Objective: The purpose of this study was to investigate the ultrastructural and proteomic alteration of the superficial masseter muscle (SM) after lower jaw sagittal advancement in a rat model.
Methods: Six 35-day-old male Sprague-Dawley rats were utilized in a transmission electron microscopy study, and six more in a proteomic study. The rats were randomly allocated to two experimental and two control groups (n=3). The experimental groups were fitted with fixed devices that protruded the mandible, whereas the control groups were not fitted with the devices. The rats were sacrificed 3 days after mandibular advancement. Transmission electron microscopy analysis, Western blot analysis, comparative proteomic analysis, and matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF) mass spectrometry (MS) were applied to profile qualitative and quantitative differences in the morphology and proteome of rat SM.
Results: Under the transmission electron microscope, morphological changes of the SM were associated with deregulation of the myofibrillar network accompanied by fibre rupture, interlaced myofilaments, and changes in the mitochondria and sarcoplasmic reticulum cisternae. The shape of mitochondria changed. Proteomic analysis identified fourteen differentially expressed proteins involved in metabolism, contraction or the cytoskeleton, and transport. Amongst these, the expression of 13 proteins increased and 1 decreased. According to Western blot analysis, myosin heavy chain II was down-regulated.
Conclusion: Three days after functional mandibular advancement, the ultrastructure of rat SM had changed. This change paralleled changes in metabolic, contractile and cytoskeletal, and transport proteins, which affected SM physiology. The energy metabolism of SM increased, and the muscle velocity and force of contraction decreased.
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