The primary signal conveyed by MAMs is Ca²⁺. As a crucial second messenger, Ca²⁺ plays a vital role in cellular functions and the maintenance of cell viability. Intracellular Ca²⁺ is predominantly sequestered within the ER, where its concentration significantly exceeds that found in the cytosol and mitochondria (Jiang et al., 2023). As a significant site for Ca²⁺ transportation, MAMs is essential for the translocation of Ca²⁺ from the ER to the mitochondria (Prinz et al., 2020).

It is widely accepted that Ca²⁺ are translocated through a protein complex that includes inositol 1,4,5-trisphosphate receptors (IP₃Rs) located on the ER, as well as the glucose-regulated protein of 75 kDa (GRP75) and voltage-dependent anion channel 1 (VDAC1) situated on the mitochondria (Szabadkai et al., 2006; Giorgi et al., 2018). Subsequently, Ca²⁺ is translocated into the mitochondrial matrix from OMM via the mitochondrial calcium uniporter (MCU). Consequently, the IP₃R-GRP75-VDAC1-MCU axis constitutes a Ca²⁺ transfer pathway between the ER and mitochondria, which is regulated by MAMs. In a recent study, DJ-1 was also shown to be present in MAMs and could interact with IP₃R-GRP75-VDAC proteins complex and become an essential component (Basso et al., 2020) (Figure 2a). Secondly, mitofusin 2 (MFN2), which is situated on both the ER surface and the OMM, is classified as a resident protein in MAMs. It serves dual functions as a critical mediator of mitochondrial fusion and as a facilitator of the interaction between IP₃R and VDAC1 (De Brito and Scorrano, 2008; Filadi et al., 2015). Furthermore, Sigma-1 receptor (Sig-1R) is recognized as a marker of the MAMs, which facilitates the extension of Ca²⁺ signaling from the ER to the mitochondria. After the activation of IP₃Rs, Sig-1R associates with IP₃R3 and dissociates from BiP. This interaction serves to stabilize IP₃R3 at the MAMs, thereby facilitating enhanced Ca²⁺ fluxes to the mitochondria mediated by IP₃R3. Moreover, elevated levels of mitochondrial Ca²⁺ enhanced the production of NADH via enzymes involved in the tricarboxylic acid (TCA) cycle, thereby facilitating an increase in ATP synthesis and the generation of reactive oxygen species (ROS) (Rizzuto et al., 2012). Therefore, the Ca²⁺ transportation, regulated by MAMs, is both associated with mitochondrial respiration and energy metabolism.

However, Ca²⁺ functions as a double-edged sword, possessing the capacity to enhance metabolic processes while simultaneously triggering cellular apoptosis. Sustained accumulation of Ca²⁺ within mitochondria has detrimental effects on cellular viability. An overabundance of mitochondrial Ca²⁺ uptake initiates the opening of the mitochondrial permeability transition pore (mPTP), which subsequently leads to the release of cytochrome c. This sequence of events results in mitochondrial dysfunction and ultimately culminates in cellular apoptosis (Giorgi et al., 2018; Chen et al., 2023; Jiang et al., 2023).

Owing to the significance of Ca²⁺ in cellular process, the level of Ca²⁺ in cells is strictly controlled. And further, altered Ca²⁺ homeostasis may lead to different pathological conditions, depending on what kind of cell involved (Liu Y. et al., 2020). Collectively, MAMs-mediated Ca²⁺ homeostasis has robust connection with various cellular processes, which is a central hub in the pathogenesis of cancers, neurodegenerative disorders, and cardiovascular diseases and so on.

It has been reported that, alongside vesicular transport, lipid transport between organelles may transpire via tight junctions or through protein-mediated interactions between membranes (Vance, 2020). The proteins that are characteristic of MAMs promote the lipid synthesis and transport in the MAMs via the later (Anastasia et al., 2021).

The synthesis of lipids in the MAMs necessitates the participation of associated enzymes situated on the membranes of both organelles. Furthermore, various intermediates involved in this process are exchanged repeatedly in the MAMs (Jiang et al., 2023). For example, in the ER, phosphatidic acid (PA) is synthesized through the action of the enzyme phosphatidylserine synthases 1 and 2 (PSS1 and PSS2), which convert it into phosphatidylserine (PS). Subsequently, PS is transported to the mitochondria, where it undergoes conversion into phosphatidylethanolamine (PE) via the enzymatic activity of phosphatidylserine decarboxylase (PISD) located on the inner mitochondrial membrane (IMM). Ultimately, PE is transported back to the ER, where it undergoes conversion to phosphatidylcholine (PC) through the action of the enzyme PEMT2. Subsequently, the PC is returned from the ER to the mitochondria (Janikiewicz et al., 2018) (Figure 2b ).

Additionally, enzymes involved in cholesterol biosynthesis are localized within MAMs. the MAMs fraction exhibits an enrichment of cholesterol and ceramides, particularly in hepatocytes (Csordás et al., 2018). ACAT1, present in MAMs, plays a crucial role in the esterification of intracellular free cholesterol into cholesterol esters. This process is essential for maintaining the balance between bound and free cholesterol in resting states (Shi et al., 2021). Research indicates that modifications in cholesterol levels can influence the interaction between the ER and mitochondria. Specifically, a reduction in cellular cholesterol levels leads to an enhanced connection between the mitochondria and the ER (Fujimoto et al., 2012). Consequently, MAMs’ lipid and proteins play an important role in determining mitochondrial lipid composition and facilitating lipid transport among organelles. And the dysregulated lipid synthesis and transport will influence lipid homeostasis and MAMs’ functions, which exert important impact on various diseases.

Mitochondrial dynamics encompasses the processes of fusion, fission, and transport, which are essential for the proper functioning of signal transduction and metabolic pathways (Chen et al., 2023).

