The ER Ca²⁺ channel inositol 1,4,5-triphosphate receptors (IP3Rs) physically connect with OMM voltage-dependent anion channels (VDACs) via the cytoplasmic chaperone glucose-regulated protein 75 (GRP75), forming a tripartite complex to modulate ER-mitochondria juxtaposition (8). In mouse primary neurons, GRP75 promotes ER-mitochondria tethering and mitochondrial Ca²⁺, thus enhancing ATP production (9).

The ER membrane protein vesicle-associated membrane protein associated protein B (VAPB), binds to the OMM protein tyrosine phosphatase-interacting protein-51 (PTPIP51), forming VAPB-PTPIP51 tethering complex which regulates ER-mitochondria Ca²⁺ transmission (10). Disruption of their interaction causes MAMs dissociation, and disturbs mitochondrial Ca²⁺ import and ATP synthesis (11). Besides, oxysterol-binding protein-related protein 5/8 (ORP5/8), enriched at MAMs in mammalian cells, physically interacts with PTPIP51. Inhibition of ORP5/ORP8 contributes to mitochondria morphology defects and respiratory dysfunction (12).

Mitofusin2 (MFN2), mitochondrial fusion regulator, is recognized as an important constituent of MAMs. ER-resident MFN2 forms homodimer or heterodimer with either mitofusin (MFN1) or MFN2 on the OMM (13). MFN2 depletion promotes the ER-mitochondria connections and mitochondrial Ca²⁺ uptake from ER, indicating that MFN2 is more than a physical tether (14, 15).

During the apoptotic process, the ER-located B-cell receptor-associated protein 31 (BAP31) is associated with OMM protein, the mitochondrial fission 1 protein (Fis1), acting as another tether for MAMs to induce apoptosis (16). Besides, phosphofurin acidic cluster sorting protein 2 (PACS-2), the first MAM protein identified to be involved in MAM formation, regulates the tethering of mitochondria with ER in a BAP31-dependent manner (17).

MAMs are abundant in proteins involved in lipid metabolisms including phosphatidylserine synthase (PSS), fatty acid CoA ligase 4 (FACL4), phosphatidylethanolamine N-methyltransferase 2 (PEMT2), phosphatidylserine decarboxylase (PSD) (19), as well as in phospholipid, triacylglycerol synthesis and steroidogenesis (20). Caveolin-1 (CAV-1) is an integral component distributed abundant on MAMs, which promotes lipid and cholesterol metabolism (21).

Ca²⁺ is a second messenger modulating multiple cellular activities such as cell metabolism and apoptosis (22). As mentioned above, the effective Ca²⁺ transmission at MAMs is regulated by multiple protein complexes. IP3R1-GRP75-VDAC1 tethering complex forms a Ca²⁺ regulatory axis with mitochondrial calcium uniporter (MCU), which mediates Ca²⁺ transmission from ER to mitochondria (23). In addition, ER chaperone proteins like Sigma-1 receptor (Sig-1R) physically associate at MAM, and regulate Ca²⁺ transmission through IP3R3 (24).

Mitochondrial dynamics include mitochondrial fission and fusion, which are crucial for maintaining cellular homeostasis. Proteins involved in mitochondrial dynamics are enriched in MAMs (25). MFN1 and MFN2 regulate OMM fusion, while Optic atrophy 1 (OPA1) modulates IMM fusion. The mitochondrial fission process is regulated by dynamin-related protein1 (Drp1), Fis1, mitochondrial fission factor (Mff), and mitochondrial dynamic proteins of 49 and 51 kDa (MiD49/51). A recent study emphasized that MAMs are the origin sites of mitochondrial fission (26).

Many autophagic proteins are located in MAMs and autophagosomal membranes may originate from MAMs (27). Under starvation, the pre-autophagosome marker autophagy-related 14-like (ATG14L) promotes relocalization of the autophagy induction factor mammalian target of rapamycin complex 2 (mTORC2) to MAMs and initiates autophagosome formation (28). It is reported that decreased MAMs tethering complexes result in abnormal hippocampal autophagy in rats (29). Moreover, MAMs modulate apoptosis via Ca²⁺ regulation. Evidence showed that Ca²⁺ overload can induce mitochondrial permeability transition pore (mPTP) opening and apoptosis (30). BAP31-Fis1 tethering complex recruits procaspase-8, which promotes the release of Ca²⁺ stores in ER and causes apoptosis (16).

MI is an acute syndrome of CVD with high death rate (31). Myocardial ischemia/reperfusion (I/R) injury, a pathophysiological status after the ischemic myocardium returns to normal perfusion, is commonly deemed as a serious risk factor for coronary artery disease (32). MI is characterized by cardiac injury, myocardial cell death and abnormal cardiac function, which will lead to heart failure and death (33). Myocardial I/R injury is mainly associated with oxidative stress, ER stress and mitochondrial dysfunction (34). Among these, calcium overload is the leading cause of disordered oxidative phosphorylation and contributes to mitochondrial dysfunction (25).

