Muscle development genes: their relevance in neuromuscular disorders

Global Quantitative Proteomics Analysis Reveals the Downstream Signaling Networks of Msx1 and Msx2 in Myoblast Differentiation

The msh homeobox 1 (Msx1) and msh homeobox 2 (Msx2) coordinate in myoblast differentiation and also contribute to muscle defects if altered during development. Deciphering the downstream signaling networks of Msx1 and Msx2 in myoblast differentiation will help us to understand the molecular events that contribute to muscle defects. Here, the proteomics characteristics in Msx1- and Msx2-mediated myoblast differentiation was evaluated  using isobaric tags for the relative and absolute quantification labeling technique (iTRAQ). The downstream regulatory proteins of Msx1- and Msx2-mediated differentiation were identified. Bioinformatics analysis revealed that these proteins were primarily associated with xenobiotic metabolism by cytochrome P450, fatty acid degradation, glycolysis/gluconeogenesis, arginine and proline metabolism, and apoptosis. In addition, our data show Acta1 was probably a core of the downstream regulatory networks of Msx1 and Msx2 in myoblast differentiation.

Supplementary Information

The online version contains supplementary material available at 10.1007/s43657-022-00049-y.

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.

Possible correlation between selenoprotein W and myogenic regulatory factors in chicken embryonic myoblasts

The biological function of selenium (Se) is mainly elicited through Se-containing proteins. Selenoprotein W (SelW), one member of the selenoprotein family, is essential for the normal function of the skeletal muscle system. To investigate the possible relationship of Se in the process of differentiation in chicken myoblasts and the expression of SelW, the cultured chicken embryonic myoblasts were incubated with sodium selenite at different concentrations for 72 h, and then the mRNA levels of SelW and myogenic regulatory factors (MRFs) in myoblasts were determined at 12, 24, 48, and 72 h, respectively. Furthermore, the correlation between SelW mRNA expression and MRF mRNA expression was assessed. The results showed that the sodium selenite medium enhanced the mRNA expression of SelW, Myf-5, MRF4, and myogenin in chicken myoblasts. The mRNA expression levels of MRFs were significantly correlated with those of SelW at 24, 48, and 72 h. These data demonstrate that Se is involved in the differentiation of chicken embryonic myoblasts, and SelW showed correlation with MRFs.

Proteomics-based investigation in C2C12 myoblast differentiation

Skeletal muscle cell differentiation is a multistage process extensively studied over the years. Even if great improvements have been achieved in defining biological process underlying myogenesis, many molecular mechanisms need still to be clarified.

To further highlight this process, we studied cells at undifferentiated, intermediate and highly differentiated stages, and we analyzed, for each condition, morphological and proteomic changes. We also identified the proteins that showed statistical significant changes by a ESI-Q-TOF mass spectrometer. This work provides further evidence of the involvement of particular proteins in skeletal muscle development. Furthermore, the high level of expression of many heat shock proteins, suggests a relationship between differentiation and cellular stress. Intriguingly, the discovery of myogenesis-correlated proteins, known to play a role in apoptosis, suggests a link between differentiation and this type of cell death.

Dystroglycan and mitochondrial ribosomal protein L34 regulate differentiation in the Drosophila eye

Mutations that diminish the function of the extracellular matrix receptor Dystroglycan (DG) result in muscular dystrophies, with associated neuronal migration defects in the brain and mental retardation e.g. Muscle Eye Brain Disease. To gain insight into the function of DG in the nervous system we initiated a study to examine its contribution to development of the eye of Drosophila melanogaster. Immuno-histochemistry showed that DG is concentrated on the apical surface of photoreceptors (R) cells during specification of cell-fate in the third instar larva and is maintained at this location through early pupal stages. In point mutations that are null for DG we see abortive R cell elongation during differentiation that first appears in the pupa and results in stunted R cells in the adult. Overexpression of DG in R cells results in a small but significant increase in their size. R cell differentiation defects appear at the same stage in a deficiency line Df(2R)Dg(248) that affects Dg and the neighboring mitochondrial ribosomal gene, mRpL34. In the adult, these flies have severely disrupted R cells as well as defects in the lens and ommatidia. Expression of an mRpL34 transgene rescues much of this phenotype. We conclude that DG does not affect neuronal commitment but functions R cell autonomously to regulate neuronal elongation during differentiation in the pupa. We discuss these findings in view of recent work implicating DG as a regulator of cell metabolism and its genetic interaction with mRpL34, a member of a class of mitochondrial genes essential for normal metabolic function.

Opposing control of rhabdomyosarcoma growth and differentiation by myogenin and interleukin 4

Rhabdomyosarcoma is a tumor of striated muscle origin that displays defective myogenic differentiation. Terminal myogenesis switches off cell proliferation and migration, hence, the promotion of rhabdomyosarcoma differentiation should antagonize tumor growth and metastasis. Terminal myogenesis is controlled by cell-intrinsic myogenic transcription factors like myogenin and environmental mediators like interleukin 4 (IL-4). We studied whether the expression of myogenin or exposure to IL-4 could promote the myogenesis of poorly differentiating human rhabdomyosarcoma cells RD/12. Forced expression of myogenin amplified myosin expression and the formation of myotube-like elements, inhibited cell migration, and reduced the growth of local tumors and liver metastases in immunodepressed mice. In contrast, exposure to IL-4 promoted cell proliferation and survival, especially at high cell density, inhibited myogenin expression, and myogenesis. Moreover, IL-4 stimulated the directed migration of cells with low myogenin levels, but not of cells with higher (spontaneous or forced) levels. Thus, IL-4, which was known to promote late stages of normal myogenesis, favors growth and migration, and inhibits further differentiation of the myogenic stages attained by rhabdomyosarcoma cells. Strategies to increase myogenin expression and block IL-4 could simultaneously reduce growth and migration, and enhance terminal differentiation of rhabdomyosarcoma, thus contributing to the control of tumor growth and metastatic spread.

