Adult phenotypes are the result of both genetic and environmental factors acting to program the form and function of embryonic tissues during development. Skeletal muscle development is known to be plastic for many vertebrate species and is affected by a variety of environmental factors such as temperature and hypoxia. The development of skeletal muscle tissue can be broadly divided into two phases, progenitor cell (myoblast) proliferation which increases the number of developing muscle cells and terminal differentiation which is marked by the expression of a definitive muscle cell phenotype. Both of these events are regulated by insulin-like growth factor (IGF) signaling through the same cellular receptor. Despite using the same receptor, cell proliferation and differentiation are mutually exclusive events in the development of a myoblast. Ren and colleagues at the University of Michigan set out to identify how the same signaling receptor can regulate these two processes and how environmental hypoxia might alter the outcome of IGF signaling in developing muscle cells.
Knowing that oxygen deprivation has a dramatic effect on muscle development, the team tested the effect of reduced oxygen on myoblast (muscle precursor) development. They exposed mouse myoblast cells to normoxia (20% O2) and hypoxia (1% O2) for up to 96 h in the presence or absence of IGF-II and measured cell number and the extent of cellular differentiation in the cultures. As hypoxia-inducible factor 1α (HIF-1α) regulates cellular responses to hypoxia, they also tested the possible effect of HIF-1α activity in mediating the action of IGF-II under hypoxia by blocking HIF-1α expression under normoxic and hypoxic conditions. Finally, the team assessed the role of three signaling cascades (Akt-mTOR, p38 and Erk1/2 MAPK), which are known to be associated with the IGF receptor, using pharmacological inhibitors to block their activity under normoxia and hypoxia, and in response to IGF-II activation.
The team discovered that exposure to IGF-II caused the myoblasts to undergo differentiation (myogenesis – the expression of a skeletal muscle-specific cellular phenotype) under normoxic conditions, but blocked differentiation and promoted cell division under hypoxic conditions. This effect was mediated by HIF-1α, because blockage of its activity caused myogenesis under normoxia and hypoxia. Pharmacological studies suggest that myogenesis is blocked by IGF signaling under hypoxia by suppression of the Akt-mTOR pathway. In addition, IGF signaling favored cell division under hypoxia by altering the activity of Erk1/2 and p38 MAPK signaling activities. Thus, this study illustrates that the same extracellular signal can initiate different intracellular signaling events depending on oxygen levels.
This study is one of the first to detail the mechanisms behind the effect of hypoxia on muscle cell development. The finding that hypoxia can alter IGF signaling through a differential interaction with HIF-1α and specific downstream signaling cascades offers a powerful model for testing the physiological and ecological impacts of altered muscle development in the face of hypoxia. Exposure to hypoxia may inhibit muscle differentiation and favor myoblast proliferation during development. If hypoxic stress is encountered during critical periods of muscle development, this could have profound effects on the adult muscle phenotype and thus perhaps physiological performance and maybe even fitness.