Several components of ER/SR-mitochondria tethering complex participate in Ca²⁺ homeostasis and mediate mPTP opening and I/R damage. Cyclophilin D (CypD), encoded by Ppif, is located in the mitochondrial matrix and acts as a crucial regulator for mPTP opening and necrosis. CypD interacts synergistically with the VDAC1-Grp75-IP3R1 complex and enhances ER Ca²⁺ efflux into mitochondria (Table 1) (35). CypD overexpression induced mPTP opening without stimulating cell death, whereas CypD inactivation significantly reduced myocardial infarct size during I/R and ameliorated myocardial injury via impeding Ca²⁺ overload (36, 37). Notably, the protective effect of preconditioning was absent in Ppif^–/– mice, which were more susceptible to heart failure, indicating that CypD plays a dual function in I/R (38, 39).

Glycogen synthase kinase-3 beta (GSK-3β), a new Ca²⁺ regulator located in the SR/ER, specifically interacts with the IP3R Ca²⁺-channeling complex and regulates Ca²⁺ transfer in cardiomyocytes (40). GSK-3β inhibition diminished Ca²⁺ overload and reduced myocardial apoptosis resulted from I/R, thereby providing cardioprotection (40). Moreover, GSK-3β inhibitors attenuated infarct size in mice and rabbits, indicating drug administration was a feasible method (41). VAPB-PTPIP51 is a widely accepted tethering complex in MAMs. PTPIP51 is markedly increased in mice I/R hearts. PTPIP51 overexpression mediated excessive mitochondrial Ca²⁺ uptake, but reversed by MCU inhibition, which protected cardiomyocytes against PTPIP51-mediated apoptosis (42). Cardiac knockdown of PTPIP51 strikingly alleviates cardiac injury after myocardial I/R, indicating that PTPIP51 might be a potential target for ischemic heart disease. Furthermore, downregulation of VAPB or PTPIP51 promotes autophagy by reducing mitochondrial Ca²⁺ levels (43).

Sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA), the main pump for Ca²⁺ uptake in the ER, regulates calcium homeostasis by interacting with calnexin. SERCA ameliorated reperfusion-induced myocardial injury by employing gene delivery strategies targeting SERCA (44, 45). Mitochondrial dynamics is crucial in myocardial I/R by regulating mPTP opening (46). MFN1-MFN2 complex is implicated in modulating mitochondrial fission and maintaining ER-mitochondria microdomain (13). Acute deletion of Mfn1 and Mfn2 prevented myocardial I/R injury and reduced infarct size (47). Mfn2 knockout hearts exhibited resistance to I/R injury, however long-term Mfn2 deletion contributed to cardiac dysfunction (47, 48).

Cardiomyopathy, myocardial disorder with abnormal cardiac muscle, can be either acquired or inherited. Hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) are the most common cardiomyopathies. Among these, DCM is the most common cause of heart failure, which is characterized by structural thinning and dilation of heart chambers with a progressively defected cardiac function (49).

It has been reported that cardiac deficiency of Mfn1 and Mfn2 exhibits progressive DCM and heart failure in succession (50). Structural and functional abnormal mitochondria were observed in Mfn2-deficient mice, which induced respiratory dysfunction and caused DCM (51). Moreover, mice with Drp1 mutation demonstrated cardiomyopathy with punctuate calcification in the heart (52). Cardiac depletion of Drp1 induced DCM after birth and rapid death in mice (53, 54). Increased mitochondrial connection, accumulated ubiquitinated proteins as well as reduced respiration was observed in Drp1 knockout cardiomyocytes. These studies indicate that Drp1 is crucial in regulating mitochondrial quality and myocardial survival.

Adult cardiac deletion of a multifunctional regulator GSK-3 contributed to severe DCM due to cell cycle dysregulation, indicating that GSK-3 is involved in maintaining cardiac homeostasis (49, 55). VDAC, the most abundant mitochondrial outer membrane protein, contains three subtypes-VDAC 1, 2, and 3 in mammalian cells (56). It has been reported that VDAC1 is upregulated in the left ventricle of HCM patients (57). VDAC1 inhibition significantly attenuated mitochondrial Ca²⁺ overload and protected cells from hypoxia-reoxygenation (H/O) (58). Cardiac VDAC2 knockout mice showed decreased ejection fraction and increased brain natriuretic peptide (BNP) level and cardiac fibrosis, which was consistent with DCM features (56).

