The Role of Mitochondrial Dysfunction and Inflammatory Response in the Pathogenesis of Sepsis-Induced Myocardial Injury: A Mechanistic Study

Introduction

Sepsis is a severe infectious disease caused by the invasion of pathogens such as bacteria, viruses, and fungi into human tissues. In its most severe form, sepsis can progress to systemic inflammatory response syndrome (SIRS), multiple organ failure, and ultimately, death.^1,2 Given its high mortality rate and the frequent onset of complex complications, sepsis represents a substantial global economic and social burden. In 2017, an estimated 48.9 million individuals worldwide were affected by sepsis, with 11 million sepsis-related deaths, accounting for 19.7% of all global fatalities.³ In the United States, over 1.7 million adults develop sepsis annually, resulting in nearly 350,000 deaths each year.⁴ A cross-sectional study conducted across 44 hospitals in China revealed a 90-day mortality rate of approximately 35.5% among patients hospitalized for sepsis.⁵

Sepsis typically begins with SIRS to infection, which can be partially managed by controlling the underlying infection. However, as the condition progresses, it frequently leads to multi-organ dysfunction or failure, involving critical organs such as the heart, brain, kidneys, and lungs. This multi-organ involvement is a central factor contributing to the high mortality observed in sepsis patients.^6,7 Notably, sepsis can also result in additional complications such as acute respiratory distress syndrome (ARDS), renal failure, disseminated intravascular coagulation (DIC), and septic shock.^8,9 These complications further exacerbate the severity of sepsis and pose significant challenges in its management. One of the key complications of sepsis is sepsis-induced myocardial injury (SIMI), which is associated with a markedly increased mortality rate, ranging from 30% to 70%, compared to non-cardiogenic septic patients.^10,11 Traditional therapeutic strategies for SIMI include fluid resuscitation, infection management, vasopressor therapy, and organ support, yet their effectiveness remains constrained. In recent years, advanced treatment modalities such as extracorporeal membrane oxygenation (ECMO) and remote ischemic conditioning (RIC) have developed, offering heart-lung replacement therapy for sepsis patients. Despite their promise, these approaches are costly and add considerable financial strain on patients, and are typically restricted to intensive care units (ICUs) in large tertiary hospitals, limiting their widespread application. Furthermore, the multifactorial nature of SIMI complicates both its early diagnosis and effective treatment. Consequently, further research into the pathophysiological mechanisms underlying SIMI is essential to the development of targeted therapeutic strategies aimed at enhancing patient survival. Studies indicate that dysregulated systemic immune responses play a pivotal role in the development of myocardial injury in sepsis, although mitochondrial dysfunction, metabolic disturbances, myocardial edema, and cell death are also key contributors to its pathogenesis.^12–14 For instance, exogenous fetuin-A can protect against sepsis-induced cardiac dysfunction in vivo via suppression of inflammation and oxidative damage.¹⁵ Additionally, interactions between these mechanisms may exist, necessitating a comprehensive approach to fully elucidate the condition.^12–14 While much of the research has focused on these individual mechanisms in isolation, there is an increasing need to integrate and synthesize these findings to establish a robust scientific framework for targeted treatment strategies for SIMI. In this review, we will further explore the potential pathophysiological mechanisms of SIMI from the perspective of mitochondrial dysfunction, with particular emphasis on the role of dysregulated inflammatory responses, calcium homeostasis imbalance, and metabolic reprogramming in the pathogenesis of SIMI. Finally, we will provide a concise overview of therapeutic strategies targeting mitochondrial dysfunction and potential therapeutic targets for SIMI.

Mitochondrial Dysfunction Induces Dysregulation of the Inflammatory Response in SIMI

Mitochondria play a crucial role in regulating the inflammatory response, thereby contributing to the onset and progression of infectious diseases and tumors. First, mitochondria are believed to be evolutionary remnants of modern Gram-negative bacteria, specifically α-proteobacteria. Consequently, some mitochondrial components share similarities with bacterial molecules, suggesting that certain mitochondrial components could serve as ligands for pattern recognition receptors (PRRs).¹⁶ Secondly, both the inner mitochondrial membrane (IMM) and outer mitochondrial membrane (OMM) provide a double-membrane structure that controls the separation of mitochondrial damage-associated molecular patterns (mtDAMPs) and their homologous PRRs.¹⁷ Mitochondrial mtDAMPs are a series of molecules released into the cytoplasm or extracellular environment when the mitochondria are damaged or dysfunctional. These include mitochondrial DNA (mtDNA), mitochondrial peptides (such as N-formyl peptides), and adenosine triphosphate (ATP). These molecules possess damage-associated molecular pattern (DAMP) characteristics and can activate immune responses in host cells, leading to the induction of inflammation. Thirdly, and more extensively studied, mitochondria regulate various forms of programmed cell death, including apoptosis, pyroptosis, autophagy, and ferroptosis, which in turn trigger inflammatory responses and contribute to pathological states in the body.¹⁸ Sepsis is characterized by the acute release of multiple inflammatory mediators, including tumor necrosis factor (TNF)-α, interleukin (IL)-6, and IL-1β. The excessive release of these mediators damages tissues and organs. The overactive inflammation triggered by damage-associated molecular patterns (DAMPs) and pathogen-associated molecular patterns (PAMPs) contributes to the disruption of endothelial barrier integrity and plays a key role in the pathogenesis of sepsis and sepsis-induced myocardial injury.¹⁹ Next, we will focus on the mechanisms by which mitochondrial dysfunction-induced dysregulation of inflammatory responses, through processes such as pyroptosis apoptosis, mitophagy, and ferroptosis, contributes to the development of septic cardiomyopathy.

Pyroptosis in SIMI

Pyroptosis is an emerging form of regulated cell death, characterized by progressive cell swelling culminating in membrane rupture, which results in the release of cellular contents and the subsequent activation of a potent inflammatory response.²⁰ This process is primarily mediated by gasdermin proteins and plays an essential role in the innate immune response during both infection and inflammation. In the context of SIMI pathogenesis, mitochondrial membrane integrity is compromised, leading to the release of DAMPs that activate gasdermin-mediated immune responses.²¹ These reactive oxygen species(ROS) are subsequently released into the cytoplasm, where they interact with nucleotide- binding oligomerization domain, leucine- rich repeat and pyrin domain- containing 3(NLRP3), thereby activating the NLRP3 inflammasome.²² Furthermore, the excessive buildup of ROS activates Toll-like receptor (TLR)-mediated inflammatory pathways, exacerbating myocardial injury in the context of sepsis.²³ Notably, Wang et al demonstrated that oxycodone mitigates pyroptosis through the modulation of the Nrf2/HO-1 signaling pathway, thereby alleviating inflammatory dysregulation and reducing lipopolysaccharide(LPS)-induced myocardial injury.²⁴ In a similar vein, inhibition of the NLRP3 inflammasome/GSDMD signaling axis significantly dampens the inflammatory response and myocardial pyroptosis, yielding substantial therapeutic benefits in SIMI.²⁵ In the subsequent sections, we will further explore the mechanistic underpinnings of pyroptosis in SIMI, with particular emphasis on its canonical pathways and critical molecular events.

The Classical Pathways of Pyroptosis in SIMI

In the pathogenesis of SIMI, pyroptosis primarily relies on the activation of inflammasomes, which trigger the activation of specific caspase family proteins. These caspases cleave gasdermin proteins, leading to the activation of gasdermin, which translocates to the membrane and forms pores, causing cell swelling, cytoplasmic leakage, and ultimately leading to cell membrane rupture and pyroptosis.²⁶ Based on whether caspases are involved, pyroptosis is classified into two main pathways: the caspase-1-dependent and caspase-1-independent pyroptotic pathways. In the Caspase-1-dependent pyroptotic pathway, upon the invasion of various pathogens, inflammasomes such as NLRP3, NLRC4, AIM2, and Pyrin can recognize these signals and become activated. These inflammasomes subsequently interact with the adapter protein ASC and pro-caspase-1 to form a complex, activating caspase-1.²⁷ On one hand, the activated caspase-1 cleaves Gasdermin D (GSDMD), exposing the N-terminal fragment of GSDMD, which interacts with phospholipids on the cell membrane, forming pores that release cellular contents and induce pyroptosis.²⁰ On the other hand, activated caspase-1 also cleaves and activates the precursors of IL-1β and IL-18, which are then released extracellularly, thereby amplifying the inflammatory response. In the caspase-1-independent pyroptotic pathway, upon exposure to LPS, human caspases-4 and -5, as well as murine caspase-11, can directly bind to LPS and become activated. These caspases then cleave GSDMD, exposing its N-terminal fragment and initiating pyroptosis. Additionally, the activation of caspase-4/5/11 stimulates the activation of the Pannexin-1 channel, which facilitates the efflux of K⁺ ions, triggering the NLRP3 inflammasome. This in turn activates caspase-1, further promoting the caspase-1-dependent pyroptotic pathway. Thus, these two distinct pyroptotic pathways, whether caspase-1-dependent or caspase-1-independent, play crucial roles in mediating pyroptosis and driving the inflammatory processes that contribute to the myocardial injury observed in SIMI²⁸ (Figure 1).

