferation and characteristics of mesenchymal stem cells. GMSCs are promising sources for tissue engineering and may be obtained during routine procedures under local anesthesia. Further research is needed to evaluate the potential of GSMCs' proliferation and cryopreservation.

Effects of Asiasari radix on the morphology and viability of mesenchymal stem cells derived from the gingiva

Medicinal herbs used in traditional Oriental medicine, which have been in use clinically for thousands of years, are attractive sources of novel therapeutics or preventatives. Asiasari radix (A. radix) has been suggested for use in the treatment of dental diseases, including toothache and aphthous stomatitis. The aim of this study was to evaluate the effects of A. radix extracts on the morphology and viability of human stem cells derived from the gingiva. An Asiasarum heterotropoides extract was centrifuged and freeze-dried in a lyophilizer. Stem cells derived from the gingiva were grown in the presence of A. radix at concentrations ranging between 0.1 µg/ml and 1 mg/ml (0, 0.1, 1, 10, 100 and 1,000 µg/ml). Cell morphology was evaluated with an optical microscope and the viability of the cells was quantitatively analyzed with a cell counting kit-8 (CCK-8) assay for up to seven days. The untreated control group exhibited normal fibroblast morphology. The shapes of the cells following 0.1, 1, 10 and 100 µg/ml A. radix treatments were similar to those of the control group. However, a significant change was noted in the 1,000 µg/ml group on day 1, when compared with the untreated group. Furthermore, on day 7, the shapes of the cells following 100 and 1,000 µg/ml A. radix treatments were rounder and fewer cells were present, when compared with those of the control group. The cultures that grew in the presence of A. radix did not exhibit any changes in the CCK‑8 assay on day 2; however, significant reductions in cell viability were noticed following 100 and 1,000 µg/ml A. radix treatment on days 5 and 7. Within the limits of this study, A. radix influenced the viability of the stem cells derived from the gingiva. Thus, the direct application of A. radix to oral tissues may produce adverse effects at high doses. Therefore, the concentration and application time of A. radix requires meticulous control to obtain optimal results. These effects require consideration, if the use of A. radix is planned for the treatment of dental diseases.

HnRNP C, YB-1 and hnRNP L coordinately enhance skipping of human MUSK exon 10 to generate a Wnt-insensitive MuSK isoform

Muscle specific receptor tyrosine kinase (MuSK) is an essential postsynaptic transmembrane molecule that mediates clustering of acetylcholine receptors (AChR). MUSK exon 10 is alternatively skipped in human, but not in mouse. Skipping of this exon disrupts a cysteine-rich region (Fz-CRD), which is essential for Wnt-mediated AChR clustering. To investigate the underlying mechanisms of alternative splicing, we exploited block-scanning mutagenesis with human minigene and identified a 20-nucleotide block that contained exonic splicing silencers. Using RNA-affinity purification, mass spectrometry, and Western blotting, we identified that hnRNP C, YB-1 and hnRNP L are bound to MUSK exon 10. siRNA-mediated knockdown and cDNA overexpression confirmed the additive, as well as the independent, splicing suppressing effects of hnRNP C, YB-1 and hnRNP L. Antibody-mediated in vitro protein depletion and scanning mutagenesis additionally revealed that binding of hnRNP C to RNA subsequently promotes binding of YB-1 and hnRNP L to the immediate downstream sites and enhances exon skipping. Simultaneous tethering of two splicing trans-factors to the target confirmed the cooperative effect of YB-1 and hnRNP L on hnRNP C-mediated exon skipping. Search for a similar motif in the human genome revealed nine alternative exons that were individually or coordinately regulated by hnRNP C and YB-1.

Repairing damaged tendon and muscle: are mesenchymal stem cells and scaffolds the answer?

Mesenchymal stem cells (MSCs) have become an area of intense interest in the treatment of musculoskeletal conditions, such as muscle and tendon injury, as various animal and human trials have demonstrated that implantation with MSCs leads to improved healing and function. However, these trials have usually been relatively small scale and lacking in adequate controls. Additionally, the optimum source of these cells has yet to be determined, partly due to a lack of understanding as to how MSCs produce their beneficial effects when implanted. Scaffolds have been shown to improve tissue-engineering repairs but require further work to optimize their interactions with both native tissue and implanted MSCs. Robust, well-controlled trials are therefore required to determine the usefulness of MSCs in musculoskeletal tissue repair.