Mitochondria are semi-autonomous organelles harboring their own DNA (mtDNA) and require continuous repair and renewal of components to sustain functionality. The regulation of mitochondrial dynamics is crucial for the optimal functioning of mitochondria and the determination of cellular outcomes (Tilokani et al., 2018). Mitochondrial dynamics facilitate the removal of damaged components from mitochondria, and dysfunctional mitochondria can be completely eliminated through the process of mitophagy, thereby mitigating the risk of additional cellular damage (Youle and van der Bliek, 2012). In a certain sense, mitophagy can be considered a component of mitochondrial dynamics. In detail, fission is essential for mitochondrial quality control as it enables the elimination of damaged or dysfunctional mitochondria. Additionally, it contributes to the induction of apoptosis in response to substantial cellular stress. In contrast, mitochondrial fusion promotes the mixing and exchange of intramitochondrial components among mitochondria, thereby playing a crucial role in the preservation of mitochondrial functionality (Youle and van der Bliek, 2012).

Research has demonstrated that MAMs play an important role in regulating mitochondrial morphology, including the size, shape, and length, by maintaining the balance between mitochondrial fission and fusion processes (Scorrano, 2013) (Figure 2c). Mitochondrial fusion requires the coordinated merging of both the OMM and IMM, a process that is predominantly regulated by three GTPases.: mitofusin 1 (MFN1), MFN2, and optic atrophy type 1 (OPA1) (Vezza et al., 2022). The fusion of the OMM is facilitated by the proteins MFN1 and MFN2 (Chen et al., 2003; Detmer and Chan, 2007), while the fusion of the inner mitochondrial membrane is governed by OPA1 (Al Ojaimi et al., 2022). MFN1 and MFN2 are located on the OMM and are instrumental in the fusion process of mitochondria, resulting in the establishment of interconnected mitochondrial networks (Filadi et al., 2018). It is noteworthy that MFN2 is the first protein identified as directly facilitating the tethering of the ER to mitochondria in mammals (De Brito and Scorrano, 2008), serving as a crucial regulator of ER-mitochondrial contact site tethering (Tilokani et al., 2018). In addition to its presence in the OMM, MFN2 is also localized at MAMs (Filadi et al., 2018; Hu et al., 2021). Within MAMs, MFN2 facilitates the interaction between the ER and mitochondria by forming physical associations with either MFN1 or MFN2 situated on the OMM (Naon et al., 2016). Research has demonstrated that the deletion of MFN2 in murine fibroblasts results in the disruption of ER-mitochondrial contact sites, consequently leading to an increased spatial separation in the MAMs (De Brito and Scorrano, 2008).

The ER exhibits significant interaction with mitochondria, particularly in its role in facilitating mitochondrial fission through the encapsulation of mitochondrial segments. This observation implies that MAMs play a crucial role in the process of mitochondrial fission (Friedman et al., 2011; Jiang et al., 2023). Mitochondrial fission is initiated at locations where ER tubules exert constrictive forces on the mitochondria (Friedman et al., 2011). Research has demonstrated that proteins located on MAMs play an important role in the regulation of mitochondrial division. The primary proteins include fission protein 1 (Fis1) (Losón et al., 2013), MiD49 and MiD51 (Palmer et al., 2013), mitochondrial fission factor (MFF) (Otera et al., 2010), dynamin-related protein 1 (DRP1) (Smirnova et al., 2001). These proteins are recruited to the contact sites in the MAMs, where they facilitate the constriction of mitochondrial membranes, thereby promoting the division process (Lee et al., 2016).

Typically, to enhance mitochondrial function, mitochondria regulate a homeostatic equilibrium between the processes of fusion and fission (Wu et al., 2023). In contrast, dysregulated mitochondrial dynamics are linked to various diseases that exhibit deficiencies in mitochondrial function and abnormal cellular responses.

Autophagy is a meticulously regulated biological process that facilitates the degradation and recycling of misfolded proteins and damaged organelles, thereby contributing to the maintenance of cellular homeostasis (Mizushima et al., 2011). As a core mechanism for maintaining cellular homeostasis, autophagy encapsulates abnormal components within a double-membrane structure known as autophagosomes and achieves nutrients recycling through lysosomal degradation (Mizushima et al., 2011). This process involves multiple physiological functions, including protein quality control and stress response (Ichimiya et al., 2020).

Empirical studies have shown that the integrity of MAMs is crucial for the process of autophagosome formation (Li Y. E. et al., 2022). The interaction between the ER protein VAPB and the mitochondrial protein PTPIP51 regulates Ca²⁺ transport between the ER and mitochondria. Moreover, when any protein is absent, the interaction between IP₃R and VDAC decreases, leading to impaired mitochondrial Ca²⁺ uptake, which in turn activates autophagy (Gomez-Suaga et al., 2017). Additionally, research has indicated that vesicle-associated membrane protein-associated proteins (VAPs), including VAPA and VAPB, enhance autophagy by modulating the interactions between the ER and other organelles or by promoting the formation of MAMs, as well as by directly engaging with autophagy-related (ATG) proteins (Zhao et al., 2018). This suggests that different tethering proteins may differentially regulate the formation of autophagosomes and the specific structures targeted for autophagic degradation.

As a typical representative of selective autophagy, mitophagy maintains cellular homeostasis by eliminating dysfunctional or excess mitochondria, preventing abnormal metabolism and apoptosis (Poole and Macleod, 2021). Its core mechanism involves two MAMs-related pathways. Firstly, under hypoxic conditions, the OMM protein FUN14 domain containing 1 (FUNDC1) localizes to MAMs by interacting with the ER membrane protein calnexin (Wu et al., 2016a). As mitophagy is initiated, the dissociation of FUNDC1 from calnexin allows it to recruit DRP1 to MAMs, driving mitochondrial fission (Wu et al., 2016b). Subsequently, when autophagy is activated, the PINK1/Parkin complex accumulates at MAMs, promoting the formation of autophagosomes by enhancing ER-mitochondria interactions. These two pathways together reveal the important role of MAMs as a hub for mitochondrial quality control, with their dynamic restructuring directly determining the process and efficiency of mitophagy.

