FUNDC1 is an outer mitochondrial membrane protein with a conserved sequence ranging from Drosophila melanogaster to Homo sapiens, discovered and named by the State Key Laboratory of Membrane Biology, Institute of Zoology, Chinese Academy of Sciences, in 2012 (Liu et al., 2012). The human FUNDC1 protein contains 155 amino acids and contains three transmembrane fragments, with the C-terminal extending into the membrane gap and the N-terminal (AAs1-50) exposed in the cytoplasm, and is widely expressed, especially in the heart (Figure 1) (Liu et al., 2012). It was shown that autophagic receptors always target cargoes to be degraded (damaged organelles, protein aggregates, or invading pathogens) with microtubule-associated protein 1 light chain 3 (MAP1LC3) members (including MAP1LC3A, MAP1LC3B, MAP1LC3B2, and MAP1LC3C) or homologs (GABARAP, GABARAPL1, and GABARAPL2) interact through the LC3 interaction region (LIR), tethering them to the autophagosomal membrane, where typical LIRs include a (W/F/Y)XX (L/I/V) core pattern interacting with MAP1LC3, and two hydrophobic pockets of LIR docking sites in the homologs anchored to the autophagosomal membrane (Zhang et al., 2021c). Related studies tested 30 LIRs, 12 of which (40%) were selective for GABARAP, but only one LIR motif (Y18-E19-V20-L21) FUNDC1 preferentially interacted with LC3, and mutations in the LIR motif would impair its interaction with LC3 and subsequently impede the process of mitophagy (Liu et al., 2012; Kuang et al., 2016; Rogov et al., 2017; Zhang, 2021; Liu et al., 2022).

In addition to inducing mitophagy, FUNDC1 has a critical role in the maintenance of normal mitochondrial morphology and function in cardiomyocytes, interacting with inositol-1,4,5-triphosphate receptor 2 (IP3R2) in modulating Ca²⁺ release from the endoplasmic reticulum (ER) into the mitochondria and cytoplasm, and disruption of its interaction decreases Ca²⁺ levels in the mitochondria and cytoplasm, triggering abnormal mitochondrial fission, mitochondrial dysfunction, cardiac dysfunction, and HF (Wu et al., 2017a). Under basal conditions, FUNDC1 binds to the mitochondria-associated ER membranes (MAMs) protein calnexin (CNX), and with mitophagy, the FUNDC1/CNX association decreases, and the FUNDC1-exposed cytoplasmic loop interacts with dynamin-related protein 1 (DRP1), which is recruited to MAMs, promoting fission of mitochondria and preventing them from experiencing hypoxic stress (Chen et al., 2016; Wu et al., 2016; Chai et al., 2021). FUNDC1 also has several other functions, such as activating the unfolded protein response (UPR) of mitochondria (UPR_mt) for maintaining mitochondrial quality control, participating in analgesia with hyperbaric oxygen, promoting adaptive thermogenesis, and regulating body metabolism (Wu et al., 2019a; Liu et al., 2020; Liu et al., 2021b; Ji et al., 2022). It was demonstrated that endogenous UPR_mt and mitophagy may be mildly activated in response to myocardial stress and that both act together to maintain mitochondrial performance and cardiac function, whereas exogenous UPR_mt is a downstream signal of mitophagy and serves as a compensatory role in maintaining mitochondrial homeostasis in the presence of mitophagy inhibition, and mitophagy coordinates UPR_mt to attenuate inflammation-mediated myocardial injury (Wang et al., 2021).

FUNDC1 is expressed at a high level in the myocardium, providing support for its critical role in cardiac function (Wu and Zou, 2019). The FUNDC1, a mitophagy receptor, occupies a critical role in mitochondrial quality control by regulating mitophagy and is also strongly associated with the development of some CVD (Li et al., 2021a; Liu et al., 2021a). It was shown that FUNDC1-mediated mitophagy is activated primarily in cardiomyocytes and is essential for mitochondrial network remodeling in the process of cardiac progenitor cell homeostasis and differentiation (Lampert et al., 2019; Zhang, 2021). FUNDC1 deficiency aggravated doxorubicin-induced cardiac dysfunction, mitochondrial damage, and cardiomyocyte PANoptosis (Bi et al., 2022). The mechanism of transcriptional regulation of mitophagy remains unclear (Li et al., 2023). It was shown that miR137 mimics introduced under hypoxic conditions could inhibit mitophagy by targeting FUNDC1 (Hu et al., 2020). Overexpression of miR-137 triggers a series of molecular alterations such as Nix, LC3B, and FUNDC1, leading to fragmentation and densification of mitochondrial ultrastructure, and ultimately leading to aberrant mitophagy (Khadimallah et al., 2021). miR-137 may reduce FUDC1-LC3 by inhibiting the overexpression of fundc1 (CDS+3UTR) rather than fundc1 (CDS), and thus inhibiting FUDC1-LC3 interaction, which in turn inhibits mitophagy (Li et al., 2014). A different view also pointed out that no downregulation of FUNDC1 was found in miR-137-transfected pancreatic cancer cells PANC-1, but downregulated the mRNA and protein levels of endogenous ATG5 (Wang et al., 2019b). It suggests that miR137 may have multiple functions. However, the post-translational regulation of FUNDC1 has been more clearly defined.