Figure 1 Main pathway of pyroptosis in SIMI. During the onset of SIMI, various stress stimuli lead to mitochondrial dysfunction, with mtDAMPs being released through the mPTP, thereby inducing pyroptosis. Concurrently, extracellular DAMPs and PAMPs can also trigger pyroptosis. Pyroptosis can be classified into two types based on its dependence on caspase-1. In caspase-1 dependent pyroptosis, the process is initiated by the assembly of inflammasomes. Conversely, in caspase-1 independent pyroptosis, it is triggered by the interaction between caspase-4, caspase-5, or caspase-11 (depending on the species) and LPS. Solid black arrows present for direct processes. Black colored dotted squared box present for complex. Abbreviations: ASC, apoptosis-associated speck like protein containing a CARD; NLRP3, nucleotide-binding oligomerization domain, leucine-rich repeat and pyrin domain- containing 3; GSDMD, Gasdermin D.

The Key Factors of Pyroptosis in SIMI

Activation of the NLRP3 inflammasome, formation of GSDMD pores, and secretion of pro-inflammatory cytokines are considered key events in the occurrence of pyroptosis in SIMI. The NLRP3 inflammasome is a large molecular protein complex, with an approximate molecular weight of 700,000 Da, consisting of NLRP3, the adaptor protein ASC, and the effector protein Caspase-1. The assembly of the NLRP3 inflammasome requires the interaction between the NLRP3 receptor, the apoptosis-associated speck-like protein (ASC) adapter, and pro-caspase-1. Under normal physiological conditions, NLRP3 is in an auto-inhibited state. In SIMI, mitochondrial dysfunction, as well as the release of PAMPs or DAMPs, leads to the deactivation of this auto-inhibition. The N-terminal pyrin domain (PYD) of NLRP3 recruits the ASC adaptor protein, which also contains a PYD domain, while the caspase recruitment domain (CARD) of ASC recruits pro-caspase-1, which contains a CARD domain, thereby facilitating the assembly of the inflammasome. ASC, composed of both PYD and CARD domains, is primarily located in the nucleus of human monocytes/macrophages, but rapidly translocates to the cytoplasm under stress, linking NLRP3 and pro-caspase-1 to promote the activation of the NLRP3 inflammasome. Caspase-1, also known as IL-1β-converting enzyme, is the effector protein of the NLRP3 inflammasome. It is activated through the auto-cleavage of the precursor molecule pro-caspase-1, resulting in the formation of active caspase-1. Its primary function is to facilitate the cleavage of pro-IL-1β and pro-IL-18 into their mature forms, IL-1β and IL-18. Clinical studies have shown that levels of NLRP3, GSDMD, IL-1β, and IL-18 are elevated in sepsis patients.^29–31 In a SIMI model induced by intraperitoneal LPS injection in mice, targeting the NLRP3 inflammasome effectively inhibited pyroptosis and alleviated cardiac oxidative stress and inflammation.³²

GSDMD belongs to the gasdermin (GSDM) family and comprises a cytotoxic N-terminal domain and a C-terminal inhibitory domain linked by a flexible linker. Upon cleavage of the C-terminal domain of GSDMD, the N-terminal fragment (GSDMD-N) is recruited to the cell membrane, where it interacts with lipids to form an intermediate structure known as the pre-pore. This pre-pore undergoes conformational rearrangement to form oligomeric arcs, which subsequently transition into gap-like structures. Finally, these gaps expand to form a ring-shaped oligomer, resulting in the formation of membrane pores. Electron microscopy reveals that the pore formed by GSDMD-N has an inner diameter of 10–15 nm, allowing the passage of certain pro-inflammatory cytokines such as IL-1β and IL-18, thus triggering pyroptosis.^33,34 Yang et al observed that in a septic mouse model, GSDMD knockout (GSDMD^−/−) mice exhibited alleviated organ damage compared to wild-type mice.³⁵ Further mechanistic studies revealed that during sepsis, mitochondrial dysfunction leads to the release of ROS, and GSDMD-N directly interacts with oxidized cardiolipin, inducing cardiac dysfunction.³⁶ Additionally, caspase-4/11 activates GSDMD-N pore formation, which in turn amplifies ROS production and upregulates the NLRP3 inflammasome, further enhancing GSDMD-N pore formation.³⁶

Pro-inflammatory cytokines, including IL-1β and IL-18, play a central role in this process. In a cecal ligation and puncture (CLP)-induced sepsis mouse model, elevated levels of cardiac troponin I (cTnI), IL-1β, and TNF-α were observed, alongside mitochondrial dysfunction.³⁷ Moreover, studies have demonstrated that Irisin can reduce the expression of IL-1β and decrease serum lactate dehydrogenase (LDH) and creatine kinase MB (CK-MB) levels, alleviating LPS-induced damage to H9C2 cells by upregulating mitochondrial ubiquitin ligase (MITOL) in a GSDMD-dependent manner.³⁸

In summary, during sepsis, mitochondrial structural or functional dysfunction releases mitochondrial-derived DAMPs, which activate the NLRP3 inflammasome and promote GSDMD pore formation. This leads to the release of IL-1β and IL-18 into the bloodstream, inducing myocardial pyroptosis and widespread inflammatory responses, ultimately contributing to sepsis-induced myocardial injury. Therefore, targeting mitochondrial dysfunction and cell pyroptosis to mitigate excessive inflammatory responses represents a promising therapeutic strategy for SIMI.

Apoptosis in SIMI

Research has shown that LPS can disrupt the ultrastructure of mitochondria, leading to mitochondrial membrane potential (MMP) impairment. This disturbance increases the production of LDH, ROS, and malondialdehyde (MDA), while simultaneously decreasing the levels of superoxide dismutase (SOD).³⁹ These changes trigger cell apoptosis and dysregulated inflammatory responses, ultimately contributing to SIMI.³⁹ Apoptosis refers to a programmed, orderly cell death process that occurs under both physiological and pathological conditions to maintain homeostasis within the body. It is characterized by a series of morphological and biochemical alterations, including nuclear condensation, DNA fragmentation, cell membrane remodeling and blebbing, cellular shrinkage, and the formation of apoptotic bodies. The apoptotic cells are then engulfed by macrophages for removal.⁴⁰ Apoptosis is a major feature of inflammatory system dysfunction, representing a hallmark of both human sepsis and experimental sepsis models in cells and animals.⁴¹ It has been observed that during SIMI, the accumulation of ROS leads to the oxidative modification of large molecular proteins and DNA structures, thereby disrupting mitochondrial integrity. This increases the permeability of the mitochondrial membrane, which in turn activates apoptotic pathways through the release of cytochrome c. This cascade of events results in myocardial cell apoptosis.⁴² This section will discuss the classic pathways and key molecules involved in apoptosis during the development and progression of SIMI.

