Esophageal striated muscle hypertrophy and muscle fiber type transformation in MSTN knockout pigs

Myostatin (MSTN) is a member of the transforming growth factor-β superfamily that inhibits skeletal muscle growth and development. The esophagus is composed of skeletal muscle and smooth muscle, but the effect of MSTN on esophagus striated muscle (ESM) is unknown. The present study investigated the role of MSTN in ESM using MSTN mutant pigs through histological, gene and protein expression analysis in ESM of MSTN knockout (MSTN-/-) pigs and their wild type (WT) littermates. Hematoxylin-eosin staining showed that the fiber cross-sectional areas in ESM of MSTN-/- pigs were significantly larger than WT pigs (P < 0.05). Immunofluorescence staining showed that the percentage of type I muscle fibers in MSTN-/- pigs were significantly lower than WT pigs (P < 0.01) and type IIA muscle fibers in MSTN-/- pigs were significantly higher than WT pigs (21% higher, P < 0.01). However, type IIB muscle fibers were not detected in the ESM of MSTN-/- or WT pigs indicating that muscle fiber types in pig ESM was composed of type I and IIA. The mRNA levels of myogenic regulatory factors (MRFs) including myogenic differentiation (MyoD), myogenin (MyoG), myogenic factor 5 (Myf5) and myogenic regulatory factor 4 (MRF4) in ESM of MSTN-/- pigs showed a significant increase (P < 0.05 at least) when compared to WT pigs while mRNA level of myocyte enhancer factor 2C (MEF2C) displayed a decrease (P < 0.001). Protein expression of myosin heavy chain I (MHC-I) in MSTN-/- ESM was decreased and myosin heavy chain IIA (MHC-IIA) was increased (P < 0.01, P < 0.05). These findings indicate that MSTN plays an important role in esophageal striated muscle development and regulates muscle fiber types.