It was shown that, on the one hand, 45 min of ischemia significantly reduced the inhibitory phosphorylation by the SRC at the Tyr18 site of FUNDC1, and on the other hand, the interaction of PGAM5 with the Ser13 site of FUNDC1 during hypoxia dephosphorylated FUNDC1, which combined to enhance the interaction of FUNDC1 with LC3 and thus activate mitophagy (Chen et al., 2014). In response to I/R stress, cardiac structure, and function could be maintained by reducing the effects of CK2 and upregulating FUNDC1-dependent mitophagy, while an increase in CK2 is induced after cardiac IRI (Zhou et al., 2018). FUNDC1-mediated mitophagy is also regulated by other factors, such as activation of the AMPKα1/ULK1/FUNDC1/mitophagy pathway can attenuate cardiac microvascular IRI, and a study on the Danqi pill also showed that Danqi pill could enhance FUNDC1-mediated mitophagy by modulating ULK1 and PGAM5 to protect HF after acute MI (Wang et al., 2022b; Cai et al., 2022). Similarly, a study on alpha-lipoic acid showed that it could protect the heart from pressure overload-induced HF by activating FUNDC1-mediated mitophagy (Li et al., 2020). FUNDC1-mediated mitophagy may also act synergistically with other responses, such as BAX inhibitor-1 (BI-1) can ameliorate myocardial injury in type 3 cardiorenal syndrome by activating UPR_mt and FUNDC1-associated mitophagy (Wang et al., 2022a). It has also been shown that mammalian sterile 20-like kinase 1 (MST1) can promote cardiac IRI by inhibiting FUNDC1-dependent mitophagy by inhibiting the mitogen-activated protein kinase (MAPK)/ERK-CREB pathway, while the genetic ablation of MST1 can reverse this FUNDC1-involved mitophagy, thereby eliminating mitochondrial damage and cardiomyocyte death and ultimately preventing IRI in the heart (Yu et al., 2019; Shang et al., 2022). It indicates that MST1 may be one of the upstream regulators of FUNDC1-mediated mitophagy. A cellular-level study showed that irisin in lipopolysaccharide-stimulated H9c2 cardiomyocytes could abrogate mitochondrial dysfunction, oxidative stress, and apoptosis through FUNDC1-related mitophagy, and thus act as a treatment for infectious cardiomyopathy (Jiang et al., 2021b). These studies provide definite evidence for modulating mitophagy and provide a reference for drug development.

After IRI, progressively increased CK2 in the heart inhibits protective mitophagy by post-transcriptional inactivation of FUNDC1, which in turn promotes mitochondrial apoptosis in cardiomyocytes and the progression of myocardial IRI (Zhou et al., 2018). Utilizing Beclin1^+/−, FUNDC1 gene knockout, and FUNDC1 transgenic mice combined with starvation and MI model, it was found that after MI, the FUNDC1 knockout group caused more severe mitochondrial and cardiac dysregulation than the Beclin1^+/− group, suggesting that mitophagy but not macroautophagy promotes cardioprotection primarily by regulating mitochondrial function (Xu et al., 2022). The same results were seen with baseline genetic ablation of FUNDC1, as evidenced by reduced early to late ventricular filling velocity, prolonged left ventricular isovolumic diastole, and reduced ejection fraction in mice and these phenotypic alterations suggest that FUNDC1 knockout mice are susceptible to HF (Wu et al., 2017a; Zhu et al., 2021). It has also been shown that serine/threonine-protein kinase 3 (RIPK3) can directly bind to FUNDC1 and inhibit mitophagy (Zhou et al., 2017b). Deletion of RIPK3 in cardiomyocytes or microvascular endothelial cells in IRI reduces cardiomyocyte apoptosis, ROS production, and mitochondrial fragmentation and activates mitophagy (Zhou et al., 2017b). Deletion of RIPK3 in vivo also improved cardiac function, whereas overexpression of RIPK3 exacerbated cardiac dysfunction by inhibiting FUNDC1-mediated mitophagy (Zhou et al., 2017b).

