Physical exercise has substantial beneficial effects not only on physical health but also on brain function. Most studies suggested the importance of exercise on the brain, particularly the hippocampus and hypothalamus. For example, long-term exercise can prevent cognitive dysfunction induced by obesity (13) or aging(14) and improve spatial learning and memory ability. Endurance exercise can alter the gene expression status of the hippocampus, thereby affecting human cognitive function(15). In addition, exercise ameliorates the hypothalamic leptin resistance(16) and insulin resistance(17) to affect the energy balance. However, the molecular mechanisms through which exercise affects brain function are unclear. The development of high-throughput sequencing provides a beneficial tool to study the role of exercise in regulating the biological processes in the brain by altering the gene expressions.
The molecular mechanisms of exercise that regulates brain function were investigated. An exercise mouse model was constructed to observe the effects of exercise on gene expression in the hippocampus and hypothalamus. Using high-throughput sequencing technology, significant differences were observed in the expression levels of 102 and 64 genes in the hippocampus and hypothalamus, respecively, in mice 12-week exercise training (Fig. 3). Using GO functional enrichment (Fig. 4a and 4b) and KEGG signaling pathway (Fig. 4c and 4d) analyses, differential genes were found to be involved in many important cellular functions and signaling pathways. For example, some enriched functions of the differentially expressed genes in the hippocampus were associated with the synaptic transmission process (GO: 0099025, GO: 0099029, GO: 0099576, GO: 0060080, GO: 0099151, and GO: 0051932), indicating that exercise may regulate synaptic activity. In addition, some enriched functions of the differentially expressed genes in the hypothalamus were associated with the neural function (GO: 0032809 and GO: 0043005), neurogenesis (GO: 0021626 and GO: 0014037), and glucagon secretion regulation (GO: 0070029), suggesting that exercise promotes hypothalamic health and its function. Regarding the KEGG pathway, the “cell adhesion molecules signaling pathway” plays a crucial role in the hippocampal neuronal survival, differentiation, axonal growth, and synaptic development(18, 19).
Recently, the significance of epigenetic regulation in various biological functions and disease pathogenesis has increased. As an epigenetic marker, the reversible m6A is the most prevalent post-transcriptional regulation of mammalian gene expression. m6A is abundant in the nervous system, and the cellular dynamics of m6A are associated with neural function, neurogenesis, and neuronal survival(20–22). The dysregulation of m6A is related to many biological processes, including neurodevelopment and neurodegenerative diseases. Reportedly, the upregulation of m6A occurs with brain maturation(23), behavioral experience(24), and memory formation(25). In this study, a high level of m6A was observed in the hippocampus and hypothalamus of mice in the exercise group (Fig. 5a). Since the dynamic equilibrium of m6A is governed by m6A-related components, such as methylesterases, demethylases, and reading proteins, the expression of 13 m6A RNA methylation regulator genes, including METTL3, METTL14, WTAP, RBM15, ZC3H13, FTO, ALKBH5, YTHDF1, YTHDF3, YTHDC2, YTHDF2, YTHDC1, and HNRNPC were analyzed in the hippocampus and hypothalamus of mice in the exercise group. The result showed that only Fto was down-regulated in the hippocampus and hypothalamus of the mice in exercise group (Fig. 5b and 5c). In addtion, western blot experiment was performed, confirming the finding (Fig. 5d).
FTO as an m6A demethylase is a crucial component of m6A modification(26, 27). Several studies suggested that FTO knockdown with siRNA increased the amount of m(6)A in mRNA, and FTO overexpression decreased the amount of m(6)A in human cells(28). The above evidence proves that FTO expression may contribute to m6A levels. Hence, presumably, elevated levels of m6A in the hippocampus and hypothalamus after exercise are due to the downregulation of FTO. Hence, presumably, elevated levels of m6A in the hippocampus and hypothalamus after exercise are due to the downregulation of FTO.
