Introduction

Mitsugumin-53 (MG53), also known as TRIM72, is a cell membrane repair protein that is part of the tripartite motif family of proteins. Similar to other proteins in the TRIM family, MG53 contains the prototypical tripartite motif that includes ring, B-box, and coiled-coil moieties, as well as a SPRY domain at the carboxy terminus [1,2,3]. It is a striated muscle protein, which is highly expressed in skeletal muscles and to a lesser extent cardiac muscles.

Following acute membrane damage, MG53 senses an oxidized intracellular environment and forms an oxidation-dependent oligomerization repair complex by tethering to phosphatidylserine domains present on intracellular vesicles and in the inner aspect of the plasma membrane [4]. A local elevation of Ca2+ enables MG53-tethered intracellular vesicles to fuse with the disrupted plasma membrane, leading to the formation of a repair patch [4]. This process is also facilitated by the interaction of MG53 with muscle specific proteins dysferlin and caveolin-3 (Cav3) [5]. MG53 knock out (KO) mice show progressive myopathy and reduced exercise capacity that is associated with a defect in its membrane repair capability [4].

In addition to its function in skeletal muscle, MG53 has been shown to have protective effects on various forms of cardiac muscle injury. Since cardiomyocytes are terminally differentiated cells with limited self-renewal capacity, and membrane rupture is a major cause of cardiomyocyte cell death following injury, membrane repair is a necessary process for preserving cardiomyocyte viability [6]. In this review, we summarize the biological function of MG53 with its potential mechanisms in cardiac tissue (Fig. 1), discuss current debates regarding the role of MG53 in diabetic cardiomyopathy (Table 1), and potential clinical applications of recombinant MG53 protein in the management and treatment of heart diseases (Table 2).

Fig. 1
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Signal pathways that are associated with the regulatory role of MG53 in heart

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Table 1 List of controversial studies on the role of MG53 in cardiomyopathy
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Table 2 List of studies for the applications of rhMG53 in the management and treatement of heart diseases
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Beneficial effects of MG53 in heart disease

Cardioprotective effects after ischemia/reperfusion injury

Ischemic preconditioning (IPC) was first reported in 1986 by Murry et al. and is an intrinsic process through which repeated short episodes of ischemia are instituted to protect the myocardium against subsequent ischemic insults [7]. Cardiac ischemia is modelled in vitro through the application of hypoxic and oxidative stress. Ischemia/reperfusion (I/R) in mouse hearts and hypoxia/oxidative stress in neonatal rat cardiomyocytes have been associated with a downregulation of MG53. IPC can prevent IR-induced decrease in MG53 expression [8]. MG53 KO mice lack IPC-mediated cardioprotection as evidenced by a failure of IPC to reduce IR-induced myocardial infarct size. IPC suppressed IR-induced infarction in wild type (WT) mouse hearts whereas overexpression of GFP-MG53 fusion protein reduced hypoxia- or H2O2-induced cell death [8]. However, adenovirus-mediated shRNA targeting MG53 downregulated MG53 expression in rat cardiomyocytes, exacerbating hypoxia-induced cell death and eliminating the protective effect of GFP-MG53 overexpression [8].

IPC activates the reperfusion injury salvage kinase (RISK) and survivor activating factor enhancement (SAFE) pathways to protect the heart against IR injury. The RISK pathway consists of PI3K-Akt-GSK3β and ERK1/2 signaling events (Fig. 1), whereas the SAFE pathway involves activation of tumor necrosis factor-α (TNF-α) and the JAK-STAT3 axis [9,10,11,12]. Overexpression of MG53 significantly increased phosphorylation levels of several key pro-survival kinases including Akt, GSK3β, and ERK1/2 over their respective controls [13,14,15,16]. However, IPC did not enhance the phosphorylation levels of these key kinases in MG53 KO mouse hearts [8]. This suggests that IPC-induced elevation of PI3K-Akt-GSK3 and ERK1/2 signals is MG53 dependent and that suppression of either pathway fully prevents IPC-induced cardioprotection (Fig. 1). Contrastingly, MG53 has not been demonstrated to activate the SAFE pathway [17].

Interestingly, a recent study from ** physiological functions in insulin signaling capabilities [58]. Results from a proteomic study [59] assessed IRS-1 protein interactions in skeletal muscles from normal individuals, obese insulin-resistant nondiabetic control subjects, and patients with type 2 diabetes, before and after insulin infusion. They failed to identify any changes in MG53 protein interaction with IRS-1 across all groups after insulin infusion [59].

