As a serious central nervous system injury, the impact of SCI on patients is inestimable. The effects of FES, an important adjuvant therapy on SCI, on Sarcopenia and weakness in SCI patients have been confirmed by many studies. But the exact mechanism of the effects of FES is not known. It has been reported in previous studies that mitochondria and energy metabolism play an important role in this process, which has been confirmed again in our study.
Through PPI network, we identified four genes including CYCS, SUCLG1, ATP5B and ATP5C1, which may be the key genes for FES to improve muscle function in patients with SCI. Around these genes, CYCS has been suggested to be closely related to the improvement of muscle function. Previous studies have confirmed that the reduction of exercise after SCI can reduce the ratio of fusion proteins and lead to mitochondrial dysfunction(9, 10), while exercise training can improve the volume, quantity, density, dynamics and quality control of skeletal muscle mitochondrial(11, 12).
As an indispensable component in the oxidative respiratory chain, CYCS is the core component of mitochondrial electron transport chain and the proteins encoded by CYCS combines with the mitochondrial inner membrane to act as the electronic carrier of complexes Ⅲ to complex Ⅳ in the electron transport chain(13). Oxidative stress caused by long-term inactivity plays an important role in muscle atrophy, which can reduce mitochondrial respiratory function. The main source of oxidants is reactive oxygen species (ROS) produced by mitochondria and high concentration of ROS can damage fast and slow muscle fibers and mitochondrial function, and lead to muscle fiber atrophy(14). CYCS can clear ROS and reduce the damage caused by oxidative stress to improve muscle function(15, 16). At the same time, exercise can increase the abundance of mitochondrial RNA and the ability of mitochondria on producing ATP (17). SUCLG1 is mainly composed of the catalytic enzyme of mitochondrial energy generation. ATP5B and ATP5C1 are mainly involved in the subunit of ATP syntheses in mitochondria, and the three genes participate in promoting energy generation. Therefore, FES may improve muscle quality and strength by promoting the production of the above four genes.
We analyzed the CYCS gene that is highest ranked in degree method by GSEA combined with the enrichment of KEGG pathway in DAVID analysis. The results showed that the up regulation of CYCS was related to myocardial contraction, Parkinson's disease, Alzheimer's disease, oxidative phosphorylation and TCA cycle. It is confirmed that CYCS is closely related to energy metabolism such as oxidative phosphorylation, TCA cycle and so on. 95% of the energy of cell activity comes from ATP produced by oxidative phosphorylation (18). Oxidative phosphorylation is composed of electron transport chain and ATP synthase. The former generates mitochondrial proton power by pumping protons across the inner mitochondrial membrane and the latter returns protons to the matrix to couple with ADP and phosphate to ATP (18). One of the main protein control the signal of mitochondria oxidative phosphorylation is cytochrome c, and the function of which was regulated by phosphorylation of several residues(18, 19). So, as the final step of electron transport chain, CYCS is an important place for the oxidation phosphorylation of electron transport chain. The TCA cycle is the final metabolic pathway of the three major nutrients as lipids, carbohydrates, and amino acids, and is the main site for energy metabolism in the mitochondria. So, as the metabolic link of three substances, TCA cycle is the key link of energy metabolism and the main place is mitochondria. Therefore, all the above evidence supports that FES is associated with mitochondria and energy metabolism.
The results of GO enrichment analysis suggests that the DEGs mainly focus on mitochondrial electron transport, mitochondrial respiratory chain complex I assembly, mitochondrial ATP synthesis coupled proton transport, TCA cycle, response to reactive oxygen species and so on. These results are consistent with our differential analysis and KEGG pathway, which mainly focus on mitochondrial energy metabolism.
As muscle samples are more difficult to obtain, bioinformatics methods were used for DEGs analysis as a favorable tool to explore the mechanism of FES on muscle function recovery of SCI patients. But due to the small sample size of these two datasets, some causative genes may not show significant differences and we selected a smaller difference. As the number of datasets in the GEO database increasing and relevant techniques and algorithm improved, the mechanisms about FES on treating SCI patients may be further explored.