Heart failure is a complicated pathophysiological syndrome of cardiac pumping failure (5). Conditions such as ischemia, pressure or volume overload, cardiac hypertrophy, cardiomyopathy, will eventually lead to heart failure (59). It is well recognized that mitochondrial function and Ca²⁺ homeostasis are significant in cardiac remodeling and heart failure (60–62). The cardiac rhythmicity and contraction require energy, which is driven by mitochondrial oxidative phosphorylation (63). Besides, SR, a membrane system with a high density of Ca²⁺-ATPases, maintains optimal calcium levels for myocardial contraction. Disruption of Ca²⁺ homeostasis can trigger ER stress and energy metabolism defects, affecting the development of heart failure (64, 65).

Mitochondrial Ca²⁺ dysregulation is involved in cardiac remodeling and heart failure. IP3R is a ligand-gated calcium channel located in the ER/SR, with isoform IP3R-2 predominant in the heart. IP3R is associated with cardiac remodeling in response to various stress that cause hypertrophy (66). The expression and activity of IP3R is enhanced under pathological conditions such as cardiac hypertrophy and heart failure. Moreover, inhibition of IP3R1 alleviates myocardial injury and heart failure (67). FUNDC1, a highly conserved OMM protein, maintains MAM formation by interacting with IP3R2 and regulates mitophagy (68). Compared with healthy group, the expression level of FUNDC1 and the number of SR-mitochondria contacts are dramatically reduced in heart failure patients. The decreased FUNDC1 level in MAMs contributed to impaired SR Ca²⁺ transportation to mitochondria through inhibition of IP3R2 ubiquitin-dependent degradation, resulting in perturbation of the CREB/Fis1 pathway and eventually compromising cardiac function. Besides, FUNDC1 knockout mice showed diastolic and systolic dysfunction (69). In contrast, FUNDC1 overexpression elevates both cytosolic and mitochondrial Ca²⁺ levels in cardiomyocytes, and lowers SR Ca²⁺ levels. Sig-1R disassociated from the binding immunoglobulin protein (BiP) and prolonged mitochondria Ca²⁺ uptake via IP3R under ER stress (70). Sig-1R regulates Ca²⁺ transfer into mitochondria to promote ATP production (71). Sig-1R knockout mice displayed mitochondrial dysfunction and cardiac remodeling, causing cardiac dysfunction. Besides, Fluvoxamine, possessing high Sig-1R affinity, alleviated heart failure in both mice and rat models subjected to TAC (72).

Mitochondrial dynamics is participated in cardiac hypertrophy and heart failure progression. Cardiac depletion of Mfn1/2 in mice showed impaired heart function with increased left ventricular end-diastolic volume and decreased fractional shortening (48, 73). Accumulating studies have demonstrated that MFN2 was downregulated in heart failure models induced by spontaneously hypertensive rats (SHR) or TAC (74). Consistently, MFN2 was decreased in hypertrophic cardiomyocytes induced by Angiotensin II (Ang II), accompanied by the alterations of mitochondria morphology (75). A study showed that MFN1/MFN2 double knockout mice died at the embryonic stage due to heart failure (50). Besides, cardiac deletion of MFN2 mice developed cardiac hypertrophy and moderate diastolic dysfunction (76). Conversely, MFN2 overexpression alleviated Ang-II induced cardiac hypertrophy (77). Intriguingly, sex hormones (estrogen and testosterone) can increase cardiac expression of Mfn1 and Mfn2, suggesting that further study is needed on the regulatory effect of the sex hormones, and their cardioprotective effects (78). Drp1, mitochondrial fission regulator, is upregulated in damaged cardiac tissues induced by doxorubicin. A study showed that Drp1 inhibitor will be a promising pharmacological agent, which inhibits the excessive mitochondrial fission mediated by doxorubicin and ameliorates its cardiotoxicity (79). Drp1 deficiency in adult mouse hearts showed the pathophysiological consequences of progressive ventricular enlargement and functional decompensation, resulting in heart failure (80). Drp1-dependent mitochondrial autophagy exerts a protective role in mitochondrial dysfunction and heart failure resulted from pressure overload (81). At present, microRNAs (miR) are being studied as therapeutic targets for CVDs. It is reported that miR 499 protects heart against MI by inhibiting mitochondrial fission mediated by Drp1 (82). OPA1, mediates IMM fusion and acts as a crucial regulator of morphological change in cardiac physiology. Altered OPA1 function was proposed to lead to the pathogenesis of heart failure (83). Studies have shown that protein levels of OPA1 were reduced in both rat and human heart failure models accompanied with mitochondrial fragmentation. Reduced OPA1 promoted apoptosis and mitochondria fragmentation, which may contribute to heart failure progression with progressive loss of cardiac myocytes (84). Together, mitochondrial dynamics are essential to maintain cardiac structure and function, which may act as a potential strategy to prevent myocardial hypertrophy and heart failure.