When the protein folding demand of the ER exceeds its capacity, unfolded or misfolded proteins accumulate in the lumen, triggering ER stress and activating the unfolded protein response (UPR) (Fan and Simmen, 2019; Qin et al., 2023). The initiation of the UPR is characterized by the activation of three ER transmembrane proteins: activating transcription factor 6 (ATF6), protein kinase R-like endoplasmic reticulum kinase (PERK), and inositol requiring enzyme 1 (IRE1) and inter-organelle communication facilitated by MAMs (Figure 2d).

Initially, ER stress can induce a perinuclear redistribution of both ER and mitochondrial networks, resulting in an increase in ER-mitochondrial contacts (Fan and Simmen, 2019). MAMs-enriched molecular chaperones, including GRP78 and calnexin, are directly involved in UPR signaling (Gutiérrez and Simmen, 2018), while Ca²⁺ signaling regulatory proteins such as GRP75 and MFN2 participate in stress adaptation by maintaining Ca²⁺ homeostasis (Yuan et al., 2020) and regulating the PERK pathway (Muñoz et al., 2013). Notably, all core UPR sensors are localized in MAMs, creating a unique regulatory microenvironment. The presence of IRE1 promotes ATP production by regulating IP₃R-mediated mitochondrial Ca²⁺ influx (Carreras-Sureda et al., 2019). Furthermore, the ubiquitination of IRE1α inhibits apoptosis induced by ER stress (Chern et al., 2019). The PERK associates with MFN2 to augment the connection between MAMs and to mediate mitochondrial ROS-induced apoptosis (Muñoz et al., 2013). Upon depletion of MFN2, the activation of PERK initiates ER stress signaling via the PERK-eIF2α-ATF4-CHOP pathway (Liu X. et al., 2021). Additionally, ATF6 extends cellular lifespan by modulating Ca²⁺ transfer and signaling via indirectly inhibiting the interaction between ERS-induced VAPB and PTPIP51(De Vos et al., 2012; Zhang et al., 2020). Moreover, VAPB overexpression inhibits ATF6 transcription, blocking the UPR response (Gkogkas et al., 2008).

MAMs maintains cellular communication between the mitochondria and ER by the hydrogen peroxide (H₂O₂) nanodomains, producing the ROS, then influencing Ca²⁺ signaling and mitochondrial functionality (Booth et al., 2016). Endoplasmic reticulum oxidoreductin 1-α (ERO1) and p66Shc are two key proteins which come from MAMs that can directly produce ROS. ERO1 elevated expression enhances ROS production by facilitating Ca²⁺ release from IP₃Rs (Anelli et al., 2012). Conversely, p66Shc associated with ROS generation, undergoes phosphorylation at Ser36 under oxidative stress, leading to isomerization and subsequent dephosphorylation, ultimately resulting in its translocation to MAMs to mediate ROS production (Pinton et al., 2007).

Moderate activation of ER stress and oxidative stress can enhance cellular adaptability, but sustained stress leads to a vicious cycle that promotes disease progression. Targeting the intersection of the two may provide new therapeutic strategies for cancer, neurodegenerative diseases, and diabetes mellitus.

Inflammation functions as an essential immune defense mechanism in living organisms, reacting to abnormal pathological stimuli including ischemia, heightened immune responses. Meantime, it is also acknowledged as a major pathological mechanism contributing to a range of conditions, including cardiovascular diseases, cancer, and metabolic disorders. Research has confirmed that the MAMs participates in the inflammatory response by regulating the nucleotide-binding oligomerization domain-like receptor family pyrin domain-containing-3 (NLRP3) inflammasome (Kelley et al., 2019).

MAMs are not only implicated in the initiation of inflammatory processes but also function as a structural platform for the assembly of the NLRP3 inflammasome. In a resting state, NLRP3 is distributed in the ER membrane and cytoplasm; upon activation, it translocates to MAMs with the adaptor protein apoptotic speck protein containing a CARD (ASC), completing inflammasome assembly by sensing the ROS produced from mitochondrial damage, thereby activating interleukin 1β (IL-1β) and IL-18 to induce inflammation (Zhou et al., 2011). The unique localization of MAMs allows for efficient capture of damage signals originating from mitochondria. Furthermore, excessive mitophagy, dysregulated Ca²⁺ transport, and ER stress may result in mitochondrial impairment, the generation of ROS, exposure of mtDNA, and subsequently trigger the activation of the NLRP3 inflammasome (Missiroli et al., 2018).

In-depth analysis of the interaction mechanism between NLRP3 and MAMs will provide new targets for the treatment of inflammatory diseases.

The innate immune system is an evolutionarily conserved system that functions as the first line of defense against invading microbial pathogens and other potential threats to the host. The innate immune system is modulated by a variety of germline-encoded receptors known as pattern recognition receptors (PRRs) (Paludan and Bowie, 2013). PRRs are capable of identifying pathogen-associated molecular patterns (PAMPs) that are characteristic of various microorganisms, including viruses, bacteria, parasites, and fungi. Furthermore, PRRs are involved in the detection of endogenous danger signals through the recognition of damage-associated molecular patterns (DAMPs). Certain studies indicate that MAMs may play a significant role in the immune response to viral infections.

In the antiviral innate immune defense system, mitochondrial antiviral-signaling protein (MAVS) plays a central role. MAVS is a protein located on the outer membrane of mitochondria, and also localized at the MAMs (Seth et al., 2005). As a central hub for signal transduction initiated by retinoic acid inducible gene (RIG-I)-like receptors, MAVS interacts with RIG-I to active NF-κB and interferon regulatory factor 3 (IRF3) pathways (Rajput et al., 2011). When a virus invades a cell, intracellular pattern recognition receptors (such as RIG-I-like receptors) can quickly recognize the virus’s RNA. The RIG-I-like receptor family includes RIG-I, melanoma differentiation-associated gene 5 (MDA5), and RIG-I-like receptor LGP2 (Yoneyama et al., 2005). In details, upon activation, MAVS recruits several TRAF family members (Bender et al., 2015). This recruitment leads to the phosphorylation and nuclear translocation of IRF3, as well as activation of the transcription factor NF-κB.