In contrast to the view that activation of mitophagy facilitates cardiac function, some investigators have suggested that excessive mitochondrial elimination induced by I/R increases the death of cardiomyocytes (Lesnefsky et al., 2017; Ji et al., 2021). A study showed that melatonin could effectively inhibit platelet activation by restoring peroxisome proliferator-activated receptor gamma (PPARγ) levels in platelets, thereby blocking FUNDC1-mediated mitophagy to protect the heart from IRI (Zhou et al., 2017a). The effect of moxibustion in relieving chronic HF may also be related to the inhibition of FUNDC1-mediated mitophagy (Xia et al., 2022). The removal of FUNDC1-dependent mitophagy could render the myocardium resistant to paraquat-induced contractile dysfunction (Peng et al., 2022). A similar result was reported in the study of electroacupuncture preconditioning, which attenuated myocardial IRI by inhibiting mTORC1/ULK1/FUNDC1 pathway-mediated mitophagy (Xiao et al., 2020). This was also supported by another study, which showed that I/R increased the expression of FUNDC1 and LC3II/LC3I ratio and decreased the expression of p-mTORC1/mTORC1, while electroacupuncture preconditioning reversed this trend and suggested that electroacupuncture preconditioning could reduce brain deficit IRI by inhibiting mitophagy (Mao et al., 2020).

IPC has previously been clinically demonstrated to exhibit cardioprotective effects, and EP has been both experimentally and clinically demonstrated to exhibit cardioprotective effects due to its similar effects to IPC (Quindry and Hamilton, 2013). EP can be divided into early EP (EEP) and late EP (LEP), both of which have a myocardial protective effect (Thijssen et al., 2018). Appropriate exercise has a variety of functions, including but not limited to enhancing memory, improving cognition, suppressing obesity, and reducing inflammation (Chen et al., 2021; De Miguel et al., 2021; Li et al., 2022b). Exercise is an effective instrument for the prevention, intervention, and treatment of metabolic diseases, and although there are many possible mechanisms by which EP mediates cardioprotection, it appears that mitochondria exert an essential role in some of these mechanisms, with growing evidence supporting the protection induced by exercise mediated by autophagy, mitophagy, and mitochondrial biogenesis (Roberts and Markby, 2021; Li et al., 2022b; Ma et al., 2023). Exercise also induces FUNDC1-mediated mitophagy, removes damaged mitochondria, reverses mitochondrial dysfunction, stimulates mitochondrial biogenesis, and has a significant effect on mitochondrial fission and fusion via the AMPK-ULK1 pathway (Laker et al., 2014; Moreira et al., 2017; Gao et al., 2020; Yu et al., 2020; Memme et al., 2021). An acute exercise study concluded that PINK1 was not associated with skeletal muscle exercise-induced mitophagy because no significant PINK1 was present in mitochondria separated from skeletal muscle at any time points after acute exercise, but there was unambiguous evidence of stable PINK1 in mitochondria after treatment of HeLa cells with the uncoupling agent CCCP (Drake et al., 2019). This may be related to FUNDC1-mediated mitophagy, but whether similar results are found in the heart requires further studies to confirm. One study suggests that FUNDC1-mediated mitophagy may be influenced by the type of exercise (Laker et al., 2017).

