Phagocytosis of apoptotic cells in homeostasis

Abstract

Human bodies collectively turn over about 200 billion to 300 billion cells every day. Such turnover is an integral part of embryonic and postnatal development, as well as routine tissue homeostasis. This process involves the induction of programmed cell death in specific cells within the tissues and the specific recognition and removal of dying cells by a clearance 'crew' composed of professional, non-professional and specialized phagocytes. In the past few years, considerable progress has been made in identifying many features of apoptotic cell clearance. Some of these new observations challenge the way dying cells themselves are viewed, as well as how healthy cells interact with and respond to dying cells. Here we focus on the homeostatic removal of apoptotic cells in tissues.

Fig. 1

Homeostatic clearance of apoptotic cells via different phagocytes. In many tissues of the body, clearance of apoptotic cells is performed by the professional phagocytes (P), which include the tissue resident macrophages and immature dendritic cells. Many non-hematopoietic cells also have phagocytic functions in ex vivo or in vitro systems. These non-professional phagocytes (NP) include epithelial cells, hepatocytes and endothelial cells of the liver, astrocytes, oligodendrocytes and neuronal progenitor cells of the central nervous system^–, or the Muller’s glia of the eye. Satellite cells of the skeletal muscle have also been reported to engulf apoptotic myoblasts. Specialized phagocytes (SP) are multifunctional cells that engulf apoptotic cells and include retinal pigment epithelial cell (RPE), and Sertoli cells in the testes.

Fig. 2

Steps during phagocytosis of apoptotic cells. When a cell initiates the apoptotic program, it releases soluble ‘find-me’ signals that attract phagocytes. The apoptotic cell is distinguished from the nearby living cell via the exposure of ‘eat-me’ signals, the most prominent of which is the phosphatidylserine (PtdSer). Eat-me signals are recognized by different engulfment receptors on the phagocytes, resulting in signaling events that facilitate the apoptotic corpse uptake. Engulfment also elicits the transcriptional up-regulation of the cholesterol efflux transporter ABCA1, and increased expression of engulfment receptors. Within the mitochondria, the levels of the uncoupling protein UCP2 are increased, enabling the continued uptake of apoptotic corpses. Anti-inflammatory mediators are expressed and secreted, contributing to tissue homeostasis and inhibition of local inflammation. PtdSer, phosphatidylserine; P2Y2, purinergic P2 receptor Y2; CX3CR1, CX3C chemokine receptor-1; G2A, G protein-coupled receptor G2A; S1PR, sphingosine-1-phosphate receptor; ATP, adenosine triphosphate; UTP, uridine triphosphate; CX3CL1, chemokine (C-X3-C motif) ligand-1, also known as Fractalkine; LPC, lysophosphatydilcholine; S1P, sphingosine-1-phosphate; LXR, liver X receptor; RXR, retinoid X receptor; PPAR, peroxisome proliferator activated receptor; BAI1, brain angiogenesis inhibitor-1; TIM4, T cell immunoglobulin and mucin domain containing molecule-4; RAGE, receptor for advanced glycation end products; TREM-2, triggering receptor expressed on myeloid cells-2; TAM, Tyro Axl Mer family receptor; LRP1, low density lipoprotein receptor related protein-1; SCARF-1 is also known as SREC-1, scavenger receptor expressed by endothelial cell-1; Gas6, growth arrest specific-6; MFG-E8, milk fat globule EGF factor-8; TSP1, thrombospondin-1; ICAM3, intracelullar adhesion molecule-3; CRT, calreticulin.

Fig. 3

Signaling pathways elicited by three PtdSer recognition receptors. Binding of the apoptotic cell to the phagocyte triggers signaling pathways. BAI1 is a 7-transmembrane receptor that directly binds the PtdSer on the surface of an apoptotic cell, resulting in the recruitment of the Engulfment and cell motility (ELMO)/Downstream of Crk (DOCK) complex, which functions as a guanine exchange factor for the small GTPase Rac. Rac activation promotes actin cytoskeleton remodeling required for the engulfment of the apoptotic corpse. Integrins α_Vβ₃ or α_Vβ₅ and the Tyro Axl Mer (TAM) family receptors bind apoptotic cells indirectly, via PtdSer-bound bridging molecules MFG-E8, Gas6 or ProteinS, resulting in the activation of the focal adhesion kinase (FAK) and contributing to the activation of Rac. TAM receptors are tyrosine kinases that also activate cell signaling pathways involving the kinases Src and phosphatidylinositol-3-kinase (PI3K) and phospholipase C (PLC)^, . TIM4 functions as a tethering receptor bringing the apoptotic cell in contact with signaling engulfment receptors, and signal through co-receptors. The extent of the connection between the signals elicited by different engulfment receptors awaits further characterization.

Fig. 4

Additional and non-obvious functions of apoptotic cells. (a) Regeneration: In the metazoan Hydra, tissue injury can lead to apoptosis of the cells, which stimulates regenerative processes in the nearby viable tissues via a process called ‘apoptosis-induced compensatory proliferation’. Apoptosis is required for the re-growth of the new Hydra head. (b) Caspase-dependent inhibition of interferon production: In the context of a viral infection, apoptosis leads to the activation of caspases that is linked to inhibiting interferon-α/β (IFN-α/β) production induced by the mitochondrial DNA (mtDNA)-mediated activation of the cGAS-STING pathway. (c) Myoblast fusion: During muscle development and regeneration after muscle injury, apoptosis of myoblasts triggers BAI1 signaling through the ELMO-DOCK complex, leading to Rac activation. This pathway contributes to the fusion of healthy myoblasts with the nascent myotube and promotes muscle development and regeneration. (d) Pathogen exploitation of engulfment receptors: Bacteria, parasites and even viruses have evolved to utilize the apoptotic cell engulfment receptors for the cellular entry and induction of anti-inflammatory signaling in the host cell (phagocyte), aiding in the establishment and persistence of infection.