Regulation of skeletal muscle differentiation in fibroblasts by exogenous MyoD gene in vitro and in vivo

Insights into the epitranscriptomic role of N6-methyladenosine on aging skeletal muscle

The modification of RNA through the N6-methyladenosine (m6A) has emerged as a growing area of research due to its regulatory role in gene expression and various biological processes regulating the expression of genes. m6A RNA methylation is a post-transcriptional modification that is dynamic and reversible and found in mRNA, tRNA, rRNA, and other non-coding RNA of most eukaryotic cells. It is executed by special proteins known as "writers," which initiate methylation; "erasers," which remove methylation; and "readers," which recognize it and regulate the expression of the gene. Modification by m6A regulates gene expression by affecting the splicing, translation, stability, and localization of mRNA. Aging causes molecular and cellular damage, which forms the basis of most age-related diseases. The decline in skeletal muscle mass and functionality because of aging leads to metabolic disorders and morbidities. The inability of aged muscles to regenerate and repair after injury poses a great challenge to the geriatric populace. This review seeks to explore the m6A epigenetic regulation in the myogenesis and regeneration processes in skeletal muscle as well as the progress made on the m6A epigenetic regulation of aging skeletal muscles.

Advances in Genetic Reprogramming: Prospects from Developmental Biology to Regenerative Medicine

The foundations of cell reprogramming were laid by Yamanaka and co-workers, who showed that somatic cells can be reprogrammed into pluripotent cells (induced pluripotency). Since this discovery, the field of regenerative medicine has seen advancements. For example, because they can differentiate into multiple cell types, pluripotent stem cells are considered vital components in regenerative medicine aimed at the functional restoration of damaged tissue. Despite years of research, both replacement and restoration of failed organs/ tissues have remained elusive scientific feats. However, with the inception of cell engineering and nuclear reprogramming, useful solutions have been identified to counter the need for compatible and sustainable organs. By combining the science underlying genetic engineering and nuclear reprogramming with regenerative medicine, scientists have engineered cells to make gene and stem cell therapies applicable and effective. These approaches have enabled the targeting of various pathways to reprogramme cells, i.e., make them behave in beneficial ways in a patient-specific manner. Technological advancements have clearly supported the concept and realization of regenerative medicine. Genetic engineering is used for tissue engineering and nuclear reprogramming and has led to advances in regenerative medicine. Targeted therapies and replacement of traumatized , damaged, or aged organs can be realized through genetic engineering. Furthermore, the success of these therapies has been validated through thousands of clinical trials. Scientists are currently evaluating induced tissue-specific stem cells (iTSCs), which may lead to tumour-free applications of pluripotency induction. In this review, we present state-of-the-art genetic engineering that has been used in regenerative medicine. We also focus on ways that genetic engineering and nuclear reprogramming have transformed regenerative medicine and have become unique therapeutic niches.

The requirement of Mettl3-promoted MyoD mRNA maintenance in proliferative myoblasts for skeletal muscle differentiation

Myogenic progenitor/stem cells retain their skeletal muscle differentiation potential by maintaining myogenic transcription factors such as MyoD. However, the mechanism of how MyoD expression is maintained in proliferative progenitor cells has not been elucidated. Here, we found that MyoD expression was reduced at the mRNA level by cell cycle arrest in S and G2 phases, which in turn led to the absence of skeletal muscle differentiation. The reduction of MyoD mRNA was correlated with the reduced expression of factors regulating RNA metabolism, including methyltransferase like 3 (Mettl3), which induces N⁶-methyladenosine (m⁶A) modifications of RNA. Knockdown of Mettl3 revealed that MyoD RNA was specifically downregulated and that this was caused by a decrease in processed, but not unprocessed, mRNA. Potential m⁶A modification sites were profiled by m⁶A sequencing and identified within the 5' untranslated region (UTR) of MyoD mRNA. Deletion of the 5' UTR revealed that it has a role in MyoD mRNA processing. These data showed that Mettl3 is required for MyoD mRNA expression in proliferative myoblasts.

Down-regulation of myogenin can reverse terminal muscle cell differentiation

Certain higher vertebrates developed the ability to reverse muscle cell differentiation (dedifferentiation) as an additional mechanism to regenerate muscle. Mammals, on the other hand, show limited ability to reverse muscle cell differentiation. Myogenic Regulatory Factors (MRFs), MyoD, myogenin, Myf5 and Myf6 are basic-helix-loop-helix (bHLH) transcription factors essential towards the regulation of myogenesis.

Our current interest is to investigate whether down-regulation of MRFs in terminally differentiated mouse myotubes can induce reversal of muscle cell differentiation. Results from this work showed that reduction of myogenin levels in terminally differentiated mouse myotubes can reverse their differentiation state. Down-regulation of myogenin in terminally differentiated mouse myotubes induces cellular cleavage into mononucleated cells and cell cycle re-entry, as shown by re-initiation of DNA synthesis and increased cyclin D1 and cyclin E2 levels. Finally, we provide evidence that down-regulation of myogenin causes cell cycle re-entry (via down-regulation of MyoD) and cellularisation through separate pathways. These data reveal the important role of myogenin in maintaining terminal muscle cell differentiation and point to a novel mechanism by which muscle cells could be re-activated through its down-regulation.

5-Aza-2'-deoxycytidine treatment induces skeletal myogenic differentiation of mouse dental pulp stem cells

OBJECTIVE

Tissue stem cells in dental pulp are assumed to possess differentiation potentials similar to mesenchymal stem cells (MSCs). The aim of this in vitro study is to examine the differentiation potentials of mouse dental pulp stem cells (DPSCs) and develop the appropriate differentiation assay systems for skeletal myogenic differentiation of these cells.

METHODS

Dental pulps were extracted from mandible sections of C57/BL6 mice, and adherent dental pulp cells were isolated in culture. These cells were cultured in osteogenic or adipogenic induction medium to induce osteogenic and adipogenic differentiation. On the other hand, the skeletal myogenic differentiation potential of these cells was investigated using different conditions, such as serum-free medium, Myod1 overexpression, or 5-Aza-2'-deoxycytidine (5-Aza) treatment for DNA demethylation. Muscle-specific transcriptional factor expression was evaluated by RT-PCR, and myotube formation and myosin heavy chain expression were evaluated by phase-contrast microscopy and immunofluorescence staining, respectively.

RESULTS

The adherent dental pulp cells exhibited a proliferative capacity and they showed osteogenic and adipogenic differentiation as seen in previous studies. Although the expression of Myod1 mRNA and myotube formation was not detected in serum-free conditions, the forced expression of Myod1 up-regulated the expression of Myogenin and Pax7 mRNA. However, myotube formation was not confirmed. Interestingly, myosin heavy chain expression and myotube formation were observed following 5-Aza treatment of these cells.

CONCLUSIONS

These results demonstrated that mouse DPSCs possess MSC-like differentiation potential. DNA demethylation induced by 5-Aza treatment resulted in the skeletal muscle differentiation in mouse DPSCs, suggesting that DNA demethylation might trigger this differential induction of mouse DPSCs.