The Classical Pathways of Apoptosis in SIMI

The process of apoptosis in SIMI primarily involves the extrinsic death receptor pathway, the intrinsic mitochondrial pathway, and the intrinsic endoplasmic reticulum (ER) pathway.^43,44 The extrinsic death receptor pathway is mainly activated through the TNF receptor-associated death domain (TRADD) receptor. The activation of death receptors on the cell surface primarily involves the TNF superfamily and TNF-related apoptosis-inducing ligands, which specifically recognize and activate these receptors, leading to the activation of caspase-8. The intrinsic ER pathway is mainly triggered by various stress factors that cause calcium ions to be pumped out of the ER, which activates caspase-12 and, in turn, activates the effector caspase-3, initiating the apoptotic cascade. Notably, in SIMI, the pathway associated with mitochondrial dysfunction is the intrinsic mitochondrial pathway, which is primarily activated by stress-induced stimuli. When the body undergoes various stress conditions such as activation of oncogenes, DNA damage, hypoxia, or deprivation of growth factors, pro-apoptotic proteins from the Bcl-2 family, such as Bax, are upregulated. Bax is generally found in the cytoplasm, and when it receives apoptotic signals, it translocates to the mitochondrial membrane. At the mitochondrial surface, Bax forms pores across the mitochondrial membrane, leading to a decrease in membrane potential and increased membrane permeability.⁴⁵ This results in the release of apoptotic factors such as cytochrome c and second mitochondria-derived activator of caspases (SMAC), which then induce apoptosis.⁴⁵ Once cytochrome c is released into the cytoplasm, it interacts with Apaf-1, and with the assistance of ATP and dATP, forms the apoptosome. The apoptosome recruits and activates pro-caspase-9, forming the caspase-9 holoenzyme. Caspase-9 further activates the effector caspases-3 and −7, initiating the caspase cascade and cleaving over 100 substrates in the cell, such as α-tubulin, actin, PARP, and lamin, ultimately leading to cell apoptosis. Additionally, in sepsis, cytochrome c oxidase exerts a non-competitive inhibitory effect on cardiomyocytes, mainly by interrupting mitochondrial oxidative phosphorylation and inhibiting ATP production, which ultimately leads to SIMI.⁴⁶ During SIMI, with changes in the mitochondrial membrane potential, other factors such as apoptosis-inducing factor(AIF) and endonuclease G (ENDOG) are also released into the cytoplasm.^47,48 AIF and ENDOG are transported to the cell nucleus, where they induce chromatin condensation and DNA fragmentation, triggering apoptosis^47,48 (Figure 2).

Figure 2 Main pathway of apoptosis in SIMI. The apoptotic pathway can be categorized into three distinct types: the extrinsic death receptor pathway, the intrinsic mitochondrial pathway, and the intrinsic endoplasmic reticulum (ER) pathway. The extrinsic apoptotic cascade is primarily activated through the specific activation of FADD receptors and caspase-8. The intrinsic ER pathway is primarily triggered by various stress factors that lead to the efflux of calcium ions from the ER, activating caspase-12, which in turn activates the effector caspase-3 and initiates the apoptosis cascade. The intrinsic mitochondrial apoptotic pathway is activated by the release of cytochrome c and SMAC, which leads to the upregulation of pro-apoptotic protein Bax and the induction of apoptosis. Additionally, the interaction between cytochrome c, Apaf-1, and caspase-9 can also trigger the extrinsic apoptotic cascade. Black arrows present for direct processes. Black colored dotted squared box present for mitochondrial dysfunction process. Abbreviations: FADD, Fas-associating protein with a novel death domain; SMAC, second mitochondria-derived activator of caspases; Apaf-1, apoptotic protease activating factor-1.

The Key Factors of Apoptosis in SIMI

Caspases, or cysteine-aspartic acid proteases, are specialized proteases that possess a cysteine residue at their active site and cleave their substrates at the C-terminal side of aspartic acid residues. The caspase family plays a critical role in inhibiting cell survival pathways and specifically activating pro-apoptotic factors. Caspases are essential in myocardial cell apoptosis. In SIMI, the extrinsic apoptotic pathway is activated through the binding of ligands to death receptors, such as FAS, leading to the recruitment, dimerization, and activation of caspase-8 with the help of adapter proteins like FAS-associated death domain (FADD) and TRADD. Activated caspase-8 then cleaves and activates effector caspases (caspase-3, -6, and -7), directly initiating apoptosis, or it cleaves BID to activate the intrinsic apoptotic pathway and induce effective cell death. On the other hand, the intrinsic mitochondrial apoptosis pathway can be activated by various cytokines, leading to the release of cytochrome c from the mitochondria and the formation of the apoptosome composed of APAF1, cytochrome c, ATP, and caspase-9, which activates caspase-9. Activated caspase-9 subsequently cleaves and activates effector caspases, such as caspase-3, -6, and -7, which then execute apoptosis. Studies have shown that in sepsis, the molecular mechanisms of myocardial injury include apoptosis and increased caspase activity.⁴⁹ Wang et al found that docosahexaenoic acid (DHA) enhances cell viability by reducing the release of LDH, expression levels of cleaved caspase-3, and caspase-3 activity.⁵⁰ DHA also decreases the production of ROS and increases superoxide dismutase activity, thus alleviating LPS-induced myocardial cell apoptosis and mitochondrial damage. Similarly, the use of the broad-spectrum caspase inhibitor Z-Val-Ala-Asp (OMe)-FMK (SAD) to inhibit caspase activity significantly restored myocardial function in septic cardiomyopathy mouse models.⁵¹ Hayakawa et al demonstrated that using the multi-caspase inhibitor IDN-1965 in transgenic Gα(q) mouse models reduced caspase-3 activity, resulting in improved myocardial function and a marked reduction in mortality.⁵²

The Bcl-2 family is a vital group of proteins involved in regulating mitochondrial apoptosis, primarily consisting of pro-apoptotic and anti-apoptotic proteins, which work in coordination during the apoptotic process. They mediate the signaling pathways through the mitochondrial pathway to determine whether a cell will undergo apoptosis. Among them, pro-apoptotic proteins can be further classified into those with BH1-3 domains and those with only the BH3 domain. Members of the pro-apoptotic group, such as Bak, and anti-apoptotic proteins, such as Bcl-2 and Bcl-xL, are primarily located on the mitochondrial membrane, while other members, such as Bid and Bad, are mainly present in the cytoplasm. In the pathogenesis of SIMI, the pro-apoptotic protein Bax plays a significant role. Under normal conditions, Bax exists in an inactive form in the cytoplasm. Two hypotheses explain the activation of Bax. One hypothesis suggests a direct activation model, where BH3-only proteins are divided into activators and sensitizers. In the absence of apoptotic signals, activators bind to anti-apoptotic proteins, inhibiting the activation of Bax. Upon receiving apoptotic signals, sensitizer proteins bind to the anti-apoptotic proteins, releasing the activators, which directly activate Bax. The other hypothesis proposes an indirect activation model, where BH3-only family members, upon receiving apoptotic signals, inhibit the activity of anti-apoptotic proteins, thereby indirectly activating Bax. Once activated, Bax relocates to the mitochondrial membrane, forming pores across the mitochondrial membrane, leading to a loss of membrane potential, increased membrane permeability, and the release of apoptotic factors. Studies have shown that during sepsis-induced myocardial cell apoptosis, the pro-apoptotic gene Bax mediates the opening of the mitochondrial permeability transition pore (mPTP) on the inner mitochondrial membrane, which inhibits the expression of the anti-apoptotic factor Bcl-2.⁵³ The Bax/Bcl-2 ratio increases, and the myocardial cell apoptosis index progressively increases.⁵³ Additionally, Bax can form homodimers under stress conditions, disrupting the protein hydrolysis cascade and causing the mitochondria to release a large amount of pro-apoptotic factors, ultimately leading to myocardial cell apoptosis. Research has indicated that mitochondrial damage induced by excessive ROS is the primary source of myocardial cell apoptosis. Dehydrocorydaline (Deh) has shown significant anti-inflammatory, anti-apoptotic, and antioxidant effects by inhibiting the TRAF6/NF-κB pathway, reducing levels of IL-1β, IL-6, TNFα, and IFNγ, lowering ROS levels, and upregulating Nrf2/HO-1, SOD, and GSH-PX expression.⁵⁴ Zhang et al have found that by regulating the activity of extracellular signal-regulated kinase 1/2 (ERK1/2) and c-Jun N-terminal kinase (JNK), inhibiting the expression of Bax while upregulating the expression of Bcl-2, mitochondrial membrane potential is reduced, ATP generation and mtDNA expression are enhanced, thus alleviating oxidative stress and improving mitochondrial damage, providing therapeutic benefits for SIMI.⁵⁵

Inhibiting the expression of pro-apoptotic proteins and their associated pathways, while promoting the expression of anti-apoptotic proteins and their respective pathways, represents a critical therapeutic approach for SIMI. For example, in SIMI mouse models, an increase in Bcl-2 protein expression, accompanied by a reduction in the expression of Bid, t-Bid, and caspase-9, effectively inhibits myocardial cell apoptosis.⁴² Li et al demonstrated that Irisin enhances Bcl-2 levels and decreases the expression of Bax and its downstream effector caspase-3 in septic myocardial cells, thereby mitigating sepsis-induced apoptosis and offering therapeutic potential for SIMI.⁵⁶ Furthermore, Shao et al observed in SIMI mice that Gastrodin decreased the expression of Caspase-1, IL-1β, and Bax in myocardial cells while upregulating Bcl-2 expression, thereby reducing LPS-induced myocardial apoptosis and improving cell viability.⁵⁷

In summary, during sepsis, the accumulation of ROS and mitochondrial dysfunction, induced by endotoxins and various stress stimuli, leads to an upregulation of pro-apoptotic proteins and a downregulation of anti-apoptotic proteins. Simultaneously, Bax is relocalized to the mitochondrial surface, forming pores in the mitochondrial membrane, which decreases membrane potential and increases membrane permeability. This facilitates the release of pro-apoptotic factors, thereby triggering cell apoptosis. These events contribute to the myocardial damage observed in sepsis. Collectively, these findings provide a robust theoretical foundation for the targeted treatment of myocardial cell apoptosis in SIMI.

