Muscle fibre transition and transcriptional changes of horse skeletal muscles during traditional Mongolian endurance training

Background: Traditional Mongolian endurance training is an effective way to improve the athletic ability of the horse for endurance events and is widely used. This incorporates aerobic exercise and intermittent fasting and these altered physiologic conditions are associated with switches between muscle fibre types. Objectives: To better understand the adaption of horse skeletal muscle to traditional Mongolian endurance training from muscle fibre characteristics and transcriptional levels and to explore possible molecular mechanisms associated with the endurance performance of horses. Study design: Before-after study. Methods: Muscle fibre type switches and muscle transcriptome changes in six Mongolian horses were assessed during 4 weeks of training. Transcriptomic and histochemical analyses were performed. The activities of oxidative and glycolytic metabolic enzymes were analysed and we generated deep RNA-sequencing data relating to skeletal muscles. Results: A fast-to-slow muscle fibre transition occurred in horse skeletal muscles, with a concomitant increase of oxidative enzyme activity and decreased glycolytic enzyme activity. Numerous differentially expressed genes were involved in the control of muscle protein balance and degradation. Differential alternative splicing events were also found during training which included exon-skipping events in Ttn that were associated with muscle atrophy. Differentially expressed noncoding RNAs showed connections with muscle protein balance-related pathways and fibre type specification via the post-transcriptional regulation of miRNA. Main limitations: The study focuses on horse athletic ability only from the aspect of muscular adaptation. Conclusion: Traditional Mongolian endurance training-induced muscle fibre transition and metabolic and transcriptional changes. Muscle-specific non-coding RNAs could contribute to these transcriptomic changes during training.