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

Congenital myopathies often have an unclear aetiology. Here, we studied a novel case of a severe congenital myopathy with a failure of myotube formation. Polymerase chain reaction-based analysis was performed to characterize the expression patterns of the Desmin, p21, p57, and muscle regulatory factors (MRFs) MyoD, Myf4, Myf5 and Myf6 in differentiating skeletal muscle cells (SkMCs), normal human fibroblasts and patient-derived fibroblasts during trans-differentiation. The temporal and spatial pattern of MRFs was further characterized by immunocyto- and immunohistochemical stainings. In differentiating SkMCs, each MRF showed a characteristic expression pattern. Normal trans-differentiating fibroblasts formed myotubes and expressed all of the MRFs, which were detected. Interestingly, the patient's fibroblasts also showed some fusion events during trans-differentiation with a comparable expression profile for the MRFs, particularly, with increased expression of Myf4 and p21. Immunohistochemical analysis of normal and patient-derived skeletal musculature revealed that Myf4, which is downregulated during normal fetal development, was still present in patient-derived skeletal head muscle, which was also positive for Desmin and sarcomeric actin. The abnormal upregulation of Myf4 and p21 in the patient who suffered from a severe congenital myopathy suggests that the regulation of Myf4 and p21 gene expression during myogenesis might be of interest for further studies.

Perinuclear localization of slow troponin C m RNA in muscle cells is controlled by a cis-element located at its 3' untranslated region

The process of mRNA localization within a specific cytoplasmic region is an integral aspect of the regulation of gene expression. Furthermore, colocalization of mRNAs and their respective translation products may facilitate the proper assembly of multi-subunit complexes like the thick and thin filaments of muscle. This postulate was tested by investigating the cytoplasmic localization of three mRNAs-the alpha-actin, slow troponin C (sTnC), and slow troponin I (sTnI), which encode different poly-peptide partners of the thin filament. Using in situ hybridization we showed that all three thin filament mRNAs are localized in the perinuclear cytoplasm of cultured C2C12 muscle cells. Their localization differs from that of the nonmuscle beta-actin mRNA, which is localized in the peripheral region of both proliferating nondifferentiated myoblasts and the differentiated myocytes. Analysis of the localization signal of the sTnC mRNA showed that a 40-nucleotide-long region of the sTnC mRNA 3' UTR is sufficient to confer the perinuclear localization on a heterologous reporter beta-Gal mRNA. This localization signal showed tissue specificity and worked only in the differentiated myocytes, but not in the proliferating myoblasts or in HeLa cells. The predicted secondary structure of the localization signal suggests the presence of multiple stem and loop structures in this region of the 3' UTR. Mutations within the stem region of the localization signal, which abolish the base pairing in this region, significantly reduced its perinuclear mRNA localization activity. Using UV-induced photo-cross-linking of RNA and proteins we found that a myotube-specific 42-kDa polypeptide binds to the localization signal.

Large-scale gene expression analysis of human skeletal myoblast differentiation

To study pathways involved in human skeletal myogenesis, we profiled gene expression in human primary myoblast cells derived from three individuals using both oligonucleotide and cDNA microarrays. Following stringent statistical testing (false-positive rate 0.4%), we identified 146 genes differentially expressed over time. Interestingly, 86 of these genes have not been reported to be involved in myogenesis in mouse cell lines. This demonstrates the additional value of human primary cell cultures in the study of muscle differentiation. Many of the identified genes play a role in muscle regeneration, indicating the close relationship of this process with muscle development. In addition, we found overlap with expression profiling studies in muscle from Duchenne muscular dystrophy patients, confirming ongoing muscle regeneration in Duchenne muscular dystrophy. Further study of these genes can bring new insights into the process of muscle differentiation, and they are candidate genes for neuromuscular disorders with an as yet unidentified cause.

Expression profiling and identification of novel genes involved in myogenic differentiation

Skeletal muscle differentiation is a complex, highly coordinated process that relies on precise temporal gene expression patterns. To better understand this cascade of transcriptional events, we used expression profiling to analyze gene expression in a 12-day time course of differentiating C2C12 myoblasts. Cluster analysis specific for time-ordered microarray experiments classified 2895 genes and ESTs with variable expression levels between proliferating and differentiating cells into 22 clusters with distinct expression patterns during myogenesis. Expression patterns for several known and novel genes were independently confirmed by real-time quantitative RT-PCR and/or Western blotting and immunofluorescence. MyoD and MEF family members exhibited unique expression kinetics that were highly coordinated with cell-cycle withdrawal regulators. Among genes with peak expression levels during cell cycle withdrawal were Vcam1, Itgb3, Itga5, Vcl, as well as Ptger4, a gene not previously associated with the process of myogenesis. One interesting uncharacterized transcript that is highly induced during myogenesis encodes several immunoglobulin repeats with sequence similarity to titin, a large sarcomeric protein. These data sets identify many additional uncharacterized transcripts that may play important functions in muscle cell proliferation and differentiation and provide a baseline for comparison with C2C12 cells expressing various mutant genes involved in myopathic disorders.