Mitophagy in SIMI

Autophagy is a process that involves the sequestration of damaged proteins and organelles within double-membrane vesicles known as autophagosomes, which are subsequently delivered to lysosomes for degradation. It plays a key role in cellular homeostasis and is also implicated in mechanisms of cell death. In sepsis animal models induced by CLP or LPS, activation of autophagy and mitochondrial depletion have been observed in the heart.^58–60 The role of autophagy in sepsis and organ damage remains a subject of controversy. In SIMI, autophagy activation has been shown to mitigate myocardial cell damage and aid in the restoration of cardiac function.^61,62 Conversely, some studies have found that inhibition of autophagy can improve myocardial damage caused by sepsis.⁶³ The prevailing view in the literature suggests that autophagy is suppressed in SIMI, and its activation holds promise for alleviating SIMI.⁶⁴ Additionally, in sepsis animal models, a reduction in the co-localization of autophagosomes and lysosomes, coupled with impaired autophagic interaction and decreased degradation, has been observed, potentially leading to ATP depletion and cardiac dysfunction. Interestingly, stimulation with rapamycin has been shown to restore these processes.⁶⁵

LPS stimulation not only induces myocardial autophagy but also triggers a more selective form of autophagy.⁵⁸ Mitochondrial autophagy, which involves the removal of damaged mitochondria, may play a crucial role in mitigating cardiac and mitochondrial dysfunction. Mitochondrial autophagy is a selective form of autophagy that acts as a compensatory mechanism in response to the inflammatory response induced by sepsis. This process is triggered by severe stress, increased ROS expression, a higher number of damaged mitochondria, and the involvement of factors such as Bcl-2 family proteins, Parkin, and BNIP3.⁶⁶ The primary mechanism underlying mitochondrial autophagy involves the activation of the PTEN induced putative kinase 1 (PINK1)/Parkin pathway, with PINK1 enhancing this process by increasing the dynamics of ATP levels.

In the process of mitophagy, cells initiate the clearance of damaged mitochondria through multiple pathways, which include both ubiquitin-dependent and -independent mechanisms. The ubiquitin-dependent pathway is regarded as the primary route for mitophagy. In this pathway, stress-induced stimuli result in the loss of mitochondrial membrane potential and the cleavage of OPA1. PINK1 is phosphorylated on the OMM, where it facilitates the recruitment of Parkin, an E3 ubiquitin ligase.⁶⁷ PINK1-mediated phosphorylation of mitofusin 2 triggers the activation of Parkin, leading to the ubiquitination of various mitochondrial proteins, including mitofusins and voltage-dependent anion channels (VDAC).⁶⁸ This process is followed by the recruitment of mitophagy receptors through Parkin’s ubiquitin-binding domain. The ubiquitin chains, generated by Parkin, are phosphorylated by PINK1, establishing a positive feedback loop that anchors Parkin to the mitochondrial membrane and further drives mitophagy.⁶⁹ The rupture of the outer mitochondrial membrane subsequently releases IMM proteins, such as PHB2, which are recognized by autophagy receptors.^70,71 These receptors, which possess an LC3-interaction region (LIR), activate autophagic machinery, facilitating the degradation and removal of dysfunctional mitochondria.^70,71 Thus, PINK1 serves as a critical biological sensor for mitochondrial health, orchestrating the activation of Parkin to promote mitochondrial turnover (Figure 3).

Figure 3 Main pathway of mitophagy in SIMI. Mitophagy primarily consists of two pathways: the ubiquitin-dependent pathway and the ubiquitin-independent pathway. In the ubiquitin-dependent pathway, stress-induced mitochondrial membrane potential depolarization leads to the accumulation of PINK1 on the outer mitochondrial membrane (OMM), where it activates the E3 ligase activity of Parkin. Subsequently, Parkin ubiquitinates outer mitochondrial membrane proteins, and the ubiquitin chains are further phosphorylated by PINK1. The phosphorylated ubiquitin-modified outer membrane proteins are then recognized by autophagy adaptors, such as OPTN, which initiate the mitophagy process. Additionally, PINK1 can directly recruit autophagy adaptors like OPTN to the mitochondria via ubiquitin phosphorylation, thereby promoting mitophagy. In the ubiquitin-independent pathway, proteins located in the outer mitochondrial membrane, such as BNIP3, FUNDC1, and Bcl2-L-13, as well as cardiolipin in the IMM, can bind to LC3 and subsequently initiate mitophagy. Blue and black colored arrows present for direct processes. Black colored dotted squared box present for ub dependent pathways or ub independent pathways. Abbreviations: OPTN, Optineurin; ULK1, UNC-51-like kinase 1; LC3, light chain 3; BNIP3, bcl2 interacting protein 3; NIX, NIP3-like protein X; FUNDC1, FUN14 domain-containing protein 1; SMURF1, Smad ubiquitin regulatory factor 1; MUL1, mitochondrial ubiquitin ligase 1; Gp78, glycoprotein 78.

Moreover, studies have shown that melatonin treatment can enhance Parkin-dependent mitophagy by upregulating Beclin1 levels, thereby mitigating the pathological effects of cardiomyopathy and promoting the expression of PINK1 and Parkin.⁷² In addition to Parkin, mitochondrial ubiquitin ligase 1 (MUL1) also plays a role as a mitophagy receptor in the ubiquitin pathway. Research indicates that MUL1 can activate NF-κB and induce the ubiquitination and degradation of mitochondrial fusion proteins, even in the absence of Parkin.⁷³

In contrast, the ubiquitin-independent pathway involves proteins located in the outer mitochondrial membrane, such as Bcl2 interacting protein 3(BNIP3), FUN14 domain-containing protein 1 (FUNDC1), and Bcl2-L-13, as well as cardiolipin in the IMM, which can directly interact with LC3 and bind to autophagosomes under conditions of extreme stress.⁷⁴ For example, FUNDC1 facilitates Parkin-independent mitophagy in response to hypoxia by interacting with LC3, and inhibition of FUNDC1-mediated mitophagy increases mitochondrial apoptosis under pathological conditions.⁷⁵ Both BNIP3 and NIP3-like protein X(NIX) integrate into the outer mitochondrial membrane through their glycine-zipper transmembrane structures and engage with LC3, thus indirectly promoting autophagy by disrupting the Bcl-2–Beclin1 interaction.⁷⁶ Additionally, Bcl2-L-13 recruits LC3 to the mitochondrial surface via its LIR domain, driving the formation of mitophagosomes and promoting mitophagy.⁷⁷ Cardiolipin, a direct mediator of mitophagy, translocates to the outer mitochondrial membrane upon membrane rupture and binds to the N-terminal region of LC3.⁷⁸ Under conditions of severe stress, this interaction triggers the release of apoptotic factors, such as cytochrome c.⁷⁸

The regulation of mitophagy has emerged as a promising therapeutic approach for SIMI. For example, in a murine model of sepsis induced by LPS, α-ketoglutarate (AKG) has been demonstrated to facilitate the clearance of damaged mitochondria through the enhancement of mitophagy and mitochondrial fission, ultimately alleviating the cardiac dysfunction associated with septic cardiomyopathy.⁷⁹ In the context of SIMI, transmembrane Bax inhibitor motif containing 1 (TMBIM1) has been identified as a key mediator that interacts with Parkin, promoting its translocation from the cytosol to damaged mitochondria.⁸⁰ This interaction activates mitophagy, enhancing the removal of dysfunctional mitochondria and mitigating myocardial injury.⁸⁰ Additionally, in a rat model of sepsis induced by CLP, the traditional herbal medicine Po-Ge-Jiu-Xin decoction has been shown to activate the PINK1/Parkin pathway, thus facilitating mitophagy, preserving mitochondrial quality, and improving cardiac function in SIMI.⁸¹ Whereas, it is crucial to note that excessive activation of mitophagy, aimed at improving myocardial injury, should be approached with caution. Overstimulation of mitophagy may lead to residual dysfunctional mitochondria and insufficient ATP production, potentially exacerbating heart failure. For instance, 4-hydroxy-trans-2-nonenal has been reported to inhibit the expression of PINK1/Parkin by activating aldehyde dehydrogenase 2 (ALDH2), thereby preventing excessive mitophagy, reducing inflammation, and mitigating myocardial dysfunction in LPS-induced injury.⁸²