Furthermore, a recent published paper from our group [60] revealed lower serum MG53 levels in db/db mice compared with WT littermates (Table 1). Either whole-body knockout of MG53 or sustained increase of MG53 in circulation did not affect insulin signaling and glucose handling in db/db mice. Rats receiving the daily intravenous (IV) treatment of rhMG53 did not have adverse effects on glucose handling [60]. Interestingly, a recent study from ** of proteins released during necrosis and apoptosis from cultured neonatal cardiac myocytes. Am J Physiol Cell Physiol. 2014;306:C639-647." href="/article/10.1186/s13578-021-00629-x#ref-CR67" id="ref-link-section-d6318297e1652">67]. MG53 was found to have a time-dependent relative elevated expression after inducing necrosis by oxidative stress, which supports the potential use of MG53 as a marker for necrotic cellular injury. Using an in vivo murine model, Liu et al. demonstrated a low circulating level of MG53 in the blood at baseline which increased in a dose-dependent fashion after an MI [68]. A recent study showed that the serum level of MG53 was elevated in patients with stable cardiovascular disease and reach to the highest level in patients with an acute MI [69]. These findings suggest MG53 may be a potential biomarker of myocardial injury.

Cardioprotection and therapeutic potential of rhMG53

As discussed above, low levels of endogenous MG53 protein has been found in human hearts, which emphasizes the physiological significance of circulating MG53 for cardioprotection. It has been demonstrated that MG53 can be secreted from skeletal muscle in response to exercise and muscle contraction, and that MG53 circulates in the bloodstream under physiological conditions [70, 71]. Pharmacokinetic (PK) and toxicology assessments support the safety of repetitive IV administration of rhMG53 in Beagle dogs [71]. Studies in mice reported no observable toxic effects with long-term IV or subcutaneous (SQ) administration of rhMG53 [70, 71]. Therefore, therapeutic approaches to modulate MG53 function or systemic administration of rhMG53 protein can potentially act as safe biological interventions to treat cardiac pathologies. As a therapeutic protein, rhMG53 has several practical attributes. First, since native MG53 is present and expressed constitutively in humans and the protein can be found normally in circulation, there are minimal toxicological and immunological concerns for using rhMG53 as a therapeutic protein. Second, rhMG53 can be purified using E. coli fermentation, be stored as a lyophilized powder long term at room temperature, and remain soluble and biologically active upon reconstitution [70].

Administration of recombinant human MG53 (rhMG53) mitigated sepsis-induced myocardial dysfunction in rats evidenced by the improved survival rate with increased cardiac function, and reduced oxidative stress, inflammation, and myocardial apoptosis via the elevation of PPARα signal pathway (Table 2) [44]. Given the role of MG53 in IPC and PostC, therapeutic rhMG53 could potentially be used to enhance the protective effects of these maneuvers (Table 2) [18]. However, because IPC can only protect against IR injury before the occurrence of a severe ischemic episode, this limits the practical application of IPC in the clinical setting of an MI [7]. Because PostC is a series of brief ischemia and reperfusion cycles applied after the onset of reperfusion, PostC may have more practical clinic applications [72].

Liu et al. used several different animal models to identify whether rhMG53 can play a therapeutic role in treating IR injury. They demonstrated that the administration of rhMG53, either before ischemia or after reperfusion, decreased infarct size in these various in vivo models (Table 2) [68]. Furthermore, they provided evidence demonstrating that rhMG53 preferentially targets infarcted tissue and upregulates phospho-AKT and phosphor-GSK3β supporting the mechanism that rhMG53 concentrates at the site of injury by binding phosphatidylserine [68, 70]. Importantly, animals with the daily administration of rhMG53 did not exhibit side effects on glucose handling (Table 2) [60].

The administration of rhMG53 can come in various forms. The protein can potentially be directly injected at or in the site of myocardial injury to promote repair or attenuate injury. rhMG53 could also be administered by IV, SQ [70], and intramuscular (IM) injections [73]. Exogenously applied rhMG53 has been shown to be able to recognize sites of injury on the membrane of cardiomyocytes and reduce infarct size [68].

There may be potential for gene therapy applications with adeno-associated virus (AAV)-mediated delivery of MG53. For example, given the essential role of MG53 in maintaining the integrity of muscle membranes, MG53 may be used to treat different forms of muscular dystrophy. MG53 gene therapy may also have a role in treating various forms of cardiomyopathies considering the ability of MG53 to maintain the integrity of cardiomyocyte membranes [22]. He et al. demonstrated the systemic delivery and muscle-specific overexpression of human MG53 gene by recombinant AAV vectors. They reported enhanced membrane repair and improved muscle and heart function with MG53 overexpression in δ-sarcoglycan-deficient TO-2 hamsters, an animal model of muscular dystrophy and congestive heart failure [22].

Conclusions

In addition to its function in skeletal muscle, MG53 appears to play an important role in cardiac muscle as well. Its critical function as a membrane repair protein has been clearly demonstrated. It also appears to be important for mediating the cardioprotective effects of both ischemic preconditioning and post-conditioning after ischemia/reperfusion injury. MG53 may also play a role in the development of atrial fibrosis, which, in turn can promote atrial fibrillation. However, there are still debates on the potential role of MG53 in mediating and even promoting diabetes, diet-induced metabolic disorders, and diabetic cardiomyopathies. The potential utility of MG53 as a diagnostic biomarker and rhMG53 as a clinically relevant of therapeutic protein is promising and warrants further study.