The proper functioning of tissues and organs depends on their ability to self-renew and repair. Some of the tissues, like epithelia, renew almost constantly while in the others this process is induced by injury or diseases. The stem or progenitor cells responsible for tissue homeostasis have been identified in many organs. Some of them, such as hematopoietic or intestinal epithelium stem cells, are multipotent and can differentiate into various cell types. Others are unipotent. The skeletal muscle tissue does not self-renew spontaneously, however, it presents unique ability to regenerate in response to the injury or disease. Its repair almost exclusively relies on unipotent satellite cells. However, multiple lines of evidence document that some progenitor cells present in the muscle can be supportive for skeletal muscle regeneration. Here, we summarize the current knowledge on the complicated landscape of stem and progenitor cells that exist in skeletal muscle and support its regeneration. We compare the cells from two model organisms, i.e., mouse and human, documenting their similarities and differences and indicating methods to test their ability to undergo myogenic differentiation.

MiRNAs and lncRNAs do not encode proteins, but they play an important role in the regulation of gene expression. They differ in length, biogenesis, and mode of action. In this work, we focus on the selected miRNAs and lncRNAs involved in the regulation of myogenesis and muscle regeneration. We present selected miRNAs and lncRNAs that have been shown to control myogenic differentiation and show that manipulation of their levels could be used to improve myogenic differentiation of various types of stem and progenitor cells. Finally, we discuss how physical activity affects miRNA and lncRNA expression and how it affects muscle well-being.

Background: Pluripotent stem cells present the ability to self-renew and undergo differentiation into any cell type building an organism. Importantly, a lot of evidence on embryonic stem cell (ESC) differentiation comes from in vitro studies. However, ESCs cultured in vitro do not necessarily behave as cells differentiating in vivo. For this reason, we used teratomas to study early and advanced stages of in vivo ESC myogenic differentiation and the role of Pax7 in this process. Pax7 transcription factor plays a crucial role in the formation and differentiation of skeletal muscle precursor cells during embryonic development. It controls the expression of other myogenic regulators and also acts as an anti-apoptotic factor. It is also involved in the formation and maintenance of satellite cell population. Methods: In vivo approach we used involved generation and analysis of pluripotent stem cell-derived teratomas. Such model allows to analyze early and also terminal stages of tissue differentiation, for example, terminal stages of myogenesis, including the formation of innervated and vascularized mature myofibers. Results: We determined how the lack of Pax7 function affects the generation of different myofiber types. In Pax7−/− teratomas, the skeletal muscle tissue occupied significantly smaller area, as compared to Pax7+/+ ones. The proportion of myofibers expressing Myh3 and Myh2b did not differ between Pax7+/+ and Pax7−/− teratomas. However, the area of Myh7 and Myh2a myofibers was significantly lower in Pax7−/− ones. Molecular characteristic of skeletal muscles revealed that the levels of mRNAs coding Myh isoforms were significantly lower in Pax7−/− teratomas. The level of mRNAs encoding Pax3 was significantly higher, while the expression of Nfix, Eno3, Mck, Mef2a, and Itga7 was significantly lower in Pax7−/− teratomas, as compared to Pax7+/+ ones. We proved that the number of satellite cells in Pax7−/− teratomas was significantly reduced. Finally, analysis of neuromuscular junction localization in samples prepared with the iDISCO method confirmed that the organization of neuromuscular junctions in Pax7−/− teratomas was impaired. Conclusions: Pax7−/− ESCs differentiate in vivo to embryonic myoblasts more readily than Pax7+/+ cells. In the absence of functional Pax7, initiation of myogenic differentiation is facilitated, and as a result, the expression of mesoderm embryonic myoblast markers is upregulated. However, in the absence of functional Pax7 neuromuscular junctions, formation is abnormal, what results in lower differentiation potential of Pax7−/− ESCs during advanced stages of myogenesis.

PAX7 transcription factor plays a crucial role in embryonic myogenesis and in adult muscles in which it secures proper function of satellite cells, including regulation of their self renewal. PAX7 downregulation is necessary for the myogenic differentiation of satellite cells induced after muscle damage, what is prerequisite step for regeneration. Using differentiating pluripotent stem cells we documented that the absence of functional PAX7 facilitates proliferation. Such action is executed by the modulation of the expression of two proteins involved in the DNA methylation, i.e., Dnmt3b and Apobec2. Increase in Dnmt3b expression led to the downregulation of the CDK inhibitors and facilitated cell cycle progression. Changes in Apobec2 expression, on the other hand, differently impacted proliferation/differentiation balance, depending on the experimental model used.

MikroRNA (miRNA) nie kodują białek, a jednak odgrywają ogromną rolę w wielu procesach biologicznych, w tym w rozwoju i regeneracji mięśni szkieletowych. miRNA regulują procesy zachodzące w trakcie rekonstrukcji mięśni, począwszy od utrzymywania komórek satelitowych w stanie spoczynkowym, przez ich aktywację, proliferację i różnicowanie w mioblasty, aż po fuzję tych komórek prowadzącą do powstania miotub. miRNA wpływają także na przebieg chorób degeneracyjnych mięśni szkieletowych, a ich poziom jest zmieniony w stanach chorobowych. Z tego powodu uważa się, że mogą być wykorzystane jako biomarkery chorób lub nawet cele terapeutyczne. Co ciekawe, aktywność fizyczna również wpływa na zmiany poziomu miRNA w tkance mięśniowej oraz krwiobiegu. Obserwowane różnice w profilach ekspresji tych cząsteczek są zależne od rodzaju, czasu trwania oraz intensywności treningu.

Non-coding RNAs play a crucial role in the regulation of gene expression, impacting all processes occurring in a living cell. Among them are microRNAs (miRNAs) and long non-coding RNAs (lncRNAs), which interplay with each other modulating the expression of target genes. In the current study, we focused on lncRNAs described as involved in mesoderm specification or proliferation and differentiation of myogenic precursor cells and verified which of them may indeed be useful as candidate markers of mesodermal/myogenic differentiation of pluripotent stem cells (PSCs). As a model, we used two mouse embryonic stem cell (ESC) lines. We compared cells differentiating in the control medium with those induced to undergo myogenic differentiation by retinoic acid, insulin, transferrin, and selenium treatment. We showed that such induced ESC differentiation upregulated expression of few lncRNAs, i.e., yylncT , H19 , SRA , and linc-YY1 , in both lines. The expression of other analyzed lncRNAs changed in the ESC line specific manner. Next, we studied the impact of miR145 or miR181 , which we have previously documented to elevate myogenic gene expression, on lncRNA levels. We found that miR145 overexpression resulted in decrease in H19 in both ESC types. Another common change observed in both cell lines was decrease of H19 in response to miR181 . Again, the expression of most lncRNAs known as mesodermal and myogenic regulators and studied by us occurred in ESC line-dependent, and diverse manner. For the first time we verified if and which lncRNAs may serve as a potential indicators of up-and-coming myogenic differentiation. However, it should obviously be tested by extensive studies that include multiple different PSC lines.