Ferroptosis in SIMI

Ferroptosis is a form of iron-dependent, regulated cell death.⁸³ Under normal conditions, an oxidative system composed of iron ions, the Fenton reaction, and ROS counteracts the antioxidant defense system, which includes the cystine/glutamate antiporter (system Xc⁻), glutathione peroxidase 4 (GPX4), and glutathione (GSH), as well as other recently identified pathways. This balance helps maintain cellular and organismal homeostasis. Research indicates that ferroptosis plays a critical role in the pathogenesis of various conditions such as atherosclerosis, myocardial ischemia-reperfusion injury, SIMI, drug-induced heart failure, arrhythmias, and diabetic cardiomyopathy.^84,85

As an iron-dependent form of programmed cell death, ferroptosis typically occurs in environments characterized by metabolic dysregulation and oxidative stress.⁸⁶ For instance, during the bacterial infection phase in severe pancreatitis, ferroptosis in intestinal epithelial cells can disrupt the intestinal barrier, facilitating the entry of harmful gut bacteria and toxins into the bloodstream, thereby exacerbating systemic inflammation and organ damage in sepsis.⁸⁷ In the progression of sepsis, mitochondrial dysfunction in cardiomyocytes is considered a crucial factor in the release of inflammatory mediators and oxidative stress, representing a key mechanism of SIMI. Ferroptosis is closely associated with dysregulated inflammatory responses, and in LPS-induced myocardial injury models, morphological changes in mitochondria align with characteristic features of ferroptosis, suggesting a significant link between mitochondrial dysfunction and ferroptosis.⁸⁸ Moreover, mitochondria, as the energy factories of cells, are essential for normal cellular metabolism, and mitochondrial dysfunction often serves as a catalyst for various diseases and widespread cell death.^89,90 Thus, targeting ferroptosis may provide an effective strategy to maintain mitochondrial homeostasis in SIMI. By modulating ferroptosis, it may be possible to alleviate cellular damage and inflammatory responses in SIMI, thereby mitigating myocardial injury and improving immune function. The following sections will discuss the classical pathways of ferroptosis and the key molecules involved in its role during the development of SIMI.

The Classical Pathway of Ferroptosis in SIMI

Ferroptosis can be initiated through both exogenous (transport protein-dependent) and endogenous (enzyme-regulated) pathways. In the exogenous pathway, cellular stressors such as infection, inflammation, or oxidative stress can inhibit membrane transport proteins, particularly the cystine/glutamate antiporter (System Xc-). System Xc- is a vital component of the cellular antioxidant defense system, consisting of two subunits: solute carrier family 7 member 11(SLC7A11), responsible for the high-affinity transport of cystine and glutamate, and solute carrier family3 member 2 (SLC3A2), which functions as a chaperone. This system exchanges extracellular cystine (Cys2) for intracellular glutamate in a 1:1 ratio. Upon internalization, cystine is converted into GSH by the action of glutamate cysteine ligase (GCL) and glutathione synthetase (GSS). GSH plays a critical role as a reductive cofactor for GPX4, an enzyme involved in protecting cellular membranes from oxidative damage. Inhibition of System Xc- disrupts cystine uptake, limiting GSH synthesis and thereby impairing GPX4 activity.^91,92 This results in a weakened cellular antioxidant defense, increasing susceptibility to ferroptosis^91,92. Concurrently, transferrin (including serum transferrin or lactoferrin) facilitates iron uptake through the transferrin receptor (TFRC), and ferritin subunits (FTH1/FTL) contribute to iron accumulation via autophagic degradation, both facilitating ferroptosis.^93,94

The endogenous pathway of ferroptosis is primarily regulated by the inhibition of antioxidant enzymes, such as GPX4. The accumulation of lipid peroxides, a hallmark of ferroptosis, is catalyzed by GPX4, which reduces lipid hydroperoxides (L-OOH) to their corresponding alcohols (L-OH).^40,93 Direct inhibition of GPX4 activity, for instance by the ferroptosis inducer RSL3, suppresses its antioxidant function, leading to an increase in reactive oxygen species ROS and the accumulation of toxic lipid peroxides.⁹⁵ Alternatively, indirect inhibition occurs through the depletion of selenium-cysteine, a critical amino acid in GPX4’s active site. This reduction can result from downregulation of the mevalonate pathway, which affects the maturation of selenium-cysteine tRNA, ultimately reducing GPX4 expression and activity, thereby triggering ferroptosis.⁹⁶ In addition to GPX4 regulation, other enzymes are involved in ferroptosis. Long-chain acyl-CoA synthetase 4 (ACSL4) and lysophosphatidylcholine acyltransferase 3 (LPCAT3) are crucial for the incorporation of polyunsaturated fatty acids (PUFAs) into phospholipids, generating PUFA-containing phospholipids (PUFA-PLs). These PUFA-PLs are prone to oxidation by free radicals, a process facilitated by lipoxygenases (ALOXs). The oxidation of PUFA-PLs leads to the disruption of the lipid bilayer, impairing cellular membrane integrity and further promoting ferroptosis⁹⁷ (Figure 4).

Figure 4 Main pathway of feroptosis in SIMI. Ferroptosis occurs through both exogenous (transport protein-dependent) and endogenous (enzyme-regulated) pathways. In the exogenous pathway, cysteine is transported into the cell via the System Xc- transporter, where it participates in the synthesis of GSH. Inhibition of this pathway impedes GSH synthesis, reduces the activity of the membrane lipid repair enzyme GPX4, and decreases the cell’s antioxidant capacity, thereby promoting ferroptosis. Additionally, iron enters the cell through transferrin, elevating intracellular iron levels and triggering ferroptosis. The endogenous/enzyme-regulated pathway is primarily mediated by intracellular antioxidant enzymes, such as GPX4. Black colored arrows present for direct processes. Black colored dotted squared box present for mitochondrial dysfunction process or labile iron pool. Abbreviations: GSH, glutathione; GPX4, glutathione peroxidase 4; GLS2, glutaminase2; DXZ, dexrazoxane; SIRT1, sirtuin1.

The Key Factors of Ferroptosis in SIMI

GPX is a member of a protein superfamily, with its catalytic site containing cysteine as a redox-active residue, facilitating redox reactions.^98,99 Among the eight members of human GPX, GPX4 is considered to play a critical role in ferroptosis. GPX4 plays a pivotal regulatory role during ferroptosis, and its unique physiological function is to convert lipid hydroperoxides into non-toxic lipid alcohols, thus inhibiting lipid peroxidation. When GPX4 synthesis is insufficient or its activity is inhibited, the accumulation of peroxides within cells leads to ferroptosis. GPX4 is often a key regulatory node in ferroptosis triggered by various inducers.¹⁰⁰ Recent studies have also found the generation of ROS during sepsis, which triggers ferroptosis.¹⁰¹ In a mouse model of LPS-induced sepsis, LPS upregulates the expression of ferroptosis markers, such as prostaglandin-endoperoxide synthase 2 (PTGS2), malondialdehyde (MDA), and lipid ROS, leading to ferroptosis and mitochondrial damage, while iron chelators like Fe-1 and dexrazoxane (DXZ) alleviate this damage.¹⁰² Similarly, overexpression of GPX4 can attenuate palmitic acid-induced ferroptosis in myocardial cells, whereas low expression of GPX4 significantly reduces its protective effect against ferroptosis.¹⁰³ GPX4 is crucial in regulating ferroptosis in the pathophysiology of septic myocardial injury. For instance, Oligo-peptide I-C-F-6 activates the Nrf2/HO-1/GPX4 pathway, alleviating myocardial oxidative damage and inhibiting ferroptosis in septic mice.¹⁰⁴ Dapagliflozin, by inhibiting the translation of key proteins involved in ferroptosis such as GPX4, FTH1, and SLC7A11, suppresses ferroptosis and improves SIMI.¹⁰⁵ Therefore, targeting GPX4 appears to be a promising strategy for improving SIMI.

