Macrophage Efferocytosis in Cardiac Pathophysiology and Repair

Abstract

As an integral component of cardiac tissue, macrophages are critical for cardiac development, adult heart homeostasis, as well as cardiac healing. One fundamental function of macrophages involves the clearance of dying cells or debris, a process termed efferocytosis. Current literature primarily pays attention to the impact of efferocytosis on apoptotic cells. However, emerging evidence suggests that necrotic cells and their released cellular debris can also be removed by cardiac macrophages through efferocytosis. Importantly, recent studies have demonstrated that macrophage efferocytosis plays an essential role in cardiac pathophysiology and repair. Therefore, understanding macrophage efferocytosis would provide valuable insights on cardiac health, and may offer new therapeutic strategies for the treatment of patients with heart failure. In this review, we first summarize the molecular signals that are associated with macrophage efferocytosis of apoptotic and necrotic cells, and then discuss how the linkage of efferocytosis to the resolution of inflammation affects cardiac function and recovery under normal and diseased conditions. Lastly, we highlight new discoveries related to the effects of macrophage efferocytosis on cardiac injury and repair.

Fig. 1.. “Find-me” signals and their respective receptors for apoptotic and necrotic cells.

As cells undergo apoptosis, various “find-me” signals [i.e., nucleotides (ATP/UTP), chemokine (CX3CL1), and lipids (LPC and S1P)] are secreted or exposed on the outer leaflet of plasma membrane. Pannexin 1 (PANX1) is important membrane channel responsible for ATP/UTP export, whereas LPC is released by activated ABCA1. Upon binding to receptors, these find-me signals stimulate chemotaxis of macrophages to injured site. Meanwhile, they could promote resolution of inflammation by suppressing gene expression of proinflammatory factors (i.e., CXCL1 and CXCL2), and upregulating anti-inflammatory genes (i.e., IL-10 and VEGF) as well as pro-resolving mediators (i.e., Nr4a1, Thbs1). The efferocytic macrophage-mediated resolution of inflammation is primarily through the inhibition of NF-κB pathway and the activation of PPARγ signaling cascade. Importantly, interaction of CX3CL1 to its receptor also promotes MFGE8, a key bridging molecule of “eat-me” signal. On the other hand, necrotic cells elicit unique “find-me” signaling. For example, formylated peptides are released from damaged mitochondria and interact with FRP1 on macrophages to increase chemotaxis. LTB4 is secreted through MVBs-exosomes that can act in conjunction with formylated peptides. H₂O₂ is shown to activate Src family of kinase Lyn, leading to enhanced monocyte/macrophage recruitment. In addition, H₂O₂ can regulate other “find-me” molecules such as formylated peptide, LTB4, and IL-8. Complement proteins can be deposited to target cells or released into extracellular area to interact with neighboring cells. FPR1 indicates formylated peptide receptor 1; LPC, lysophosphatidylcholine; LTB4, leukotriene B4; MVBs, multivesicular bodies; S1P, sphingosine-1-phosphate.

Fig. 2.. Apoptotic “eat-me” signals and their respective receptors.

The best characterized “eat-me” signal is phosphatidylserine (PtdSer, or PS), which is exposed to the outer leaflet of plasma membrane on dying cells. Macrophages can directly recognize PS through its interaction with receptors such as BAI, TIM-4, and CD36. Alternatively, PS can be bound indirectly to receptors on macrophages through “bridging proteins” such as MFGE8 for integrins, GAS6-ProS dimer for TAM tyrosine kinases. Other “eat-me” molecules include CRT and CD31. In coordination with MBL and C1q, CRT interacts with LRP1 and facilitates the recognition of PS by macrophages. In addition, CD31 activates integrins after binding to the bridging molecule fibronectin (FN). Recognition of eat-me signals switches macrophages to anti-inflammatory phenotype partly through the activation of nuclear receptors such as LXR and PPARγ. BAI indicates brain-specific angiogenesis inhibitor; C1q, complement factor C1q; CRT, calreticulin; GAS6, growth arrest-specific 6; LRP1, low-density lipoprotein (LDL) receptor-related protein 1; MBL, mannose-binding lectin; MFGE8, milk fat globule-EGF factor 8; ProS, protein S; TAM, Tyro3, Axl, Mer; TIM-4, T cell immunoglobulin mucin receptor 4.

Fig. 3.. Transfer of extracellular vesicles containing miR-26a from cardiosphere-derived cells to macrophages enhances efferocytosis of dying cardiomyocytes in MI models.

Extracellular vesicles released by cardiosphere-derived cells (CDCev) transfer miR-26a to targeting macrophages, leading to inhibition of ADAM17 mRNA translation. Consequently, ADAM17-mediated cleavage of MerTK is reduced, resulting in increased levels of active MerTK. In addition, CDCev also upregulate complement factor C1q in macrophages. Thus, increased levels of MerTK and C1q promote the recognition of “eat-me” signals and stimulate the efferocytosis of dying cardiomyocytes. Furthermore, sustained MerTK inhibits pro-inflammatory responses (i.e., reduced levels of IL-6, IL-1α/β, and TNFα) in macrophages via suppression of NF-κB and MAPK pathways. Therefore, augmented efferocytosis together with reduced inflammation results in improved cardiac repair after MI injury.