Exercise may have a cardioprotective role by modifying FUNDC1 to regulate MAMs. FUNDC1, which tethers IP3R2, regulates both Ca²⁺ homeostasis and MAMs, exerts a crucial role in mitochondrial quality control, and is closely associated with the development of multiple CVD (Liu et al., 2021a). Research in septic mice found that upregulation of FUNDC1-dependent formation of MAMs promotes cardiac dysfunction (Jiang et al., 2021a). Conversely, FUNDC1 deficiency exacerbated high-fat diet-induced remodeling of the heart, mitochondrial aberrations, cell death, elevated IP3R3, and Ca²⁺ overload in FUNDC1^−/− mice (Ren et al., 2020). Diabetes may induce the formation of MAMs by downregulating AMPK, and activation of AMPK may play a part in ameliorating diabetic cardiomyopathy by downregulating FUNDC1 and FUNDC1-associated MAMs (Wu et al., 2019b; Chen Y. et al., 2022). Lack of FUNDC1 decreases IP3R2 and Ca²⁺ levels in mitochondria and cytoplasm and reduces mitochondrial dysfunction, cardiac dysfunction, and HF by inhibiting Ca²⁺-sensitive CREB-mediated fission 1(FIS1) expression (Wu et al., 2017a; Zhang et al., 2018). Although the role of MAMs in FUNDC1-mediated mitophagy needs to be further investigated, the available evidence demonstrates that MAMs provide a platform for FUNDC1 to exert its bio functions (Yang et al., 2020).

Although the merits of autophagy on myocardial ischemic injury remain controversial, the protective effect of exercise on the myocardium is clear, and modulation of mitophagy by exercise may be a promising cardioprotective strategy (Gedik et al., 2014). Although some exercise regimens are known to promote the protective effects of mitophagy in the heart, further research is still needed to determine the precise mechanisms of interaction between functional mitophagy and physical activity, to analyze and identify the possible factors influencing these mechanisms, and to investigate exercise modalities that are both convenient and induce mitochondrial adaptation, which would also greatly help to address exercise compliance (Moreira et al., 2017; Memme et al., 2021).

The promotion of FUNDC1-mediated mitophagy under hypoxic conditions can play a role in protecting cardiomyocytes (Li et al., 2018). Hypoxia activates mitophagy through the phosphorylation of ULK1 at the Ser555 site induced by AMPK, while the inhibition or knockdown of the AMPK gene inhibits mitophagy by preventing the translocation of ULK1 (Tian et al., 2015; Ponneri Babuharisankar et al., 2023). Mechanistically, the ULK1 is upregulated and translocated to the damaged mitochondria during hypoxia, which proceeds to interact with FUNDC1 and phosphorylate FUNDC1 at the Ser17 site, enhancing the binding of FUNDC1 to LC3 and promoting mitophagy (Wu et al., 2014b). Mitophagy plays a critical role in the reduction of IRI by hypoxic preconditioning, and the decrease in oxygen levels induced by hypoxia or ischemia increases mitophagy by decreasing FUNDC1 phosphorylated at the Tyr18 site, induced by mitochondrial degradation (Zhang et al., 2016). In contrast, it has been suggested that inhibition of mitophagy may protect cells from hypoxia-induced damage, such that inhibition of mitophagy by introducing miR-137 mimics targeting FUNDC1 under hypoxic conditions promotes mitochondrial mass, ATP synthesis, and mitochondrial transcriptional activity, and a similar effect was obtained by specific siRNA knockdown of FUNDC1 (Hu et al., 2020). It was recently shown that miR-130a can regulate FUNDC1-mediated mitophagy by targeting GJA1 in myocardial IRI (Yan et al., 2023). However, more studies support that activation of mitophagy is protective and suggest that the phosphorylation state of FUNDC1 at Tyr18 may serve an essential role for several main reasons: first, under normoxic/physiological conditions, FUNDC1 phosphorylation level at Tyr18 is high and only slight phosphorylation occurs at Ser13, suggesting that Tyr18 phosphorylation mainly controls the baseline activity of FUNDC1; second, during hypoxia or ischemia, the phosphorylation levels of total FUNDC1 decreased significantly, while the Ser13 phosphorylation level of FUNDC1 remained relatively unchanged, indicating that the activation of FUNDC1 was achieved through dephosphorylation of Tyr18; third, during the reperfusion phase after ischemia or hypoxia, the phosphorylated FUNDC1^Tyr18 changed slightly during the reperfusion phase after ischemia or hypoxia, whereas phosphorylated FUNDC1^Ser13 gradually increased, indicating that phosphorylation of Ser13 inactivated FUNDC1 after reperfusion; finally, in general, the total phosphorylated FUNDC1 at Tyr18 and Ser13 sites decreased during the ischemic phase and gradually increased during the reperfusion phase, which was associated with FUNDC1-dependent mitophagy activation state was negatively correlated (Zhou et al., 2017a; Zhou et al., 2017b; Zhang et al., 2017; Zhou et al., 2018; Zhang, 2021). Although hypoxia and exercise-induced hypoxia are different, the research related to hypoxia has certainly provided ideas for the study of exercise.