Herein, 40 subjects involving 33,824 genes were analyzed to investigate the relationship between SDHB and the incidence of sarcopenia in our study. The results of GSEA revealed that DEGs in both sarcopenia/non-sarcopenia control and SDHB-low/high cohorts enriched biological processes of cellular respiration, cornification, mitochondrial gene expression, mitochondrial translation, oxidative phosphorylation, respiratory electron transport chain, translational elongation, translational termination. It was reported that in NIH-3T3 fibroblasts, mitochondrial membrane potential, cellular respiration, proliferation, and mitochondrial architecture were associated with sarcopenia[25]. Also accumulation of mitochondrial (mtDNA) alterations in myofibres activates regeneration during ageing, mitochondrial oxidative phosphorylation and mitochondrial dynamics ultimately leading to sarcopenia[26–28]. SDH is a key enzyme in the tricarboxylic acid cycle and oxidative phosphorylation of the respiratory chain. The lack of SDH inhibits the tricarboxylic acid cycle and the transmission of electrons in the respiratory chain, resulting in the production of ROS and oxidative damage to cells[8, 11]. SDHB genes are normal or not will affect the function of SDH, which will eventually lead to excessive production of ROS in mitochondria[29]. Study has shown that deletion of the SDHB genes and point mutations of SDHB is associated with SDH dysfunction and increased production of reactive oxygen species (ROS) [12]. Of particular note, these processes may be associated with sarcopenia and low expression of SDHB. After that, the regulatory network and co-expression module of DEGs related to SDHB were constructed to further exploring, which could deepen the understanding of SDHB into the genome scale pathogenesis in sarcopenia.
Also in our results functional enrichment analysis showed that the turquoise module exhibited the greatest negative correlation with sarcopenia and positive correlation with SDHB in line with the GSEA results, hence supporting the involvement of DEGs in oxidative phosphorylation, thermogenesis, ribosome and TCA cycle. More specifically, TCA and oxidative phosphorylation took part in ROS and reactive nitrogen species (RNS) induced muscle atrophy[30] showing that TCA cycle and oxidative phosphorylation are involved in the pathogenesis of sarcopenia. Also the DEGs in the blue module and the brown module participated in KEGG pathway of ras signaling pathway, PI3K-AKt signaling pathway and mitophagy ,glutathione metabolism, supporting the fundamental process for sarcopenia development. Among them, Ras-extracellular signal-regulated kinase (ERK) pathway played critical roles in the inhibition of myocyte differentiation and muscle regeneration, which leads to muscle atrophy[31], also the aging phenotype was associated with repression of muscle-specific genes and activation of the epidermal growth factor receptor (EGFR)-Ras- ERK pathway[32]. And the PI3K/Akt pathway is known to regulate muscle protein synthesis and degradation[33]. Deactivation of the PI3K/Akt pathway inhibits protein synthesis and promotes protein degradation. It causes an imbalance in muscle mass maintenance and induces muscle loss[34]. An essential surveillance mechanism targeting defective and harmful mitochondria to degradation is the selective form of autophagy called mitophagy[35]. Impaired mitophagy is a primary pathogenic event underlying diverse aging-associated diseases such as sarcopenia[36].
The analysis of cross-talking pathways revealed that SDHB was jointly involved in oxidative phosphorylation, TCA cycle and thermogenesis. Less is known about the association of SDHB with thermogenesis. Brown adipose tissue (BAT) contains high mitochondrial content and generates heat via uncoupling protein 1 (UCP1)[37]. Uncoupling was essential for heat production in skeletal muscle and BAT[38]. In Kim’s research, the thermogenic effect in the skeletal muscle of the USP21-KO animals seemed to be at least partly to mitochondrial uncoupling activity[39]. Mitochondrial oxidative phosphorylation was a critical regulator of skeletal muscle mass and function. Muscle atrophy due to mitochondrial dysfunction was closely associated with the bone loss[27]. A comparison of the muscle transcriptome in sarcopenia men versus age‐matched controls from Singapore, Hertfordshire UK, and Jamaica demonstrated that the major transcriptional signature of sarcopenia was mitochondrial bioenergetic dysfunction in skeletal muscle, with down‐regulation of oxidative phosphorylation genes. Oxidative phosphorylation and mitochondrial energy homeostasis were the most perturbed biological processes associated with sarcopenia in all ethnicities, with bioenergetic alterations spanning across all mitochondrial respiratory complexes both at the level of expression and activity, also the activity of the two mitochondrial TCA cycle enzymes citrate synthase and SDH were strongly reduced in sarcopenia muscle, confirming that a global alteration of oxidative metabolism and energy production was perturbed in human sarcopenia muscle[40]. The unique property of SDH was that it participates in the oxidative phosphorylation pathway and the TCA cycle and thus has dual functions in carbon metabolism and mitochondrial respiration[41, 42]. To understand the consequences of SDHB deficiency on gene expression, a whole genome expression analysis was performed, notably, genes involved in glycolysis, hypoxia, cell proliferation, and cell differentiation were up regulated in this analysis, whereas genes involved in oxidative phosphorylation were down regulated[43]. And the other study showed that AMP-activated protein kinase (AMPK) was a known energy sensor and regulates the rate of protein synthesis rate phosphorylating mTORC1[6], SDHB silencing increased AMPK phosphorylation and enhanced levels of p‐p38 and p‐Hsp27[9]. The TCA cycle is an integral part of aerobic respiration, generating ATP and precursors for building macromolecules and reducing agents that decrease oxidative stress. SDH was the only TCA cycle enzyme connecting the TCA to the ETC as complex II[9]. SDH mediated the oxidation of succinate in the TCA cycle, which was coupled to the reduction of ubiquinone to ubiquinol in the electron transfer chain[8],and the up-regulation of SDH activity marked the acceleration of TCA and the increase of ATP[44]. TCA cycle regulator and oxidative phosphorylation gene sets were repressed in sarcopenic muscle with highly significant FDRs[40].
The turquoise module with the highest degree of correlation coefficient validated the strongest interaction between its DEG and SDHB expression according to the scatter plot between MM and GS. Subsequently, DEGs of the turquoise module were displayed in the global regulatory network to identify the cross-talking pathways of SDHB, which supported the pleiotropic roles of SDHB in sarcopenia pathophysiology responsible for oxidative phosphorylation[40], TCA cycle[40, 44] and thermogenesis. Because of the low expression of SDHB, the vulnerability of related pathways may be very obvious, leading to sarcopenia under the comprehensive pathogenic effect. The diagnostic performance of SDHB showed that the AUC value was 74.9%, suggesting a possible use of SDHB as a predictor of sarcopenia prevalence. Studies using reverse transcription polymerase chain reaction or Western blotting or a combination of both for gene expression confirmation are lacking. However, previous study using Western immunoblotting revealed that circulating small extracellular vesicles from 11 older adults with physical frailty and sarcopenia participants had lower amounts of adenosine triphosphate 5A, ubiquinone oxidoreductase subunit S3 and SDHB[45]. Future experiments are needed to validate the presented mechanisms of SDHB in sarcopenia.