Interesting, the stimulator of interferon genes (STING) has been recognized as a positive modulator of RIG-I-mediated interferon-β (IFN-β) signaling pathways (Rajput et al., 2011). STING interacts with MAVS and RIG-I to form a complex to promote the phosphorylation of IRF3 and the production of IFN (Xue et al., 2022).

Apoptosis represents one of form of programmed cell death (PCD) that is meticulously regulated to oversee cellular growth, development, and renewal. Mitochondria and the ER transmit apoptosis signals through MAMs.

In details, Fis1 forms a complex with the ER membrane protein B cell receptor-associated protein 31 (Bap31), which leads to the cleavage and generation of the pro-apoptotic protein p20Bap31 and activates caspase-8, initiating apoptosis mediated by MAMs (Liu et al., 2022).

The transport of Ca²⁺ from the ER to the mitochondria is a critical component in the process of apoptosis. B-cell lymphoma-extra-large (Bcl-xl), which is an anti-apoptotic protein belonging to the B-cell lymphoma 2 (Bcl-2) family, relocates to the MAMs under apoptotic stimuli, enhancing its interaction with IP₃R3 to reduce cytosolic Ca²⁺ release and inhibit Ca²⁺ overload (Bonneau et al., 2013). Moreover, the anti-apoptotic function of Bcl-2 is dependent on the integrity of MAMs and its interaction with TOM20 (Lalier et al., 2021). In terms of the pro-apoptotic factor, Bax, which is a pro-apoptotic member of the Bcl-2 family, translocates to the mitochondrial membrane via MAMs, promoting the release of cytochrome c by accumulating Ca²⁺ and opening the mPTP (Lee et al., 2014).

MAMs, as a core regulatory hub of apoptosis, precisely regulate cell survival and death through the integration of multiple mechanisms, including Ca²⁺ signaling, protein complex interactions. Their functional abnormalities are closely related to various pathological processes such as neurodegenerative diseases, cardiovascular diseases, and diabetes mellitus, making them potential therapeutic targets.

Diabetes mellitus (DM) is a disease caused by abnormal insulin secretion and action, resulting to the disturbances of glucose homeostasis (Eizirik et al., 2020). The evidence indicating that dysregulated Ca²⁺ signaling, ROS production, ER stress, and modifications in autophagy are associated with the insulin resistance and the function of β cells (Rieusset, 2015). Considering important functions played by MAMs, the dysfunction of MAMs has crucial relationship with DM.

MAMs play a central role in maintaining glucose homeostasis by regulating the localization and activity of key proteins of the insulin signaling pathway, including mTORC2, PKB, phosphatase and tensin homolog (PTEN), and protein phosphatase 2A (PP2A) (Figure 3g). Insulin stimulation promotes mTORC2 enrichment at MAMs, then activating PKB kinase (Betz et al., 2013). Activated PKB subsequently phosphorylates IP₃R, thereby inhibiting the IP₃R-mediated release of Ca²⁺ (Giorgi et al., 2010). Furthermore, PKB phosphorylates the resident proteins PACS2 and HK2 to preserve the structural integrity, a process that is essential for insulin signaling (Miyamoto et al., 2008; Aslan et al., 2009). Moreover, the phosphorylation effects mediated by PKB is counterbalanced by PTEN/PP2A-mediated dephosphorylation, establishing dynamic equilibrium. This process aids in the restoration of Ca²⁺ translocation, ultimately enhancing the cellular sensitivity to apoptotic stimuli (Bononi et al., 2013). The enhancement of MAMs integrity promotes insulin action, whereas disruption of the MAMs in hepatocytes results in impaired insulin signaling. Recently, research have shown that silencing the critical proteins PKB or mTORC2 which is disrupting the insulin signaling inevitably impairs the mitochondrial functions and MAMs integrity (Betz et al., 2013). The description indicates a bidirectional relationship between the integrity of MAMs and insulin signaling. It is essential to emphasize that, while there is consensus regarding the involvement of MAMs in insulin signaling, the specific mechanisms underlying this interaction remain ambiguous.

Considering that the MAMs contains various proteins integral to insulin signaling and that its structural integrity is essential for the proper function of insulin, it is not unexpected that abnormal structure and functions in MAMs are observed in the context of insulin resistance and DM (Tubbs et al., 2018). The integrity of MAMs is compromised in the livers of mice that are obese and diabetic. The liver-specific deletion of MFN2 or the inhibition of mTORC2 compromises the structural integrity of MAMs, leading to various metabolic dysregulations, such as insulin resistance and impaired glucose tolerance (Sebastián et al., 2012). In a comparable manner, the genetic deletion or pharmacological inhibition of Cyclophilin D (CypD) disrupts the integrity of the MAMs and leads to the development of hepatic insulin resistance in murine models. Conversely, the overexpression of CypD in primary hepatocytes derived from mice enhances the contacts between the mitochondria and the ER, thereby facilitating improved insulin signaling (Rieusset et al., 2016). Furthermore, the disruption of MAMs represents a significant subcellular modification that accelerates the development of muscle insulin resistance in both murine and human models.

Except for the insulin signaling and insulin resistant, MAMs also exert its effects on the modulation of the secretory mass and function of β cell. Consequently, the impaired MAMs in pancreatic β cells are regarded as a significant factor in the pathogenesis of DM. Recent findings indicated that diabetic models exhibit reduced ER-mitochondria contacts in β-cells, accompanied by ER stress and insulin secretion reduction (Thivolet et al., 2017). Moreover, modifications to MAMs proteins have implications for both the functions and mass of β cells. Mouse with depletion of IP₃R1 exhibit disrupted ER Ca²⁺ homeostasis, increased ER stress, diminished glucose tolerance, and a reduction in β cell mass (Takeda et al., 2016). Conversely, the depletion of VDAC1 in pancreatic β cells results in an increase in ATP production, which subsequently causes depolarization of the plasma membrane, elevated cytosolic Ca²⁺ levels, and enhanced insulin secretion. This mechanism serves to safeguard β cells from elevated glucose concentrations and preserves the cell’s reductive capacity (Zhang et al., 2019).