Another key protein involved in ferroptosis is the cystine/glutamate antiporter SLC7A11, which primarily functions in the reverse transport of extracellular cystine into cells, contributing to the formation of GSH and antioxidant defense.¹⁰⁶ Modulating the activity of SLC7A11 can target ferroptosis to improve sepsis-induced myocardial damage. METTL3-mediated m6A methylation of SLC7A11 enhances its mRNA methylation level, leading to mRNA decay and consequently promoting ferroptosis in SIMI.¹⁰⁷ Erastin, a specific inducer of ferroptosis, reduces intracellular GSH levels by inhibiting System Xc-, thus triggering ferroptosis.⁴⁰ In animal and cell models induced by LPS, Cao et al found that sodium hydrosulfide (NaHS) inhibits the phosphorylation of Beclin 1, an endogenous binding protein of SLC7A11, increases the expression levels of SLC7A11 and GPX4, and alleviates iron metabolism disorders and oxidative stress, thereby inhibiting ferroptosis and improving SIMI.¹⁰⁸ Additionally, the traditional Chinese medicine YiQiFuMai injection targets the SLC7A11/GPX4 axis to suppress ferroptosis and improve SIMI.¹⁰⁹ As a primary intracellular antioxidant and the source of GSH, SLC7A11 can inhibit lipid peroxidation. Similarly, in an LPS-induced sepsis mouse model, Cyclovirobuxine D upregulates the expression of SLC7A11, inhibits ferroptosis, and ameliorates sepsis-induced myocardial damage.¹¹⁰

In addition to GPX4 and SLC7A11, p53 plays an important role in ferroptosis during SIMI. Previous studies have confirmed that p53 specifically inhibits the System Xc- system by downregulating the expression of SLC7A11, thereby inducing ferroptosis.¹¹¹ Moreover, mitochondrial dysfunction in a p53-dependent manner is observed in the pathogenesis of SIMI.¹¹² Lin et al discovered that quercetin alleviates SIMI by upregulating GPX4 and ferritin levels and downregulating iron and prostaglandin-endoperoxide synthase 2 levels via the sirtuin 1 (SIRT1)/p53/SLC7A11 signaling pathway, thereby ameliorating ferroptosis.¹¹³ Other molecules also play important roles in ferroptosis. Heme oxygenase-1 (HO-1), a major source of intracellular iron, has been shown by Kwon et al to induce lipid peroxidation and consequently trigger ferroptosis.¹¹⁴

In summary, during infection, mitochondrial dysfunction, excessive ROS production, and dysregulated inflammatory responses inhibit key ferroptosis proteins, including GPX4 and SLC7A11, leading to iron overload and ferroptosis in myocardial cells, which contributes to SIMI. Thus, targeting ferroptosis and mitochondrial dysfunction represents a crucial strategy for improving SIMI.

Mitochondrial Dysfunction Induces Calcium Homeostasis Imbalance in SIMI

The precise regulation of intracellular calcium (Ca²⁺) levels is essential for cell survival, functional maintenance, and mitochondrial dynamics.^115,116 Intracellular Ca²⁺ levels are primarily controlled through two major pathways: one is Ca²⁺ influx via ion channels across the plasma membrane, and the other is the release of Ca²⁺ from intracellular stores such as the ER. The ER and mitochondria are the principal intracellular Ca²⁺ storage reservoirs. Mitochondria play a particularly crucial role in calcium homeostasis, not only serving as the primary ATP production factories within the cell but also being intimately linked to mitochondrial dynamics, function, and metabolism. Mitochondria are also involved in the regulation of microdomains within the cell, which govern key cellular processes.^117,118

Mitochondrial involvement in calcium signaling is regulated by two mechanisms.¹¹⁹ On the one hand, mitochondria possess a calcium buffering function, which allows them to handle intracellular Ca²⁺ levels ranging from 50 to 500 nM in various normal cell types.^120,121 Through this buffering capacity, mitochondria dynamically manage transient changes in calcium concentrations and buffer calcium overload associated with pathological states.¹²⁰ On the other hand, mitochondria are in close contact with the ER and plasma membrane calcium channels, forming microdomains that promote synchronized increases in mitochondrial Ca²⁺ concentrations and cytosolic Ca²⁺ signaling.¹²² Within the ER, inositol 1,4,5-trisphosphate (IP₃)-activated gated channels expose mitochondria to much higher Ca²⁺ concentrations than the cytosol, thereby increasing mitochondrial calcium levels.¹²³ Moreover, extracellular Ca²⁺ entering through plasma membrane channels can also enter mitochondria, further linking cellular activation to mitochondrial function.¹²⁰ The rapid diffusion of Ca²⁺ helps prevent calcium overload in organelles.

Furthermore, in cardiac mitochondria, calcium ion transport processes are facilitated by microdomains between the sarcoplasmic reticulum (SR) and mitochondria, which bring the calcium release and uptake sites closer together and are maintained by connecting proteins such as mitochondrial fusion proteins. This system ensures that mitochondrial calcium ions can synchronize with ATP production required for excitation-contraction coupling, thus regulating ATP generation.¹²⁴ Additionally, calcium ion delivery to mitochondria is crucial for maintaining mitochondrial antioxidant capacity and declining ROS generation caused by increased ATP synthesis.¹²⁵ Mitochondrial calcium overload is considered a major trigger for the opening of the mitochondrial permeability transition pore (mPTP), which, under oxidative stress, may lead to pathological organ damage.¹²⁶ Therefore, dysfunction of the calcium transport system plays a significant role in cardiac diseases, such as heart failure.¹²⁷

In cardiomyocytes, calcium homeostasis involves not only cytosolic Ca²⁺ but also mitochondrial Ca²⁺. Calcium channels on the IMM include the mitochondrial calcium uniporter (MCU), mitochondrial ryanodine receptors (mRyR), mPTP, mitochondrial sodium/calcium exchanger (mNCX), and hydrogen/calcium exchanger (mHCX).^128,129 Among these, MCU operates through the mitochondrial membrane potential, dependent on matrix and extracellular calcium concentrations, and is closely linked to oxidative phosphorylation processes and the activity of dehydrogenases in the tricarboxylic acid (TCA) cycle, thus providing energy for ATP synthesis.¹³⁰ The voltage-dependent anion channel (VDAC) is part of the mPTP, responsible for transporting calcium and metabolic products across the outer mitochondrial membrane.¹³¹ Notably, about 20% of the mitochondrial surface is adjacent to the ER, forming a high-calcium microenvironment known as mitochondrial-associated membranes (MAMs).¹³² During calcium-induced calcium release (CICR) in the autonomic nervous system, the ER releases large amounts of calcium through the RyR, which enters mitochondria via VDAC. Subsequently, the sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) reabsorbs calcium to prevent excessive cytosolic calcium accumulation, maintaining calcium homeostasis. Dysfunction of these calcium channels, particularly inhibition of L-type calcium channels (LTCC), SERCA inhibition, increased RyR calcium leakage, and decreased myofilament calcium sensitivity, is closely associated with the occurrence and progression of SIMI.¹³³ For instance, during SIMI, reduced sensitivity of L-type calcium channels leads to diminished RyR activity, impairing the influx of extracellular calcium into cardiomyocytes via L-type calcium channels, affecting calcium uptake and further hindering calcium binding to troponin, thereby disrupting myocardial contraction activation.¹³⁴

In sepsis, due to calcium leakage from the SR and decreased calcium uptake by organelles, mitochondrial calcium concentration increases.¹³⁵ Moreover, mitochondrial dysfunction activates NADPH oxidase 2 (NOX2), leading to abnormal calcium handling and damaging myocardial contraction. Targeting NOX2 and restoring calcium homeostasis is therefore considered a potential therapeutic strategy for improving SIMI. In murine models of sepsis induced by LPS and CLP, the NOX2 inhibitor apocynin alleviated oxidative stress in cardiomyocytes, maintained intracellular calcium homeostasis and mitochondrial function, and mitigated the contractile dysfunction induced by sepsis.¹³⁶ Targeting mitochondrial calcium efflux or calcium uptake can also alleviate SIMI. Mitochondrial calcium efflux is regulated by the PINK1-PKA-NCLX axis. In CLP-induced sepsis mouse models, reduced expression of PINK1 in cardiomyocytes disrupts the PINK1-PKA-NCLX axis, leading to disordered mitochondrial calcium efflux, mitochondrial calcium overload, and inhibition of mitochondrial autophagy, preventing the clearance of damaged mitochondria, thereby contributing to SIMI.¹³⁷ Jiang et al found that LPS increased the formation of MAMs and intracellular calcium levels, enhanced expression of Cav1.2 and RyR2, reduced mitochondrial membrane potential and intracellular ATP levels, promoted mitochondrial fragmentation, expression of mitophagy proteins, and ROS generation in H9c2 cells, while Bazedoxifene and Stattic reversed this phenomenon, alleviating SIMI.¹³⁸ Similarly, after LPS administration, rodents exhibited increased mitochondrial Ca²⁺ content or mitochondrial Ca²⁺ overload, while preventing mitochondrial Ca²⁺ overload helped improve mitochondrial and myocardial dysfunction.^136,139 Additionally, reduced mitochondrial Ca²⁺ uptake was associated with increased survival rates in septic rats.¹⁴⁰ Therefore, the role of mitochondria in calcium homeostasis may serve as a promising target for management of SIMI.

