Myogenin (Myf4) upregulation in trans-differentiating fibroblasts from a congenital myopathy with arrest of myogenesis and defects of myotube formation

From cyclins to CDKIs: Cell cycle regulation of skeletal muscle stem cell quiescence and activation

After extensive proliferation during development, the adult skeletal muscle cells remain outside the cell cycle, either as post-mitotic myofibers or as quiescent muscle stem cells (MuSCs). Despite its terminally differentiated state, adult skeletal muscle has a remarkable regeneration potential, driven by MuSCs. Upon injury, MuSC quiescence is reversed to support tissue growth and repair and it is re-established after the completion of muscle regeneration. The distinct cell cycle states and transitions observed in the different myogenic populations are orchestrated by elements of the cell cycle machinery. This consists of i) complexes of cyclins and Cyclin-Dependent Kinases (CDKs) that ensure cell cycle progression and ii) their negative regulators, the Cyclin-Dependent Kinase Inhibitors (CDKIs). In this review we discuss the roles of these factors in developmental and adult myogenesis, with a focus on CDKIs that have emerging roles in stem cell functions.

3D cellular alignment and biomimetic mechanical stimulation enhance human adipose-derived stem cell myogenesis

Determination of a stem cell source with sufficient myogenic differentiation capacity that can be easily obtained in large quantities is of great importance in skeletal muscle regeneration therapies. Adipose-derived stem cells (ASCs) are readily available, can be isolated from fat tissue with high yield and possess myogenic differentiation capacity. Consequently, ASCs have high applicability in muscle regenerative therapies. However, a key challenge is their low differentiation efficiency. In this study, we have explored the potential of mimicking the natural microenvironment of the skeletal muscle tissue to enhance ASC myogenesis by inducing 3D cellular alignment and using dynamic biomimetic culture. ASCs were entrapped and 3D aligned in parallel within fibrin-based microfibers and subjected to uniaxial cyclic stretch. 3D cell alignment was shown to be necessary for achieving and maintaining the stiffness of the construct mimicking the natural tissue (12 ± 1 kPa), where acellular aligned fibers and cell-laden random fibers had stiffness values of 4 ± 1 and 5 ± 2 kPa, respectively, at the end of 21 d. The synergistic effect of 3D cell alignment and biomimetic dynamic culture was evaluated on cell proliferation, viability and the expression of muscle-specific markers (immunofluorescent staining for MyoD1, myogenin, desmin and myosin heavy chain). It was shown that the myogenic markers were only expressed on the aligned-dynamic culture samples on day 21 of dynamic culture. These results demonstrate that 3D skeletal muscle grafts can be developed using ASCs by mimicking the structural and physiological muscle microenvironment.

Skeletal muscle microRNA and messenger RNA profiling in cofilin-2 deficient mice reveals cell cycle dysregulation hindering muscle regeneration

Congenital myopathies are rare skeletal muscle diseases presenting in early age with hypotonia and weakness often linked to a genetic defect. Mutations in the gene for cofilin-2 (CFL2) have been identified in several families as a cause of congenital myopathy with nemaline bodies and cores. Here we explore the global messenger and microRNA expression patterns in quadriceps muscle samples from cofillin-2-null mice and compare them with sibling-matched wild-type mice to determine the molecular pathways and mechanisms involved. Cell cycle processes are markedly dysregulated, with altered expression of genes involved in mitotic spindle formation, and evidence of loss of cell cycle checkpoint regulation. Importantly, alterations in cell cycle, apoptosis and proliferation pathways are present in both mRNA and miRNA expression patterns. Specifically, p21 transcript levels were increased, and the expression of p21 targets, such as cyclin D and cyclin E, was decreased. We therefore hypothesize that deficiency of cofilin-2 is associated with interruption of the cell cycle at several checkpoints, hindering muscle regeneration. Identification of these pathways is an important step towards developing appropriate therapies against various congenital myopathies.

ATOH8: a novel marker in human muscle fiber regeneration

Regenerating muscle fibers emerge from quiescent satellite cells, which differentiate into mature multinuclear myofibers upon activation. It has recently been found that ATOH8, a bHLH transcription factor, is regulated during myogenic differentiation. In this study, expression and localization of ATOH8, the other well-described regeneration markers, vimentin, nestin and neonatal myosin, and the satellite cell marker Pax7 were analyzed on protein level in human myopathy samples by immunofluorescence studies. On mRNA level, expression levels of ATOH8 and vimentin were studied by quantitative real-time PCR. ATOH8 is expressed in activated satellite cells and proliferating myoblasts of human skeletal muscle tissue. Quantitative analyses of ATOH8+, Pax7+, vimentin+, nestin+ and neonatal myosin+ muscle fibers showed the highest amount of regenerating muscle fibers in inflammatory myopathies, followed by muscular dystrophy. The relative co-expression of ATOH8 with the above-mentioned markers did not vary among the disorders. These results show that the novel regeneration marker ATOH8 contributes to muscle cell differentiation in healthy and diseased human muscle tissue.