While the study underscores the significance of MAMs in the context of DM, the regulatory mechanisms governing MAMs remain inadequately understood and necessitate additional research. The growing acknowledgment of the significance of ER-mitochondria interactions in the impairment of pancreatic islets and peripheral tissues suggests that maintaining the optimal functionality of MAMs may offer a new therapeutic strategy for diabetes mellitus.

Recent studies have highlighted the central regulatory role of MAMs in cardiovascular pathologies. In myocardial ischemia-reperfusion injury (IRI), heart failure (HF), and diabetic cardiomyopathy (DCM), MAMs contribute to disease progression through shared mechanisms including Ca²⁺ dyshomeostasis, mitochondrial dynamics abnormalities, and energy metabolism disorders.

MAMs plays a pivotal role in myocardial damage by Ca²⁺ transport (Figure 3h). During IRI, ER-mitochondrial Ca²⁺ overload which mediated by the IP₃Rs-GRP75-VDAC protein complex activates the mPTP via CypD-dependent pathways, triggering cardiomyocyte apoptosis (Elrod and Molkentin, 2013). Additionally, emerging evidence suggests that CypD has the capacity to interact with IP₃Rs-GRP75-VDAC during the transfer of Ca²⁺ (Paillard et al., 2013). Research has demonstrated that pharmacological inhibition of CypD or GSK-3β effectively mitigates Ca²⁺ overload and reduced both cell death and the extent of infarct area in hearts subjected to reperfusion (Wang K. et al., 2015; Gong et al., 2021). Furthermore, the downregulation of MFN2 expression may alleviate mitochondrial Ca²⁺ overload and cellular damage by disrupting the interaction between CypD and the IP₃R1-VDAC1 complex. This highlights the critical importance of MAMs in the regulation of the mPTP and Ca²⁺ channels, positioning them as potential therapeutic targets for the management of IRI. In HF, study has shown that there is an observed increase in the distance between the ER and mitochondria within myocardial cells, accompanied by a reduction in mitochondrial Ca²⁺ uptake (Gutiérrez et al., 2014). Hence, the disturbances in Ca²⁺ homeostasis may represent a critical characteristic of HF(Chaanine, 2021). Protein complexes including FUNDC1, IP₃R, MFN2, and Sig-1R have been implicated in the pathogenesis of HF. Suppression of the FUNDC1-CREB-Fis1 axis causes a marked decrease in Ca²⁺ concentrations and abnormal mitochondrial fission, thereby exacerbating cardiac dysfunction and contributing to the progression of HF (Wu et al., 2017). The inhibition of Sig-1R, another critical molecule has been shown to promote autophagy in cardiomyocytes when subjected to oxidative stress (Wang et al., 2018), while Sig-1R agonists enhance Ca²⁺ signaling to improve ATP synthesis (Ortíz-Rentería et al., 2018). Additionally, Sig-1R associates with RyR to form another complex, which diminishes Ca²⁺ leakage, mitigates myocardial hypertrophy, and enhances ATP synthesis (Tagashira et al., 2013). In DCM, under conditions of elevated glucose levels, the inactivation of AMPK triggers the activation of FUNDC1, promoting the formation of MAMs and abnormal mitochondrial autophagy. Concurrently, FUNDC1 interacts with IP₃R2, obstructing its ubiquitination, enhancing Ca²⁺ release into mitochondria, thereby exacerbating mitochondrial dysfunction and adversely affecting myocardial function (Wu S. et al., 2019). The genetic deletion of FUNDC1 markedly inhibits the formation of MAMs induced by high glucose levels, consequently inhibiting regulatory pathways and reversing the progression of DCM (Wu S. et al., 2019). Consequently, the deregulation of FUNDC1-associated MAMs may offer a novel therapeutic approach for the management of DCM.

Mitochondrial dynamics which regulate the CVDs represents another core MAMs mechanism. MFN1/2-mediated fusion and DRP1-controlled fission processes are crucial for myocardial protection. Prior studies have indicated that excessive mitochondrial fission within myocardial cells is recognized as a significant contributor to the onset of IRI (Sharp et al., 2014). MFN2 overexpression or mdivi-1 treatment preserves mitochondrial network integrity to reduce sensitivity to mPTP opening and the cellular mortality following IRI (Ong et al., 2010). Furthermore, the decrease in the expression levels of MFN1 and MFN2 has been shown to mitigate IRI in mouse myocardial tissue and to diminish the extent of infarction (Hall et al., 2016). In HF, study have illustrated that the reduction of MFN2 expression leads to mitochondrial fragmentation in both in vivo and in vitro models of heart failure (Liu J. et al., 2021; Xu et al., 2021). Furthermore, emerging evidence suggests that the upregulation of MFN2 not only mitigated the production of ROS and restored mitochondrial membrane potential, but also inhibited cardiomyocyte hypertrophy and the associated pro-hypertrophic phenotype (Sun et al., 2019).

These findings establish MAMs as a convergent pathological hub integrating Ca²⁺ signaling, mitochondrial dynamics, and energy metabolism across cardiovascular diseases. Targeting key MAMs components like MFN2, FUNDC1, and IP₃R provides a theoretical foundation for developing novel therapeutic strategies with cross-disease applicability.