Although existing research has demonstrated that disturbances in mitochondrial calcium efflux and uptake play a significant role in the exacerbation of SIMI, and that modulating this process holds potential for mitigating SIMI, the precise molecular mechanisms involved remain poorly understood. Therefore, extensive future research is required to comprehensively investigate and clarify these underlying mechanisms.

Mitochondrial Dysfunction Induces Metabolic Dysregulation in SIMI

Sepsis is a systemic inflammatory response triggered by infection, which leads to the dysfunction of multiple organs, particularly the heart. Mitochondria are crucial in cellular energy metabolism, predominantly producing ATP through oxidative phosphorylation (OXPHOS), which is essential for cell survival (Figure 5). However, during sepsis, alterations in cellular energy demands occur due to metabolic reprogramming, which subsequently affects the metabolic processes of immune cells and cardiomyocytes, ultimately leading to immune dysregulation and myocardial injury.

Figure 5 The Normal Metabolism in Myocardial Cells. Under normal conditions, myocardial cells uptake long-chain fatty acids via transport proteins, facilitating their oxidation within the mitochondria to produce acetyl-CoA and NADH. These metabolites are further involved in the tricarboxylic acid (TCA) cycle, subsequently entering the electron transport chain to generate ATP. Black or blue colored arrows present for direct processes. Black colored dotted squared box present for respiratory chain. Abbreviations: CPT1, carnitine palmitoyl transferase 1; CACT, carnitine-acylcarnitine translocase; CPT2, carnitine palmitoyl transferase 2; NADH, Nicotinamide adenine dinucleotide; TCA cycle, tricarboxylic acid cycle.

Metabolic reprogramming refers to a shift in cellular metabolic pathways that occurs in response to various environmental challenges, enabling cells to adapt their energy production mechanisms to meet the stresses or challenges posed by external factors. In the case of sepsis, almost all types of cells undergo metabolic reprogramming. Notably, in immune cells and certain organ cells, glycolysis replaces oxidative phosphorylation as the primary energy source, a phenomenon referred to as the “Warburg effect”.^141–143 This metabolic adaptation is critical for the function of immune cells during sepsis, and a similar reprogramming from oxidative phosphorylation to glycolysis has also been observed in cardiomyocytes¹⁴⁴ (Figure 6).

Figure 6 The cell shifts its metabolism from FAO-dependent oxidative phosphorylation (OXPHOS) to a predominantly glycolytic pathway in SIMI. During sepsis, cells undergo a metabolic shift from FAO-driven OXPHOS to glycolysis. Glycolysis generates ATP by producing pyruvate, which is subsequently converted to lactate, thereby fulfilling the energy demands of the immune response. Black or blue colored arrows present for direct processes. Black colored dotted squared box present for respiratory chain or aerobic glycolysis process. Abbreviations: GLUT, glucose transporter; HK2, hexokinase2; PFKFB3, 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3; PKM2, Pyruvate kinase M2; LDH, lactate dehydrogenase; MCT, medium chain triglyceride.

Research on metabolic reprogramming in SIMI has primarily focused on immune cells (such as macrophages) and cardiomyocytes.^144,145 Under normal conditions, cells uptake long-chain fatty acids through transport proteins, promoting their oxidation in mitochondria to generate acetyl-CoA, FADH2, and NADH. These metabolites participate in the tricarboxylic acid (TCA) cycle and enter the electron transport chain to produce ATP. However, during sepsis, cells undergo a metabolic shift from fatty acid oxidation (FAO)-driven OXPHOS to glycolysis. Glycolysis generates ATP by producing pyruvate, which is then converted to lactate to meet the energy demands of the immune response. Studies suggest that moderate glycolysis plays a protective role in sepsis by activating immune responses and regulating inflammation, but excessive glycolysis can lead to immune suppression, further exacerbating myocardial injury induced by sepsis.

Moderate Glycolysis Activates the Immune System, Inducing Inflammatory Responses in SIMI

PRRs are pivotal in the innate immune response by detecting PAMPs and DAMPs. In the early phases of sepsis, the activation of PRRs initiates the immune response, triggering the onset of inflammation. During sepsis and SIRS, there is a metabolic shift in immune cells from oxidative phosphorylation to glycolysis. This metabolic reprogramming supports the rapid activation and proliferation of immune cells.^146,147 Specifically, glycolysis leads to increased glucose uptake and enhances metabolic pathways, ensuring immune cells acquire sufficient energy to address inflammatory challenges.^146,148 This reprogramming ensures a rapid supply of ATP, but the resulting metabolic intermediates also serve as essential nutrients for immune cell proliferation and biosynthesis. Studies have highlighted that the enhanced expression of GLUT1 facilitates glucose uptake, thereby promoting glycolysis and further supporting immune cell functions.^149,150 Inhibition of GLUT1 expression significantly reduces glucose uptake and glycolytic flux, resulting in a marked decrease in the viability and functional capacity of effector T cells (Teff).^149,151 The increased glucose uptake and glycolytic flux, alongside the high rate of ATP production via glycolysis, ensure that ATP derived from glycolysis meets the energy demands of activated immune cells, even in the presence of impaired OXPHOS. Metabolites such as glucose-6-phosphate (G-6-P), dihydroxyacetone phosphate (DHAP), and glyceraldehyde 3-phosphate (G-3-P) generated through glycolysis provide sufficient energy and biosynthetic precursors, thereby facilitating immune activation and enabling more efficient and rapid immune responses.

Consequently, limiting glycolysis may attenuate organ damage associated with the “cytokine storm” in the early stages of sepsis.^152,153 Research has identified mTOR as a key regulator of cellular metabolism. Inhibition of mTOR through rapamycin or its derivatives activates downstream targets such as HIF-1α and GLUT1, thereby promoting glucose uptake and enhancing glycolysis.^154,155 Furthermore, rapamycin suppresses LPS-induced pro-IL-1β synthesis and the subsequent ATP-driven release of IL-1β.¹⁵⁶ HIF-1α, as a central regulator of immune cell metabolism during sepsis, drives the expression of GLUT1 and LDH, stimulating glycolytic metabolism in inflammatory cells, enhancing pro-inflammatory cytokine secretion, and triggering the cytokine storm in the early stages of sepsis. Thus, HIF-1α represents a promising therapeutic target for sepsis. SOCS1 serves as a negative regulator of the metabolic reprogramming during sepsis by inhibiting HIF-1α-induced hexokinase 1 (HK1) expression in macrophages.¹⁵⁷ Additionally, lidocaine effectively reduces the secretion of LPS-induced inflammatory cytokines by inhibiting the HIF-1α-mediated glycolytic pathway, exerting anti-inflammatory effects.¹⁵⁸ miR-223, by downregulating HIF-1α and modulating the glycolytic pathway, facilitates M2 macrophage polarization, thereby inducing an anti-inflammatory state.¹⁵⁹

Macrophages play a critical role in the immune response during sepsis and SIMI, particularly in their polarization into pro-inflammatory (M1) and immune-suppressive (M2) phenotypes.¹⁶⁰ M1 macrophages predominantly contribute to the inflammatory response through glycolysis and lipid biosynthesis. These cells are induced by LPS, TLR ligands, or interferon-γ (IFN-γ). Lipid biosynthesis is crucial for membrane remodeling and the production of inflammatory mediators in M1 macrophages.¹⁶¹ And it is transcriptionally regulated by sterol regulatory element-binding proteins (SREBPs). Notably, SREBP-1a is highly expressed in macrophages, regulating lipid biosynthesis and promoting M1 polarization.¹⁶² Fatty acid synthase (FAS), a key enzyme in fatty acid biosynthesis, is integral to the inflammatory responses in M1 macrophages. Wei et al have shown that FAS deficiency in macrophages prevents the recruitment of adipose tissue macrophages and inflammation in mice.¹⁶³

Targeting glycolytic pathways can also promote M2 macrophage polarization, presenting a potential therapeutic strategy for sepsis.¹⁵⁹ For example, methylsulfonylmethane promotes the expression of Arg1 via the lactate H3K18la pathway, inducing M2 macrophage polarization and offering protective effects against methicillin-resistant Staphylococcus aureus infections.¹⁶⁴ Additionally, Sema7A induces metabolic reprogramming in macrophages through the activation of mTOR and AKT2 signaling pathways. This reprogramming is characterized by reduced fatty acid oxidation (FAO) and oxidative phosphorylation, enhanced glycolysis and the pentose phosphate pathway (PPP), and impaired Krebs cycle activity, ultimately favoring M2 macrophage polarization.¹⁶⁵ Furthermore, exosomes derived from human bone marrow mesenchymal stem cells have been shown to downregulate glycolysis in sepsis-induced macrophages and promote M2 polarization.¹⁶⁶ In summary, metabolic reprogramming during the early stages of sepsis enhances glycolysis, facilitating appropriate inflammatory responses that provide organ protection.