Expression of myogenic regulatory factors and myo-endothelial remodeling in sporadic inclusion body myositis

Muscle repair relies on coordinated activation and differentiation of satellite cells, a process that is unable to counterbalance progressive degeneration in sporadic inclusion body myositis (s-IBM). To explore features of myo regeneration, the expression of myogenic regulatory factors Pax7, MyoD and Myogenin and markers of regenerating fibers was analyzed by immunohistochemistry in s-IBM muscle compared with polymyositis, dermatomyositis, muscular dystrophy and age-matched controls. In addition, the capillary density and number of interstitial CD34⁺ hematopoietic progenitor cells was determined by double-immunoflourescence staining. Satellite cells and regenerating fibers were significantly increased in s-IBM similar to other inflammatory myopathies and correlated with the intensity of inflammation (R > 0.428). Expression of MyoD, visualizing activated satellite cells and proliferating myoblasts, was lower in s-IBM compared to polymyosits. In contrast, Myogenin a marker of myogenic cell differentiation was strongly up-regulated in s-IBM muscle. The microvascular architecture in s-IBM was distorted, although the capillary density was normal. Notably, CD34⁺ hematopoietic cells were significantly increased in the interstitial compartment. Our findings indicate profound myo-endothelial remodeling of s-IBM muscle concomitant to inflammation. An altered expression of myogenic regulatory factors involved in satellite cell activation and differentiation, however, might reflect perturbations of muscle repair in s-IBM.

Expression and subcellular localization of myogenic regulatory factors during the differentiation of skeletal muscle C2C12 myoblasts

It is known that the MyoD family members (MyoD, Myf5, myogenin, and MRF4) play a pivotal role in the complex mechanism of skeletal muscle cell differentiation. However, fragmentary information on transcription factor-specific regulation is available and data on their post-transcriptional and post-translational behavior are still missing. In this work, we combined mRNA and protein expression analysis with their subcellular localization. Each myogenic regulator factor (MRF) revealed a specific mRNA trend and a protein quantitative analysis not overlapping, suggesting the presence of post-transcriptional mechanisms. In addition, each MRF showed a specific behavior in situ, characterized by a differentiation stage-dependent localization suggestive of a post-translational regulation also. Consistently with their transcriptional activity, immunogold electron microscopy data revealed MRFs distribution in interchromatin domains. Our results showed a MyoD and Myf5 contrasting expression profile in proliferating myoblasts, as well as myogenin and MRF4 opposite distribution in the terminally differentiated myotubes. Interestingly, MRFs expression and subcellular localization analysis during C2C12 cell differentiation stages showed two main MRFs regulation mechanisms: (i) the protein half-life regulation to modulate the differentiation stage-dependent transcriptional activity and (ii) the cytoplasmic retention, as a translocation process, to inhibit the transcriptional activity. Therefore, our results exhibit that MRFs nucleo-cytoplasmic trafficking is involved in muscle differentiation and suggest that, besides the MRFs expression level, also MRFs subcellular localization, related to their functional activity, plays a key role as a regulatory step in transcriptional control mechanisms.

Congenital myopathies

PURPOSE OF REVIEW

The aim of this review is to provide an up-to-date personal analysis of current congenital myopathy research.

RECENT FINDINGS

In the past year novel congenital myopathies have been suggested, genes have been discovered for some of the congenital myopathies for the first time (beta-tropomyosin in cap disease and perhaps skeletal muscle alpha-actin in Zebra body myopathy), further genes have been identified for congenital myopathies where other genes had already been found (cofilin in nemaline myopathy, selenoprotein N in congenital fibre type disproportion) and recessive myosin storage myopathy was associated with homozygous mutation of slow-skeletal/beta-cardiac myosin which was already known to be mutated in dominant myosin storage myopathy. There has been further clarification of the pathobiology of the congenital myopathies, including determination of the basis of epigenetic effects: silencing of the normal allele in recessive central core disease and persistence of cardiac (fetal) alpha-actin in nemaline myopathy patients with no skeletal actin.

SUMMARY

The increased understanding of the genes and pathobiology of the congenital myopathies that is developing should ultimately lead to effective treatments.