Despite the diverse pathological phenotypes of various diseases, the dysfunction of MAMs is predominantly linked to four fundamental regulatory functions: the imbalance of Ca²⁺ homeostasis, abnormal mitochondrial dynamics, metabolic reprogramming, and the dysregulation of stress responses. Notably, Ca²⁺ transport, a critical function of MAMs, demonstrates an imbalance in Ca²⁺ homeostasis in both breast cancer cells and dopamine neurons. However, the downstream consequences of this imbalance lead to distinct pathological outcomes: an increase in Ca²⁺ flux contributes to apoptosis resistance in cancer cells, whereas a decrease in Ca²⁺ flux is associated with abnormal energy metabolism. This phenomenon of “shared pathways-differentiated effects” indicates that MAMs serve as essential regulatory hubs across various diseases, necessitating targeted interventions that consider the specific tissue microenvironment. Future investigations should aim to further clarify the spatiotemporal dynamics of MAMs protein complex to uncover its molecular plasticity in different disease contexts.

Various approaches can be employed to influence cellular functions by enhancing or attenuating Ca²⁺ transport through the modulation of MAMs proteins.

Currently, the predominant factor for targeting Ca²⁺ transport is Bcl-2. Bcl-2, as an important anti-apoptotic factor in the intracellular, could yield good results if it can be targeted to inhibit its functions. Research has demonstrated that the overexpression of Bcl-2 diminishes the growth inhibition and apoptotic effects induced by cisplatin in human ovarian cancer (OC) cells (Xu et al., 2017). The TAT-fused inositol 1,4,5-trisphosphate receptor-derived peptide (TAT-IDP_S), which specifically interacts with the BH4 domain of Bcl-2, has been shown to enhance cisplatin-induced Ca²⁺ flux from the ER into both the mitochondria and cytosol (Xie et al., 2017). The finding suggests that TAT-IDP_S has the potential to augment the cytotoxic effects of cisplatin on OC cells through the facilitation of ER Ca²⁺ release (Xie et al., 2017). And other therapeutic target is BIRD-2, which also targets the BH4 domain of Bcl-2, has defined that causing the Ca²⁺ overload and opening the mPTP, resulting the death of the diffuse large b-cell lymphoma (DLBCL) and chronic lymphocytic leukemia (CLL) cell (Kerkhofs et al., 2021). Another MAMs target for treating OC is GRP75. GRP75 deficiency perturbed MAM integrity, reduced ER-mitochondrial Ca²⁺ transfer, exacerbated CP-induced mitochondrial dysfunction, and triggered marked ROS elevation with enhanced apoptosis in OC cells (Li J. et al., 2022). An additional element of the Ca²⁺ transport complex, VDAC1, has the potential to function as both a therapeutic target and a biomarker for BC, given that the overexpression was identified in a prior investigation (Shoshan-Barmatz et al., 2017). So far, there are two therapeutic methods based on the VDAC1. Initially, one strategy involves the downregulation of overexpressed VDAC1 through the application of RNA interference techniques, such as shRNA and siRNA, which ultimately leads to the induction of apoptosis in cancer cells. Furthermore, considering that HK, Bcl-2, and Bcl-xl exert their anti-apoptotic effects by interacting with VDAC1, there exist VDAC1-derived peptides specifically engineered to disrupt these interactions, thereby facilitating the induction of apoptosis (Prezma et al., 2013; Shoshan-Barmatz et al., 2017). As for the basic element of the Ca²⁺ transport complex, Kuchay et al. proposed that inhibition of IP₃R3 degradation in PTEN-dysregulated cancers could be a novel treatment strategy. This study demonstrated that PTEN competes with FBXL2 for binding to IP₃R3, thereby influencing its degradation. The findings indicate that a non-degradable IP₃R3 mutant enhances the sensitivity of PTEN-low/absent tumors to Ca²⁺-dependent photodynamic therapy (Kuchay et al., 2017). Furthermore, panaxydol was shown to induce Ca²⁺ release via IP₃R, activating the JNK signaling pathway and triggering PERK-dependent ER stress (Kim et al., 2016; Martucciello et al., 2020). As for the neurodegenerative diseases, the high-concentration IP₃R inhibitor 2-APB upregulated PGC-1α and TFAM expression, ameliorating motor and mitochondrial impairments in Parkinson’s disease murine models (Piccinin et al., 2021). Similarly, Xestospongin C alleviated Aβ-induced Ca²⁺ overload in hippocampal neurons and improved cognitive function in APP/PS1 mice (Wang et al., 2019). Additionally, β-carotene reduced intracellular Ca²⁺ levels by targeting the IP₃R-GRP75-VDAC1-MCU signaling axis, thereby repairing mitochondrial oxidative damage (Meng et al., 2023). Moreover, inhibition of the neurokinin-1 receptor (NK-1R) has emerged as a promising therapeutic target for the development of novel pharmacological agents. This inhibition has been demonstrated to induce oxidative stress and elevate mitochondrial ROS levels through a rapid Ca²⁺ flux from the ER to the mitochondria. This process subsequently leads to the induction of apoptosis in myeloid leukemia cells, both in vitro and in vivo. In recent study, two NK-1R antagonists, aprepitant and SR140333 may work synergistically with conventional chemotherapies to treat human myeloid leukemia (Ge et al., 2019). The removal of HK2 from the MAMs through the application of a specific peptide induces an overload of Ca²⁺ within the mitochondria. This event subsequently activates calpain in a Ca²⁺-dependent manner, which results in mitochondrial depolarization and ultimately leading to cell death in chronic lymphocytic leukemia B cells, as well as in breast and colon cancer cells (Ciscato et al., 2020). Furthermore, subsequent research demonstrated that miR-218 suppresses tumor growth and angiogenesis through the downregulation of HK2 and LDHA expression in vivo (Wang et al., 2024). There are a lot of the Sig-1R agonists and antagonists have received positive responses in clinical trials. SKF10047 has been demonstrated to activate the Sig-1R, leading to a substantial increase in the protein expression of the components involved in the IP₃R-GRP75-VDAC transport system, as well as an enhancement of Ca²⁺ transport associated with MAMs (Ooi et al., 2021). Consequently, SKF10047 proves to be advantageous in inhibiting NLRP3 activation and the pM1 polarization of microglia in rats subjected to stress-induced hypertension, thereby mitigating the occurrence of neuroinflammation. Comparably, the Sig-1R agonist PRE-084 has been shown to restore Ca²⁺ transfer in MAMs and enhance mitochondrial respiration. This compound may serve as a therapeutic agent for alleviating symptoms related to Wolfram syndrome in preclinical models (Crouzier et al., 2022). In contrast, CGI1746 acts as an inhibitor of Sig-1R, which leads to a reduction in the formation of MAMs and hinders the transfer of Ca²⁺ from the ER to the mitochondria. As a result, CGI1746 plays a role in alleviating mitochondrial Ca²⁺ overload and provides protective effects against cellular damage associated with ferroptosis (Zhang et al., 2024). Furthermore, research has demonstrated that overexpression of MFN2 promotes the interaction between the mitochondria and the ER by binding to the ER-resident Ca²⁺-ATPase SERCA2 in tumor-infiltrating CD8⁺ T cells (Yang et al., 2023). Enhancing the interaction through the upregulation of MFN2 in CD8⁺ T cells has been shown to augment the effectiveness of cancer immunotherapy. On the other hands, the implementation of effective strategies aimed at restoring mitochondrial network formation in lung cancer cells and hepatocellular cancer cells through the overexpression of MFN2 has been associated with a significant decrease in cancer cell proliferation and an enhancement of spontaneous apoptosis (Rehman et al., 2012; Wang W. et al., 2015). Notably, the upregulation of MFN2 can be achieved through the miR-761, and then disrupts mitochondrial function and significantly inhibits tumor growth and metastasis in both in vivo and in vitro (Zhou et al., 2016).