Excessive Glycolysis Can Induce Persistent Low-Grade Inflammation and Immune Paralysis in SIMI

Sepsis, particularly in its later stages, exhibits a complex shift in metabolic pathways that significantly affects the immune response. While glycolysis plays a crucial role in the early stages of sepsis by supporting immune cell activation, sustained glycolytic activity in the later phases can contribute to immune suppression. This immune dysregulation manifests as a reduced expression of genes encoding pro-inflammatory cytokines (such as TNF-α and IL-6) and chemokines that recruit T cells (such as CCL2), while the expression of anti-inflammatory cytokines, such as IL-4 and IL-10, is increased.^167,168 Consequently, the host becomes less capable of mounting an effective response to secondary infections. This immune suppression is associated with a metabolic shift from glucose metabolism to fatty acid oxidation (FAO), where increased FAO facilitates the transition of immune cells from a pro-inflammatory to an anti-inflammatory phenotype. For instance, PPAR-γ, by promoting β-oxidation of fatty acids, suppresses the pro-inflammatory response of macrophages, thereby contributing to immune suppression.¹⁶⁹ Sirtuins, key regulators of metabolism and energy sensing, further influence immune responses by modulating the transition between glycolysis and FAO. SIRT1 and SIRT6, through the SIRT1-RELB-SIRT6 axis, regulate metabolic reprogramming that supports the metabolic shift from glycolysis to FAO. Liu et al demonstrated that SIRT6 inhibits HIF-1α-dependent glycolysis, while SIRT1 enhances PGC-1-dependent fatty acid flux and oxidation.¹⁷⁰ This transition from glucose to fatty acid metabolism represents a critical aspect of metabolic reprogramming associated with the early and late stages of sepsis and SIMI.¹⁷⁰

In addition to its role in promoting an anti-inflammatory immune phenotype through FAO, lactate, produced during glycolysis, acts as an important signaling molecule in sepsis and organ injury.^171,172 Lactate interacts with receptors such as GPR81 and GPR132, contributing to the pathophysiology of sepsis and organ damage.^173–175 It has been shown that lactate inhibits TLR/NF-κB signaling and the production of pro-inflammatory cytokines by macrophages.¹⁷⁶ Zhang et al found that elevated intracellular lactate levels inhibit RLR-mediated IFN activation in M1 macrophages.¹⁷⁷ Liu et al demonstrated that increased lactate levels in macrophages promote an M2 phenotype through HIF-2α, inducing the expression of M2-associated genes such as vascular endothelial growth factor(VEGF), Mannose receptor C- type 1(MRC1), arginase 1(ARG1), and Retnla.¹⁷⁸ Furthermore, lactate, when transported out of the cell via the Monocarboxylate Transporter 4(MCT4), can influence immune cell migration and cytokine production, inducing “migration cessation” signals mediated by lactate transporters SLC5A12 (in CD4+ T cells) and SLC16A1 (in CD8+ T cells).¹⁷⁹ Thus, excessive lactate production exacerbates immune suppression and contributes to both sepsis progression and myocardial damage.

In summary, during sepsis and associated myocardial injury, metabolic reprogramming profoundly impacts the host’s immune response and organ function. While moderate metabolic reprogramming can effectively activate the immune system and protect organs from damage, excessive glycolysis may lead to immune paralysis and the onset of secondary infections. Therefore, targeting immune cell metabolism and limiting excessive glycolysis could represent promising therapeutic strategies in the treatment of sepsis.

Targeted Therapy of SIMI Focusing on Mitochondrial Dysfunction

The previous discussion comprehensively examined the role of mitochondrial dysfunction in Sepsis-induced Myocardial Injury (SIMI), delving into its contributions through various mechanistic pathways, including dysregulated inflammation and immune responses, disturbances in calcium homeostasis, and metabolic reprogramming. As the central organelle responsible for energy production and cellular homeostasis, mitochondria play a pivotal role in regulating these processes, and their dysfunction in the context of SIMI is emerging as a critical factor driving disease progression. Consequently, targeting mitochondrial dysfunction has been identified as a promising therapeutic strategy for mitigating SIMI, with several compounds demonstrating potential in reversing the deleterious effects of sepsis on myocardial tissue (Table 1).

Table 1 Therapeutic Strategies Based on RCD, Calcium Homeostasis Imbalance and Metabolic Reprogramming Associated with Mitochondrial Dysfunction in SIMI

For example, the selective estrogen receptor modulator Bazedoxifene and the Src kinase inhibitor Stattic have been shown to effectively counteract LPS-induced myocardial inflammation. These compounds not only improve mitochondrial membrane potential and restore intracellular ATP levels, but also reduce mitochondrial fragmentation, the expression of mitophagy-related proteins, and the production of ROS in H9c2 cells, thereby alleviating the cellular stress and dysfunction that characterize SIMI.¹³⁸ In addition, the oligo-peptide I-C-F-6, through activation of the Nrf2/HO-1/GPX4 signaling pathway, has been demonstrated to significantly attenuate myocardial oxidative damage in sepsis mice, offering a potential therapeutic avenue by inhibiting ferroptosis, a form of regulated cell death linked to iron and ROS overload.¹⁰⁴ Given the intricate interplay between dysregulated inflammatory responses, various forms of regulated cell death, calcium dysregulation, and metabolic shifts, these factors collectively contribute to the pathophysiology of sepsis and SIMI. However, targeting a single molecular pathway may prove insufficient in addressing the multifaceted nature of this condition. As such, multi-target therapeutic approaches have gained traction as a promising direction for future research. In this regard, therapeutic strategies that can simultaneously modulate mitochondrial function, restore calcium homeostasis, regulate inflammatory responses, and mitigate cellular metabolic shifts may hold the key to more effective interventions in SIMI. However, the complexity of sepsis-induced myocardial injury—characterized by a dynamic and interconnected network of factors such as inflammation, mitochondrial dysfunction, regulatory cell death, and metabolic alterations—calls for more comprehensive, multi-dimensional therapeutic approaches. Future research should prioritize the development of such multi-target strategies, combining novel molecular agents that target distinct but interconnected pathways involved in SIMI. This approach not only promises to provide a more holistic treatment framework but also has the potential to offer new theoretical insights into the prevention and management of SIMI. Ultimately, this comprehensive understanding could lead to the establishment of innovative therapeutic paradigms, enhancing the clinical outcomes of patients suffering from this devastating condition.

Conclusion

Mitochondrial dysfunction plays a central role in the pathogenesis of SIMI, directly influencing cardiomyocyte viability and cardiac function. As sepsis progresses, mitochondrial dysfunction initiates and amplifies a cascade of inflammatory responses, culminating in disturbances in calcium homeostasis, activation of regulated cell death pathways (including apoptosis, pyroptosis, mitophagy, and ferroptosis), and metabolic reprogramming. These processes contribute to the profound myocardial damage observed in SIMI, leading to significantly increased mortality rates in sepsis patients.

Initially, mitochondrial dysfunction in sepsis is marked by a reduction in mitochondrial membrane potential, impaired ATP production, and excessive ROS generation. This triggers the release of mtDAMPs, which activate abnormal inflammatory responses and further escalate cellular injury. Additionally, mitochondrial calcium overload exacerbates oxidative stress, creating a vicious cycle that accelerates myocardial injury. The reprogramming of cellular metabolism, shifting from oxidative phosphorylation to aerobic glycolysis in the early stages of sepsis, contributes to a low-grade inflammatory state and immune suppression in later stages, further worsening sepsis and systemic organ damage.

These findings underscore the multifactorial nature of SIMI and highlight the urgent need for more targeted therapeutic approaches that address both the mitochondrial dysfunction and the associated inflammatory, metabolic, and cellular pathways. While promising therapeutic strategies, including biomolecules and phytochemicals, have shown potential in preclinical studies, their clinical application remains limited. Future research should prioritize the identification of key signaling pathways involved in SIMI and the development of novel therapeutic targets. By improving our understanding of the complex interplay between mitochondrial dysfunction, inflammatory responses, and metabolic alterations, we can develop more effective strategies for treating SIMI, ultimately improving patient outcomes in sepsis.