Low Oxygen Tension Enhances Expression of Myogenic Genes When Human Myoblasts Are Activated from G0 Arrest

Insights into human muscle biology from human primary skeletal muscle cell culture

This review arises from the symposium held in honour of Prof Jenny Morgan at UCL in 2024 and the authors would like to acknowledge the outstanding contribution that Prof Morgan has made to the field of translational muscle cell biology. Prof Morgan published a review article in 2010 entitled: Are human and mice satellite cells really the same? In which the authors highlighted differences between species which are still pertinent to skeletal muscle cell culture studies today. To our knowledge there are no comprehensive reviews which outline the considerable work that has been undertaken using human primary skeletal muscle origin cells as the main model system. This review highlights the multitude of muscle biology that has been investigated using human primary cells, as well as discussing the advantages and disadvantages over other cell models. We also discuss future directions for primary cell culture models utilising the latest technologies in cell type specificity and culture systems.

Androgen action on myogenesis throughout the lifespan; comparison with neurogenesis

Androgens' pleiotropic actions in promoting sex differences present not only a challenge to providing a comprehensive account of their function, but also an opportunity to gain insights by comparing androgenic actions across organ systems. Although often overlooked by neuroscientists, skeletal muscle is another androgen-responsive organ system which shares with the nervous system properties of electrochemical excitability, behavioral relevance, and remarkable capacity for adaptive plasticity. Here we review androgenic regulation of mitogenic plasticity in skeletal muscle with the goal of identifying areas of interest to those researching androgenic mechanisms mediating sexual differentiation of neurogenesis. We use an organizational-activational framework to relate broad areas of similarity and difference between androgen effects on mitogenesis in muscle and brain throughout the lifespan, from early organogenesis, through pubertal organization, adult activation, and aging. The focus of the review is androgenic regulation of muscle-specific stem cells (satellite cells), which share with neural stem cells essential functions in development, plasticity, and repair, albeit with distinct, muscle-specific features. Also considered are areas of paracrine and endocrine interaction between androgen action on muscle and nervous system, including mediation of neural plasticity of innervating and distal neural populations by muscle-produced trophic factors.

Effects of hypobaric hypoxia during a simulated ultra-long-haul flight on inflammation and regeneration after muscle trauma and muscle trauma-hemorrhagic shock

INTRODUCTION/AIMS

Because wounded warfighters or trauma victims may receive en route care to the closest medical facility via airplane transport, we investigated the effects of extended mild hypobaric hypoxia (HB), the environmental milieu of most airplanes, on inflammation and regeneration after muscle trauma or monotrauma (MT) and muscle trauma-hemorrhagic shock or polytrauma (PT).

METHODS

Male C57BL/6N mice were assigned to one of six groups pertaining to injury (control/uninjured, MT, and PT) and atmospheric pressure exposure (HB and normobaric normoxia, NB). Body mass, blood and muscle leukocyte number by flow cytometry, immunohistochemistry, or both, and the muscle relative mRNA level of selected genes involved in inflammation and muscle regeneration were examined at ~1.7, 4, 8, and 14 days post trauma (dpt). At 14 dpt, the proportion of smaller- and larger-sized myofibers at the regenerating site of MT mice was determined.

RESULTS

Greater body mass loss, an increased number of blood and muscle leukocytes, and differential muscle relative mRNA levels were observed in MT and PT groups compared to controls. The MT+HB or PT+HB mice demonstrated more body mass loss and altered relative mRNA level than the corresponding NB mice. Additionally, a subgroup of MT+HB mice demonstrated a greater proportion of smaller myofibers (250 to 500 μm² ) than MT+NB mice at 14 dpt.

DISCUSSION

HB exposure after muscle trauma alone may prolong regeneration. Following HB exposure, therapies that promote oxygenation may be needed during this muscle recovery.

The effect of hypoxia on myogenic differentiation and multipotency of the skeletal muscle-derived stem cells in mice

Background

Skeletal muscle-derived stem cells (SC) have become a promising approach for investigating myogenic differentiation and optimizing tissue regeneration. Muscle regeneration is performed by SC, a self-renewal cell population underlying the basal lamina of muscle fibers. Here, we examined the impact of hypoxia condition on the regenerative capacity of SC either in their native microenvironment or via isolation in a monolayer culture using ectopic differentiation inductions. Furthermore, the effect of low oxygen tension on myogenic differentiation protocols of the myoblasts cell line C2C12 was examined.

Methods

Hind limb muscles of wild type mice were processed for both SC/fiber isolation and myoblast extraction using magnetic beads. SC were induced for myogenic, adipogenic and osteogenic commitments under normoxic (21% O2) and hypoxic (3% O2) conditions. SC proliferation and differentiation were evaluated using histological staining, immunohistochemistry, morphometric analysis and RT-qPCR. The data were statistically analyzed using ANOVA.

Results

The data revealed enhanced SC proliferation and motility following differentiation induction after 48 h under hypoxia. Following myogenic induction, the number of undifferentiated cells positive for Pax7 were increased at 72 h under hypoxia. Hypoxia upregulated MyoD and downregulated Myogenin expression at day-7 post-myogenic induction. Hypoxia promoted both SC adipogenesis and osteogenesis under respective induction as shown by using Oil Red O and Alizarin Red S staining. The expression of adipogenic markers; peroxisome proliferator activated receptor gamma (PPARγ) and fatty acid-binding protein 4 (FABP4) were upregulated under hypoxia up to day 14 compared to normoxic condition. Enhanced osteogenic differentiation was detected under hypoxic condition via upregulation of osteocalcin and osteopontin expression up to day 14 as well as, increased calcium deposition at day 21. Hypoxia exposure increases the number of adipocytes and the size of fat vacuoles per adipocyte compared to normoxic culture. Combining the differentiation medium with dexamethasone under hypoxia improves the efficiency of the myogenic differentiation protocol of C2C12 by increasing the length of the myotubes.

Conclusions

Hypoxia exposure increases cell resources for clinical applications and promotes SC multipotency and thus beneficial for tissue regeneration.

Effects of hyperoxia and hypoxia on the proliferation of C2C12 myoblasts

Hyperoxic conditions are known to accelerate skeletal muscle regeneration after injuries. In the early phase of regeneration, macrophages invade the injured area and subsequently secrete various growth factors, which regulate myoblast proliferation and differentiation. Although hyperoxic conditions accelerate muscle regeneration, it is unknown whether this effect is indirectly mediated by macrophages. Here, using C2C12 cells, we show that not only hyperoxia but also hypoxia enhance myoblast proliferation directly, without accelerating differentiation into myotubes. Under hyperoxic conditions (95% O₂ + 5% CO₂), the cell membrane was damaged because of lipid oxidization, and a disrupted cytoskeletal structure, resulting in suppressed cell proliferation. However, a culture medium containing vitamin C (VC), an antioxidant, prevented this lipid oxidization and cytoskeletal disruption, resulting in enhanced proliferation in response to hyperoxia exposure of ≤4 h/day. In contrast, exposure to hypoxic conditions (95% N₂ + 5% CO₂) for ≤8 h/day enhanced cell proliferation. Hyperoxia did not promote cell differentiation into myotubes, regardless of whether the culture medium contained VC. Similarly, hypoxia did not accelerate cell differentiation. These results suggest that regardless of hyperoxia or hypoxia, changes in oxygen tension can enhance cell proliferation directly, but do not influence differentiation efficiency in C2C12 cells. Moreover, excess oxidative stress abrogated the enhancement of myoblast proliferation induced by hyperoxia. This research will contribute to basic data for applying the effects of hyperoxia or hypoxia to muscle regeneration therapy.