Ca²⁺ signaling, regulated by the interactions between the ER and mitochondria via MAMs, plays a key role in the treatment of various diseases. Therapeutic strategies that focus on the modulation of Ca²⁺ signaling pathways offer multifaceted intervention methods for cancer, neurodegenerative disorders, and metabolic diseases, by precisely regulating the functions of MAMs.

MAMs serve as physical platforms facilitating the processes of mitochondrial fission and fusion. Critical proteins located on these platforms, including DRP1, MFN1/2, and OPA1, play essential roles in the regulation of mitochondrial dynamics. Research shows that restoring mitochondrial dynamics through various methods, including gene therapy that alters gene expression to influence mitochondrial dynamics processes, as well as chemotherapy targeting various mechanisms essential for mitochondrial fission and fusion, can improve tissue functionality and prolong lifespan in animal models (Jüschke et al., 2021).

As previously noted, DRP1 is a dynamin-related protein that plays a critical role in mitochondrial fission (Smirnova et al., 2001). Upon activation, DRP1 accumulates to the OMM, leading the constriction and subsequent division of the mitochondria. Fission inhibitors have been engineered to reduce the levels of DRP1, thereby inhibiting mitochondrial division and facilitating mitochondrial fusion. These inhibitors demonstrate potential in mitigating the onset and progression of various diseases. Mdivi-1 is the first identified inhibitor that specifically targets proteins involved in mitochondrial division. It selectively impedes the function of DRP1 by primarily interacting with an orthosteric domain (Cassidy-Stone et al., 2008). Multiple studies have illustrated that implementation of Mdivi-1 significantly enhances behavioral outcomes and diminishes neurodegeneration in animal models of AD and PD (Oliver and Reddy, 2019). Furthermore, Mdivi-1 mitigates excessive cellular necrosis and reduces the infarct area in cardiomyocytes during IRI (Maneechote et al., 2020). And then, it also has demonstrated efficacy in inhibiting the proliferation and dissemination of malignant cells, reversing resistance to oncological treatments, and augmenting MHC-I expression in mouse tumor models (Dai et al., 2020; Lei et al., 2022). As for P110, a small peptide that inhibits mitochondrial division, has been shown to reduce both mitochondrial division and necrosis in neurons obtained from patients with Huntington’s disease (HD) and PD (Guo et al., 2013; Qi et al., 2013). Additionally, research has shown that Dynasore mitigates cardiac pathology and diminishes neuronal injury associated with degenerative diseases by inhibiting excessive mitochondrial fission (Ong et al., 2015; Medala et al., 2021). In addition to employing compounds to target MAMs proteins, modifications in gene expression can be achieved via genetic pathways, thereby regulating mitochondrial dynamics in cell. The inhibition of DRP1 leads to compromised mitochondrial autophagy and heightened apoptosis in HCC cells subjected to hypoxic conditions. This phenomenon is correlated with a decrease in mitochondrial membrane potential, alongside the release of cytochrome c and apoptosis-inducing factor. Thus, it will be a potential therapy for the HCC patients by down-regulating DRP1 (Lin et al., 2020).

MFN1 and MFN2 (Chen et al., 2003; Detmer and Chan, 2007) are situated on the OMM, while OPA1 (Al Ojaimi et al., 2022) is found on the IMM. These proteins are crucial for the mechanism of mitochondrial fusion. In mouse models exhibiting dilated cardiomyopathy, the overexpression of heme oxygenase-1 (HO-1) was found to enhance mitochondrial fusion through the upregulation of MFN1 and MFN2 expression levels (Hull et al., 2016). Serves as an additional agent, Melatonin promotes mitochondrial fusion via stimulating the Notch1/MFN2 signaling pathway, consequently increases the expression levels of MFN2 (Pei et al., 2016). Furthermore, Hydrazone M1 is a compound that facilitates mitochondrial fusion by reinstating the fusion process in MEFs (murine embryonic fibroblasts) that are deficient in MFN1 and MFN2 (Maneechote et al., 2019). It is noteworthy that 15-oxospiramilactone (S3) has the capacity to enhance ubiquitination without leading to the degradation of MFN1/2 by USP30. This process ultimately increases the activity of MFN1/2 and initiates mitochondrial fusion (Yue et al., 2014). As for OPA1, there are also several options. In terms of promoting OPA1-STAT3 pathway, punicalagin (Fu et al., 2021) and κ-opioid receptor (Wang et al., 2020) are two compounds which can promote mitochondrial fusion by enhancing the expression of OPA1. Besides, paeonol is a compound that promotes mitochondrial fusion via the OPA1 mechanism by activating the STAT3-CK2α signaling pathway in the context of diabetic cardiomyopathy (Liu C. et al., 2021). As previously indicated, there are some methods to regulate the mitochondrial dynamics by genetic pathways. On the one hand, the overexpression of OPA1 restores mitochondrial quality control and preserves the functionality of cardiomyocytes in a model of hypoxia-induced cardiomyocyte injury (Liu Y.-N. et al., 2020). On the other hand, MFN2 overexpression exerts positive function in lung cancer cells (Rehman et al., 2012) and HepG2 cells (Gao et al., 2018).

