To be or not to be? How selective autophagy and cell death govern cell fate

Abstract

The health of metazoan organisms requires an effective response to organellar and cellular damage either by repair of such damage and/or by elimination of the damaged parts of the cells or the damaged cell in its entirety. Here, we consider the progress that has been made in the last few decades in determining the fates of damaged organelles and damaged cells through discrete, but genetically overlapping, pathways involving the selective autophagy and cell death machinery. We further discuss the ways in which the autophagy machinery may impact the clearance and consequences of dying cells for host physiology. Failure in the proper removal of damaged organelles and/or damaged cells by selective autophagy and cell death processes is likely to contribute to developmental abnormalities, cancer, aging, inflammation, and other diseases.

Figure 1. Overview of the general autophagy pathway

Shown are cellular events and selected aspects of the molecular regulation involved in the lysosomal degradation pathway of autophagy in mammalian cells. Several membrane sources may serve as the origin of the autophagosome and/or to contribute to its expansion. A “pre-initiation” complex (also called the ULK complex) is negatively and positively regulated by upstream kinases that sense cellular nutrient and energy status, resulting in inhibitory and stimulatory phosphorylations on ULK1/2 proteins. In addition to nutrient sensing kinases shown here, other signals involved in autophagy induction may also regulate the activity of the ULK complex. The pre-initiation complex activates the “initiation complex” (also called the Class III PI3K complex) through ULK-dependent phosphorylation of key components, and likely, other mechanisms. Activation of the Class III PI3K complex requires the disruption of binding of Bcl-2 anti-apoptotic proteins to Beclin 1, and is also regulated by AMPK, and a variety of other proteins not shown in figure. The Class III PI3K complex generates PI3P at the site of nucleation of the isolation membrane (also known as the phagophore) which leads to the binding of PI3P binding proteins (such as WIPI/II), and the subsequent recruitment of proteins involved in the “elongation reaction” (also called the ubiquitin-like protein conjugation systems) to the isolation membrane. These proteins contribute to membrane expansion, resulting in the formation of a closed double-membrane structure, the autophagosome, which surrounds cargo destined for degradation. The phosphatidylethanolamine-conjugated form of the LC3 (LC3-PE), generated by the ATG4-dependent proteolytic cleavage of LC3, and the action of the E1 ligase, ATG7, the E2 ligase, ATG3, and the E3 ligase complex, ATG12/ATG5/ATG16L, is the only autophagy protein that stably associates with the mature autophagosome. The autophagosome fuses with a lysosome to form an autolysosome; inside the autolysosome, the sequestered contents are degraded and released into the cytoplasm for recycling. Late endosomes or multivesicular bodies can also fuse with autophagosomes generating intermediate structures known as amphisomes, and they also contribute to the formation of mature lysosomes. Additional proteins (not depicted in diagram) function in the fusion of autophagosomes and lysosomes. The general autophagy pathway has numerous functions in cellular homeostasis (examples listed in box labeled “physiological functions”) which contribute to the role of autophagy in development and protection against different diseases.

Figure 2. Roles of autophagy proteins in the removal of unwanted organelles and in the removal of cells

The left panel shows Parkin-dependent and Parkin-independent mechanisms involved in the selective degradation of mitochondria by autophagy (mitophagy). In Parkin-dependent mitophagy, mitochondrial damage and loss of mitochondrial membrane potential (ΔΨm) leads to localization of the kinase, PINK1, on the cytoplasmic surface of the mitochondria, resulting in recruitment of the E3 ubiquitin ligase, Parkin, to the mitochondria, followed by the ubiquitination of mitochondrial proteins, and the formation of an isolation membrane that surrounds the damaged mitochondria. In Parkin-independent mitophagy, protein such as Nix (shown in figure), BNIP3, and FUNDC1 (not shown in figure) bind to LC3. Other autophagy proteins may be involved in Parkin-dependent and Parkin-independent mitophagy (discussed in text). The precise details of how an isolation membrane is formed around specific mitochondria earmarked for degradation are unclear. Other damaged/unwanted organelles such as ER, peroxisomes, and lipid droplets can also be degraded by selective autophagy; the molecular mechanisms of these forms of selective autophagy are not well understood in mammalian cells. The right panel depicts roles of LC3-associated phagocytosis (LAP) of apoptotic corpses and of live cells (entosis). In LAP, components of the autophagy initiation complex (Beclin 1, VPS34) are recruited to the phagosome, which leads to recruitment of LC3-PE, and facilitation of phagolyosomal fusion. This process requires other components of the elongation machinery, but – in contrast to general autophagy or selective autophagy – proceeds independently of the ULK pre-initiation complex.

Figure 3. Cell death pathways engaged by cellular damage

Cellular damage induces cell death by inducing expression and/or modification of pro-apoptotic BH3-only proteins of the Bcl-2 family (inset) which engage the mitochondrial pathway of apoptosis, in which MOMP releases proteins of the mitochondrial inter-membrane space. Among these is cytochrome c, which activates APAF1 to form a caspase-activation platform (the apoptosome) that binds and activates caspase-9. This then cleaves and thereby activates executioner caspases to promote apoptosis. Cellular damage can also induce the expression of death ligands of the TNF family, which bind their receptors to promote the activation of caspase-8 by FADD. The latter is antagonized by expression of c-FLIP_L, and the caspase-8-FLIP heterodimer does not promote apoptosis, but instead blocks another cell death pathway engaged by death receptors, necroptosis. Necroptosis involves the activation of RIPK1 and RIPK3, resulting in phosphorylation and activation of the pseudokinase, MLKL, which promotes an active necrotic cell death.