Apoptotic bodies: bioactive treasure left behind by the dying cells with robust diagnostic and therapeutic application potentials

Abstract

Apoptosis, a form of programmed cell death, is essential for growth and tissue homeostasis. Apoptotic bodies (ApoBDs) are a form of extracellular vesicles (EVs) released by dying cells in the last stage of apoptosis and were previously regarded as debris of dead cells. Recent studies unraveled that ApoBDs are not cell debris but the bioactive treasure left behind by the dying cells with an important role in intercellular communications related to human health and various diseases. Defective clearance of ApoBDs and infected-cells-derived ApoBDs are possible etiology of some diseases. Therefore, it is necessary to explore the function and mechanism of the action of ApoBDs in different physiological and pathological conditions. Recent advances in ApoBDs have elucidated the immunomodulatory, virus removal, vascular protection, tissue regenerative, and disease diagnostic potential of ApoBDs. Moreover, ApoBDs can be used as drug carriers enhancing drug stability, cellular uptake, and targeted therapy efficacy. These reports from the literature indicate that ApoBDs hold promising potential for diagnosis, prognosis, and treatment of various diseases, including cancer, systemic inflammatory diseases, cardiovascular diseases, and tissue regeneration. This review summarizes the recent advances in ApoBDs-related research and discusses the role of ApoBDs in health and diseases as well as the challenges and prospects of ApoBDs-based diagnostic and therapeutic applications.

Keywords: Apoptotic bodies; Diseases; Immunomodulation; Mesenchymal stem cells; Tissue regeneration.

Fig. 1

The characteristics of various EVs: exosomes, microvesicles, and ApoBDs. Created with BioRender.com

Fig. 2

Pathways involved in apoptosis. The intrinsic pathway is activated by various stresses, such as medicine, radiation, and fever. This induces the BCL-2 protein to regulate the permeability of the mitochondrial outer membrane (MOM) and then opens the mitochondrial permeability transition pore. Cytochrome C, via binding to APAF1, causes the formation of the apoptosome, and the procaspase 9 in this process cluster. Caspase 9 proteolytically activates the effector caspases 3 and 7. The extrinsic pathway is death-receptor-induced apoptosis. FAS ligand bind to FAS receptors to activate downstream signaling. This leads to the recruitment of pro-caspase 8, FADD, and TRADD, resulting in the formation of DISC. DISC catalyzes the activation of caspase 8. Caspase 8 directly or indirectly activates caspases 3 and 7. Finally, ApoBDs are formed. Created with BioRender.com

Fig. 3

The formation of ApoBDs. First, the rupture of the cytoskeletal plasma membrane causes apoptotic membrane blebbing and the loss of the phospholipid asymmetry in the plasma membrane triggers membrane blebbing, which is thought to be regulated by kinases including the PAK2, LIMK1, and ROCK1. And then, the apoptotic membrane protrudes in different types, including microtubule spikes, apoptopodia, and beaded apoptopodia. PANX1 and PlexB2 have a major influence on their nuclear contents and size. At the later stage of cell death, the apoptotic cells are fragmented and ApoBDs are formed. Created with BioRender.com

Fig. 4

Differential centrifugation for ApoBDs isolation. After induction of apoptosis by UV or staurosporine, the supernatant is centrifuged at 300–400g for 10 min at 4 ℃ to remove the large apoptotic cells and fragments. Then, the supernatant is centrifuged at 1000g–4000g for 15–30 min at 4 ℃ to separate micron-size ApoBDs. At last, nano-size ApoBDs are obtained from precipitate through centrifugation of the supernatant at 16,000g for 30 min at 4 ℃. Created with BioRender.com

Fig. 5

Functional roles of ApoBDs in cell clearance. ApoBDs recruit motile phagocytes by releasing a “find-me” signal to eliminate apoptotic cells and maintain tissue homeostasis. Many systemic diseases, such as SLE, glomerulonephritis, COPD, and atherosclerosis are related to the failure of ApoBDs clearance on time. Created with BioRender.com

Fig. 6

Functional roles of ApoBDs in intercellular communication. ApoBDs can bind to a variety of receptor cells to facilitate intercellular communication and promote proliferation, vascular protection, virus removal, genetic information exchange, and immunomodulation through signaling molecules. Created with BioRender.com

Fig. 7

ApoBDs have the potential to promote tissue regeneration. Bone regeneration: ApoBDs promote bone regeneration by stimulating osteoclast differentiation and inhibiting the M1 polarization of macrophages. Cardiovascular regeneration: ApoBDs of fibroblasts stimulate the formation of endothelial precursor clones and ApoBDs of cardiomyocytes promote clones of cardiomyocyte precursors. Skin regeneration: ApoBDs promote the proliferation of stem cells and the M2 polarization of macrophages. Created with BioRender.com

Fig. 8

The biological mechanism and functional molecules of ApoBDs. ApoBDs exert various biological functions by modulating different signal pathways and transferring miRNA and cytokines

Fig. 9

Application of ApoBDs in various diseases. As markers of cell death and carriers of dying cell material, ApoBDs have great potential to apply in disease diagnosis/prognosis, vaccines, immunomodulation, drug delivery, and targeted therapy. Created with BioRender.com

Fig. 10

Therapeutic potential of ApoBDs in tissue regeneration, inflammation modulation, and cancer treatment. A Transmission electron microscopy image showing the morphology of ApoBDs. Source: Reprinted with permission from Ref. (99). B Scanning electron microscopy image of the presence of ApoBDs from H2O2-treated cells. Source: Reprinted with permission from Ref. (8). Copyright 2019, with permission from Creative Commons CC BY. C ApoBDs promoted cutaneous wound healing. Ci Photographs of cutaneous wounds during the wound healing procedure. Cii Quantification of the wound healing rate. Source: Reprinted with permission from Ref. (66). Copyright 2020, with permission from Creative Commons CC BY. D Images (Di) and quantification (Dii) of the cytokeratin 14 expression in the skin tissue. Source: Reprinted with permission from Ref. (66). Copyright 2020, with permission from Creative Commons CC BY. E, F Mature osteoclast-derived ApoBDs have the highest osteogenic potency in ApoBDs, microvesicles, and exosomes from mature osteoclast, bone marrow macrophage, and preosteoclast. ALP activity (Ei and Fi), ALP stain (Eii), Alizarin red (Fii), and quantification of Alizarin Red activity (Eiii and Fiii). Source: Reprinted with permission from Ref. ( Copyright 2019, with permission from Creative Commons CC BY

Fig. 11

The possible approaches for modifications of ApoBDs for therapeutic applications. A ApoBDs have great potential in regeneration, anti-inflammation, and anti-infection, through drug delivery ability. B Hydrogel can be used for sustained release of ApoBDs, which has promising capability in tissue regeneration and anti-tumor therapy. C Several methods have been explored to induce the drug delivery ability, physical properties, and biocompatibility of ApoBDs, such as constructed with inorganic nanoparticles, freeze and thaw, and membrane fusion with liposome. Created with BioRender.com

Fig. 12

Advances and prospects of ApoBDs in diagnostic and therapeutic applications. Created with BioRender.com