Dysregulation of MAMs’ proteins is significantly associated with the development of these diseases. Currently, targeting the related proteins on the MAMs offers a therapeutic approach for various diseases by precisely regulating the fission-fusion equilibrium via modulating DRP1, MFN1/2, and OPA1. Consequently, the regulation of MAMs to restore cellular functions may represent a novel therapeutic approach for the management of these diseases.

Mitophagy maintains dynamic homeostasis of cells through targeting dysfunctional mitochondria for timely clearance and recycling. MAMs serve as the critical platform coordinating this process, as they are essential for autophagosome formation by providing membrane contact sites for autophagy initiation. Dysfunction in MAMs is implicated in the molecular mechanisms underlying the initiation and progression of various human diseases, therefore targeting MAMs’ mitophagy-related proteins is a viable way.

With the MAMs microenvironment, FUNDC1 is a mitochondrial receptor situated in the OMM that governs the process of mitophagy. FUNDC1-dependent mitophagy is crucial for preserving the quality and functionality of mitochondria in platelets, thereby resulting platelet activation. This process contributes to decreased blood oxygen levels, potentially resulting in myocardial infarction (Zhang et al., 2017). Consequently, the inhibition of mitophagy during the acute phase of I/R may mitigate myocardial injury. And the modulation of the peroxisome proliferator-activated receptor gamma (PPAR-γ)/FUNDC1/mitophagy pathways may serve as a viable strategy for mitigating platelet hyperactivity and IRI (Zhou et al., 2017). CK2α, an inhibitor of FUNDC1, exacerbates mitochondrial apoptosis during cardiac IRI by inactivating protective mitophagy. In contrast, the lack of CK2α has been demonstrated to maintain cardiac structure and function during I/R stress by facilitating the upregulation of FUNDC1-dependent mitophagy (Zhou et al., 2018). Moreover, studies have indicated that FUNDC1 is essential for the regulation of mitochondrial division at MAMs interfaces. It facilitates the expression of the cancer stem cell marker BMI1 (polycomb ring finger oncogene) by enhancing the influx of Ca²⁺ into the cytosol and promoting the nuclear translocation and activity of nuclear factor of activated T cell 1 (NFATC1). This mechanism subsequently stimulates the proliferation, migration, and invasion of breast cancer cells, and it also functions as a predictor of poor outcomes in patients diagnosed with BC (Wu L. et al., 2019). This indicates that the inhibition of FUNDC1 or its associated pathway may represent a potential therapeutic target for the treatment of BC. MAMs further regulate autophagy through other resident proteins. As previously noted, DJ-1, which is localized to MAMs exhibits a significant relationship with α-syn. And the absence of DJ-1 results in the accumulation of α-syn. Consequently, studies have demonstrated that the overexpression of DJ-1 can enhance the autophagic capacity of microglia, thereby improving their ability to eliminate α-syn (Gao et al., 2012). This mechanism may potentially mitigate or even reverse the symptoms experienced by patients with PD. Furthermore, research has demonstrated that the loss of PML protein leads to a decline in Ca²⁺ transport, mitochondrial respiration, and ATP synthesis. This decrease subsequently initiates autophagy via the activation of the AMPK/mTOR/Ulk1 signaling pathway. Consequently, the loss of PML from the MAMs not only heightens resistance to apoptotic stimuli but also facilitates adaptation to metabolic stress and cellular damage induced by anticancer therapies (Missiroli et al., 2016). This research presents evidence that the inhibition of autophagy, when used in conjunction with chemotherapy agents like 5-fluorouracil (5-FU), may improve the sensitivity of cancer cells to chemotherapy. Consequently, this approach could contribute to the development of anticancer strategies that simultaneously modulate both autophagy and apoptosis in cancer cells.

Collectively, these studies collectively assessed the viability of targeting proteins involved in the regulation of autophagy at the level of MAMs for therapeutic interventions in various diseases, thereby underscoring their considerable potential as therapeutic targets.

MAMs proteins which regulate the metabolic reprogramming in cells suggests that targeting metabolic pathways at the contact sites between the ER and mitochondria presents novel opportunities for disease treatment. For example, Raturi et al. demonstrated that the tumor suppressor TMX1 interacts with SERCA2b within the MAMs, promoting mitochondrial ATP synthesis and triggering apoptosis in cancer cells by influencing the ER-mitochondrial contacts and the intracellular Ca²⁺ flux between the ER and mitochondria (Raturi et al., 2016). Therefore, therapeutic strategies targeting TMX1 may further enhance cancer cell death through mitochondrial metabolic regulation. Furthermore, it has been observed that cell metabolism, regulated by MAMs, not only alters the properties of tumor cells but may also influence the function of tumor-associated immune cells. In human peripheral blood leukemic T cell line, the rapid activation of PKB by mTORC2 results in the inhibition of GSK3β which situated at MAMs. This process facilitates mitochondrial respiration by promoting metabolite flux into mitochondria. This modulation is essential for the metabolic reprogramming required for memory CD8⁺ T cells to enhance therapeutic outcomes (Bantug et al., 2018).