Parathyroid hormone and parathyroid hormone type-1 receptor accelerate myocyte differentiation

The ZHTc6-MyoD embryonic stem cell line expresses the myogenic transcriptional factor MyoD under the control of a tetracycline-inducible promoter. Following induction, most of the ZHTc6-MyoD cells differentiate to myotubes. However, a small fraction does not differentiate, instead forming colonies that retain the potential for myocyte differentiation. In our current study, we found that parathyroid hormone type 1 receptor (PTH1R) expression in colony-forming cells at 13 days after differentiation was higher than that in the undifferentiated ZHTc6-MyoD cells. We also found that PTH1R expression was required for myocyte differentiation, and that parathyroid hormone accelerated the differentiation. Our analysis of human and mouse skeletal muscle tissues showed that most cells expressing PTH1R also expressed Pax7 and CD34, which are biomarkers of satellite cells. Furthermore, we found that parathyroid hormone treatment significantly improved muscle weakness in dystrophin-deficient mdx mice. This is the first report indicating that PTH1R and PTH accelerate myocyte differentiation.

Evaluation of the cellular origins of heterotopic ossification

Heterotopic ossification (HO), acquired or hereditary, is featured by the formation of bone outside of the normal skeleton. Typical acquired HO is a common, debilitating condition associated with traumatic events. Cardiovascular calcification, an atypical form of acquired HO, is prevalent and associated with high rates of cardiovascular mortality. Hereditary HO syndromes, such as fibrodysplasia ossificans progressiva and progressive osseous heteroplasia, are rare, progressive, life-threatening disorders. The cellular origins of HO remain elusive. Some bona fide contributing cell populations have been found through genetic lineage tracing and other experiments in vivo, and various other candidate populations have been proposed. Nevertheless, because of the difficulties in establishing cellular phenotypes in vivo and other confounding factors, the true identities of these populations are still uncertain. This review critically evaluates the accumulating data in the field. The major focus is on the candidate populations that may give rise to osteochondrogenic lineage cells directly, not the populations that may contribute to HO indirectly. This issue is important not solely because of the clinical implications, but also because it highlights the basic biological processes that govern bone formation.

Differential impact of the Bisphosphonate Alendronate on undifferentiated and terminally differentiated human myogenic cells

Objectives

Alendronate, a nitrogen-containing bisphosphonate, is well established as a treatment for osteoporosis through regulation of osteoclast activity. Previously, the pharmacological effects of bisphosphonates on cells outside the bone environment have been considered irrelevant because of the bone-targeting property of bisphosphonates. However, the chronic effects of bisphosphonates on tissue-neighbouring bone, in particular skeletal muscles, should not be ignored because patients are treated with bisphosphonates for long periods.

Methods

Here, we show that the impact of alendronate on immortalized human myogenic cells depends on growth and differentiation-inducing conditions.

Key findings

Alendronate disrupted cytoskeletal structures and prevented migration, proliferation and differentiation of undifferentiated human myogenic cells that are involved in muscle regeneration. In contrast, alendronate did not affect the morphology, gene expression or survival of terminally differentiated human myotubes.

Conclusions

The present results suggest that the muscle regeneration capacity of osteoporosis patients treated with bisphosphonates for long periods may be attenuated. The present research on the pharmacological effects of alendronate on cultured human myogenic cells will contribute to improvement of therapeutic strategies and optimization of rehabilitation programmes for locomotive activity in osteoporosis patients treated with bisphosphonates.

Immature muscular tissue differentiation into bone-like tissue by bone morphogenetic proteins in vitro, with ossification potential in vivo

The objective of this study was to induce bone formation from immature muscular tissue (IMT) in vitro, using bone morphogenetic proteins (BMPs) as a cytokine source and an expanded polytetrafluoroethylene (ePTFE) scaffold. In addition, cultured IMTs were implanted subcutaneously into Sprague-Dawley (SD) rats to determine their in vivo ossification potential. BMPs, extracted from bovine cortical bones, were applied to embryonic SD rat IMT cultures, before 2 weeks culture on ePTFE scaffolds. Osteoblast-like cells and osteoid tissues were partially identified by hematoxylin-eosin staining 2 weeks after culture. Collagen type I (Col-I), osteopontin (OP), and osteocalcin (OC) were detected in the osteoid tissues by immunohistochemical staining. OC gene expression remained low, but OP and Col-I were upregulated during the culture period. In vivo implanted IMTs showed slight radiopacity 1 week after implantation and strong radiopacity 2 and 3 weeks after implantation. One week after implantation, migration of numerous capillaries was observed and ossification was detected after 2 weeks by histological observation. These results suggest that IMTs are able to differentiate into bone-like tissue in vitro, with an ossification potential after implantation in vivo.