Although polymorphisms within the intron 1 of the FTO gene were first reported to be associated with obesity(9, 29, 30), the physiological role of the FTO gene remains unclear. FTO is widely found in central and peripheral tissues of mammals(31). In peripheral tissues, FTO is related to energy metabolism(32, 33) and cancer progression(34–36). In central tissues, FTO is highly expressed in the brain and essential for development of the central nervous system (CNS) in humans(37, 38). Numerous preclinical evidence reported that altered FTO expression is partially responsible for energy balance, epilepsy, neurodevelopment, and neurodegenerative diseases (Table 1). In animal studies, FTO can activate the phosphorylation of Tau, which is one of the markers of Alzheimer's disease (AD)(39). In human studies, the genetic variation in the introns of the FTO gene possibly contributes to the risk of AD(40, 41). However, specific mechanism of the FTO gene variants that contribute to the risks of AD is still unclear and requires further research. Moreover, the FTO inhibitor can regulate the neuronal excitability with anticonvulsant activity(42),and is responsible for glioblastoma progression(43). Axonal FTO is reportedly involved in neuronal development by regulating the m6A modification of axonal mRNA(44). Decreasing FTO in the dorsal hippocampus aids in memory formation(25). However, the loss of FTO leads to impairment of neuronal differentiation and a processing defect of brain-derived neurotrophic factor (BDNF) within the hippocampus, which increasing anxiety and impairing the working memory(45). In addition, the complete or neural-specific Fto gene deletion results in postnatal growth retardation of mice(31). The m6A RNA demethylase FTO alleviates the deficits in dopaminergic neurotransmission in response to arsenite exposure(46). FTO is related to appetite and food intake in the hypothalamus(47). Further research found that mice with low expression of FTO remain sensitive to the anorexigenic effects of leptin(48). All these studies strongly suggest that FTO plays vital roles in the physiological and pathological functions of the brain.
Although most studies have focused on the impact of FTO overexpression or knockdown in the brain, the genes that related to FTO are still important as they perform many subsequent molecular functions and biological processes. It has been reported that FTO as a transcriptional coactivator promotes gene transcription, ultimately affecting adipose tissue development(33). However, the mechanism of FTO interaction with downstream genes to further regulate nerve function remain largely unknown.
FTO could be regulated not only by nutrition but also by exercise. Previous studies found that physical activity might weaken the effect of the FTO variant on BMI(49–53). In addition, gender also influences the FTO genotype on exercise for weight losss. It is observed that males carrying the FTO risk allele lose more weight after a 12-week regular exercise(54). An acute decreased skeletal muscle FTO mRNA expression was observed after high-intensity exercise by Danaher et al(55). Most researchers focus on the reducted obesity risk caused by FTO gene polymorphisms under exercise, while there are few on its function. In the present study, the level of m6A increased in the hippocampus and hypothalamus of mice performing exercise (Fig. 5a). However, the FTO expression was down-regulated (Fig. 5b, 5c and 5d). Overall, the study reported for the first time that long-term exercise can down-regulated the FTO expression in the hippocampus and hypothalamus, indicating that FTO may be a promising key player between exercise and the brain.
However, it is unclear whether exercise-induced FTO downregulation can regulate downstream target genes and the biological processes. Hence, the Fto/co-expression genes were downloaded from the database for GO enrichment and KEGG signal pathway analyses. The results showed 54 Fto/co-expression genes in the mouse brain (Fig. 6a). Based on the results of bioinformation analyses, the significant enrichment pathway primarily correlated with vasopressin-regulated signaling pathway, water reabsorption signaling pathway, synaptic vesicle cycle signaling pathway, endocrine signaling pathway, calcium reabsorption signaling pathway, protein processing in endoplasmic reticulum signaling pathway, salivary secretion signaling pathway, cAMP signaling, insulin secretion signaling pathway, and morphine addiction signaling pathway (Fig. 6c). The result suggests that FTO and its co-expression genes are involved in many important biological processes in the brain. In addtion, the known and unknown proteins co-expressed with FTO may be regulated by FTO-m6A to alter their expression and function. Hence, FTO could be a valuable therapeutic target for brain diseases in the future. Thus, exercise may regulate the expression and function of the related genes via FTO-dependent demethylation of mRNA m6A.