Critical role of the mTOR pathway in poultry skeletal muscle physiology and meat quality: an opinion paper

Skeletal muscle is the major component of meat, and is primarily composed of muscle fibers bounded by multiple connective tissue layers (Velleman and McFarland, 2014). These connective tissue layers function as an essential support and functional system, incorporating components like blood vessels and extracellular matrix macromolecules. Consequently, muscle development and growth, morphological structure, and biochemistry are crucial aspects in the determination of meat yield and quality. The modern poultry industry has had one area of focus on selection for enhanced growth performance, specifically emphasizing increased body weight and skeletal muscle yield (Havenstein et al., 2007; Collins et al., 2014). Structural abnormalities, such as diminished connective tissue spacing and reduced capillary density resulting from excessive muscle fiber hypertrophy, have been observed in the breast muscle of modern rapid-growing poultry lines (Velleman et al., 2003; Joiner et al., 2014). The loss of connective tissue spacing and presence of oversized myofibers result in direct contact between muscle fibers, and this condition is correlated with a greater occurrence of muscle fiber degeneration (Wilson et al., 1990; Velleman et al., 2003). Furthermore, insufficient capillary supply in the breast muscle could limit the removal of anaerobic respiration byproducts, such as lactic acid. The residual lactic acid in the breast muscle can lead to a decrease in pH, potentially exacerbating muscle degeneration. In addition to the structural flaws, the breast muscle of modern fast-growing poultry breeds exhibits conditions such as Wooden Breast (Sihvo et al., 2014) and White Stripping (Soglia et al., 2018), which adversely affect the quality of the breast meat. Muscle growth and structure are primarily determined by muscle cell biology and biochemistry, which are influenced by signal transduction pathways. One of the key players involved in muscle function is the mechanistic target of rapamycin (mTOR) pathway, which is critical in regulating muscle hypertrophic growth and mass accretion in poultry (Vignale et al., 2015; Ma et al., 2018). This opinion paper will discuss how the mTOR pathway modulates skeletal muscle growth, structure, and biochemistry, and ultimately can affect poultry meat yield and quality. Muscle fiber number is fixed by the time of hatch (Smith, 1963). Post-hatch muscle grows through the hypertrophy of existing muscle fibers. Accumulation of intracellular protein in existing muscle fibers is the most likely mechanism for post-hatch muscle hypertrophic growth. With regard to the molecular mechanisms, mTOR is a key regulator controlling muscle size and mass accretion in mammals and poultry (Bodine et al., 2001; Vignale et al., 2015). It has been broadly hypothesized that mTOR promotes myofiber hypertrophy by stimulating protein synthesis (Wang and Proud, 2006; Wang et al., 2015; You et al., 2019). A schematic illustration of possible mechanisms of mTOR pathway in skeletal muscle function is presented in Figure 1. Using mammalian models, the mTOR protein kinase has been found to function in two distinct multiprotein complexes: mTOR OPEN ACCESS